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Git Internals part 3: the SSH transport


This publish is the final within the “Git Internals” collection. Partially 1, we launched git’s object storage mannequin, and partially 2, we noticed how objects are saved in “packfiles” to save lots of area.

On this publish, we’ll flip our consideration from native git repositories (saved in your laptop) to distant ones (saved on a server, e.g. GitHub). We’ll examine the protocol git makes use of to speak with “remotes” and implement git fetch from the bottom up.

When you use on-line git repositories (like these on GitHub) ceaselessly, you might know you may git clone a repository utilizing both an HTTP/HTTPS URL (e.g. https://github.com/git/git.git) or an “SSH” URL (e.g. git@github.com:git/git.git). The distinction between these URLs is the protocol that git makes use of to speak with the GitHub server throughout a git clone, git fetch, git pull, or git push. git implements a number of protocols: “dumb” HTTP/HTTPS, “sensible” HTTP/HTTPS, SSH, and “git”. The dumb protocol is much less environment friendly, so it’s not often utilized in apply. The sensible protocols use the identical procedures, however differ within the underlying protocol used to connect with the server. We’ll concentrate on the SSH one as a result of it’s so widespread and is an attention-grabbing software of SSH.

The protocol is human-readable, so we’ll principally be taught the way it works by observing what the consumer and server ship one another. If you wish to have a look at git’s documentation on the subject, listed here are some good assets:

Sorry for the delay on half 3! My life has been busier than anticipated the previous few months.

The supply code for this publish will be discovered here.



The place does SSH are available?

Utilizing an SSH git URL requires you to add your SSH public key to the server. SSH keys are sometimes used to authenticate SSH connections, so that you would possibly be capable of guess that your git consumer is speaking with the git server over an SSH connection.

When you’re unfamiliar with SSH, this is a fast overview of the way it’s used. (We cannot fear about how SSH is applied, however that is additionally fascinating.) SSH lets you run terminal instructions on a distant laptop. For instance, I can run hostname to see the identify of my laptop:

csander:~ $ hostname
csander-mac.native
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I may also use SSH to open a terminal on the EC2 occasion internet hosting calebsander.com and run hostname there:

csander:~ $ ssh ubuntu@calebsander.com # ubuntu is the person to log in as
ubuntu@ip-172-31-52-11:~ $ hostname
ip-172-31-52-11
ubuntu@ip-172-31-52-11:~ $ exit
Connection to calebsander.com closed.
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By default, ssh runs a terminal course of (e.g. bash) on the server. You’ll be able to inform ssh to run a unique command as a substitute:

csander:~ $ ssh ubuntu@calebsander.com hostname
ip-172-31-52-11
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A key characteristic git will leverage is that the SSH connection is bidirectional: your native normal enter is linked to the enter of the distant course of and the distant normal output again to the native one. That is best to see when working a command like cat (copy normal enter to plain output). When you ship kind a line of textual content, it will get despatched to the cat course of working on the opposite laptop, which prints the road, inflicting it to be despatched again.

csander:~ $ ssh ubuntu@calebsander.com cat
abc # enter despatched to server
abc # output despatched again
123 # enter
123 # output
(enter Ctrl+D to finish the usual enter, terminating the method)
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SSH gives each authentication (the server checks that the consumer’s SSH key could entry the repository) and encryption (the communication is hidden from anybody snooping on the connection), which is probably going why git selected to make use of it.

If we run the command git clone git@github.com:git/git.git and use ps aux | grep ssh to listing the SSH processes whereas it is working, we are able to see the SSH command that git used:

/usr/bin/ssh -o SendEnv=GIT_PROTOCOL git@github.com git-upload-pack 'git/git.git'
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-o SendEnv=GIT_PROTOCOL is pointless, so the SSH command will be simplified to:

ssh git@github.com git-upload-pack git/git.git
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There we are able to see all of the items of the URL git@github.com:git/git.git! The half earlier than the : is the SSH login (e.g. person@area.identify) and the half after the : is the argument to the git-upload-pack executable, specifying the repository. (It might be complicated that git-upload-pack is used for a clone/fetch/pull and git-receive-pack is used for push, however that is from the attitude of the server.)

When you’re curious, the GitHub SSH server is restricted so you may’t run different instructions:

$ ssh git@github.com
PTY allocation request failed on channel 0
Hello calebsander! You've got efficiently authenticated, however GitHub doesn't present shell entry.
Connection to github.com closed.
$ ssh git@github.com echo Good day world
Invalid command: 'echo Good day world'
  You seem like utilizing ssh to clone a git:// URL.
  Make sure that your core.gitProxy config choice and the
  GIT_PROXY_COMMAND setting variable are NOT set.
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Now that we all know the SSH command, we are able to run it ourselves and see what the server sends again:

$ ssh git@github.com git-upload-pack git/git.git
014e74cc1aa55f30ed76424a0e7226ab519aa6265061 HEADmulti_ack thin-pack side-band side-band-64k ofs-delta shallow deepen-since deepen-not deepen-relative no-progress include-tag multi_ack_detailed allow-tip-sha1-in-want allow-reachable-sha1-in-want symref=HEAD:refs/heads/grasp filter object-format=sha1 agent=git/github-g2faa647c16c3
003d74cc1aa55f30ed76424a0e7226ab519aa6265061 refs/heads/foremost
003e4c53a8c20f8984adb226293a3ffd7b88c3f4ac1a refs/heads/maint
003f74cc1aa55f30ed76424a0e7226ab519aa6265061 refs/heads/grasp
003dd65ed663a79d75fb636a4602eca466dbd258082e refs/heads/subsequent
003d583a5781c12c1d6d557fae77552f6cee5b966f8d refs/heads/seen
003db1b3e2657f1904c7c603ea4313382a24af0fd91f refs/heads/todo
003ff0d0fd3a5985d5e588da1e1d11c85fba0ae132f8 refs/pull/10/head
0040c8198f6c2c9fc529b25988dfaf5865bae5320cb5 refs/pull/10/merge
...
003edcba104ffdcf2f27bc5058d8321e7a6c2fe8f27e refs/tags/v2.9.5
00414d4165b80d6b91a255e2847583bd4df98b5d54e1 refs/tags/v2.9.5^{}
0000(ready for enter)
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Okay, that is so much to unpack (pun undoubtedly meant), so let’s break it down!



Opening an SSH connection in Rust

First, we’ll see the right way to open this SSH connection in Rust. We are able to assemble the identical ssh command we ran ourselves, utilizing Stdio::piped() for the enter and output streams so we get an ssh_input that implements Write and an ssh_output implementing Learn.

use std::env;
use std::io;
use std::course of::{ChildStdin, ChildStdout, Command, Stdio};

// Utilizing the kinds and capabilities applied in earlier posts

struct Transport {
  ssh_input: ChildStdin,
  ssh_output: ChildStdout,
}

impl Transport {
  fn join(repository: &str) -> io::End result<Self> {
    // `repository` will seem like "git@github.com:git/git.git".
    // "git@github.com" is the SSH login (person "git", hostname "github.com").
    // "git/git.git" specifies the repository to fetch on this server.
    let repository_pieces: Vec<_> = repository.cut up(':').gather();
    let [login, repository] = <[&str; 2]>::try_from(repository_pieces)
      .map_err(|_| {
        make_error(&format!("Invalid SSH repository: {}", repository))
      })?;
    // Begin an SSH course of to connect with this repository.
    // We do not anticipate the `ssh` command to complete as a result of we're going to
    // talk forwards and backwards with the server by its normal enter and output.
    let mut ssh_process = Command::new("ssh")
      .args([login, "git-upload-pack", repository])
      .stdin(Stdio::piped())
      .stdout(Stdio::piped())
      .spawn()?;
    let ssh_input = ssh_process.stdin.take().ok_or_else(|| {
      make_error("Did not open ssh stdin")
    })?;
    let ssh_output = ssh_process.stdout.take().ok_or_else(|| {
      make_error("Did not open ssh stdout")
    })?;
    Okay(Transport { ssh_input, ssh_output })
  }
}

fn foremost() -> io::End result<()> {
  let args: Vec<_> = env::args().gather();
  let [_, repository] = <[String; 2]>::try_from(args).unwrap();
  let mut transport = Transport::join(&repository)?;
  // Print the SSH output
  io::copy(&mut transport.ssh_output, &mut io::stdout())?;
  Okay(())
}
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Operating this program provides the identical consequence as working the SSH command instantly:

$ cargo run git@github.com:git/git.git
014e74cc1aa55f30ed76424a0e7226ab519aa6265061 HEADmulti_ack thin-pack side-band side-band-64k ofs-delta shallow deepen-since deepen-not deepen-relative no-progress include-tag multi_ack_detailed allow-tip-sha1-in-want allow-reachable-sha1-in-want symref=HEAD:refs/heads/grasp filter object-format=sha1 agent=git/github-g2faa647c16c3
003d74cc1aa55f30ed76424a0e7226ab519aa6265061 refs/heads/foremost
003e4c53a8c20f8984adb226293a3ffd7b88c3f4ac1a refs/heads/maint
003f74cc1aa55f30ed76424a0e7226ab519aa6265061 refs/heads/grasp
003dd65ed663a79d75fb636a4602eca466dbd258082e refs/heads/subsequent
003d583a5781c12c1d6d557fae77552f6cee5b966f8d refs/heads/seen
003db1b3e2657f1904c7c603ea4313382a24af0fd91f refs/heads/todo
003ff0d0fd3a5985d5e588da1e1d11c85fba0ae132f8 refs/pull/10/head
0040c8198f6c2c9fc529b25988dfaf5865bae5320cb5 refs/pull/10/merge
...
003edcba104ffdcf2f27bc5058d8321e7a6c2fe8f27e refs/tags/v2.9.5
00414d4165b80d6b91a255e2847583bd4df98b5d54e1 refs/tags/v2.9.5^{}
(ready)
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Discovering the default distant URL

Within the instance above, we handed the specified repository URL to our program. However when utilizing git, it’s common to run git fetch/pull/push with out specifying a repository. By default, git makes use of the URL specified in the course of the preliminary git clone, so it have to be saved someplace. Exploring the .git listing, we see:

$ git clone git@github.com:git/git.git
Cloning into 'git'...
distant: Enumerating objects: 325167, carried out.
distant: Whole 325167 (delta 0), reused 0 (delta 0), pack-reused 325167
Receiving objects: 100% (325167/325167), 185.01 MiB | 7.77 MiB/s, carried out.
Resolving deltas: 100% (242985/242985), carried out.
Updating recordsdata: 100% (4084/4084), carried out.
$ cd git
$ cat .git/config
[core]
    repositoryformatversion = 0
    filemode = true
    naked = false
    logallrefupdates = true
    ignorecase = true
    precomposeunicode = true
[remote "origin"]
    url = git@github.com:git/git.git
    fetch = +refs/heads/*:refs/remotes/origin/*
[branch "master"]
    distant = origin
    merge = refs/heads/grasp
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There’s a [remote ...] part for every distant repository. By default, the repository used within the git clone command known as origin. The url parameter provides us the URL for that distant.

There’s additionally a [branch ...] part for every department, e.g. grasp, indicating which distant and distant ref identify to push and pull the department from by default.

For instance, think about working git pull with grasp checked out. The [branch "master"] and [remote "origin"] config sections translate this into fetching git@github.com:git/git.git and merging origin/grasp into grasp.

We are able to discover the URL for origin by parsing the config file after which extracting the url parameter from the [remote "origin"] part:

use std::collections::HashMap;
use std::fs::File;
use std::io::{BufRead, BufReader};

const CONFIG_FILE: &str = ".git/config";
// `r#` is helpful for string literals with quotes
const REMOTE_ORIGIN_SECTION: &str = r#"[remote "origin"]"#;
const URL_PARAMETER: &str = "url";

// A parsed .git/config file, represented as
// a map of part -> parameter -> worth
#[derive(Debug)]
struct ConfigFile(HashMap<String, HashMap<String, String>>);

impl ConfigFile {
  fn learn() -> io::End result<Self> {
    let config_file = File::open(CONFIG_FILE)?;
    let mut sections = HashMap::new();
    // The parameter values for the present part
    let mut parameters: Possibility<&mut HashMap<String, String>> = None;
    for line in BufReader::new(config_file).strains() {
      let line = line?;
      if let Some(parameter_line) = line.strip_prefix('t') {
        // The road is indented, so it is a parameter in a bit
        let (parameter, worth) = parameter_line.split_once(" = ")
          .ok_or_else(|| {
            make_error(&format!("Invalid parameter line: {:?}", parameter_line))
          })?;
        // All parameters must be beneath a bit
        let parameters = parameters.as_mut().ok_or_else(|| {
          make_error("Config parameter will not be in a bit")
        })?;
        parameters.insert(parameter.to_string(), worth.to_string());
      }
      else {
        // The road begins a brand new part
        parameters = Some(sections.entry(line).or_default());
      }
    }
    Okay(ConfigFile(sections))
  }

  fn get_origin_url(&self) -> Possibility<&str> {
    let remote_origin_section = self.0.get(REMOTE_ORIGIN_SECTION)?;
    let url = remote_origin_section.get(URL_PARAMETER)?;
    Some(url)
  }
}

fn foremost() -> io::End result<()> {
  let config = ConfigFile::learn()?;
  println!("Config file: {:#?}", config);
  let origin_url = config.get_origin_url().ok_or_else(|| {
    make_error("Lacking distant 'origin'")
  })?;
  println!("Distant 'origin' URL: {}", origin_url);
  Okay(())
}
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Operating this prints:

Config file: ConfigFile(
    {
        "[remote "origin"]": {
            "url": "git@github.com:git/git.git",
            "fetch": "+refs/heads/*:refs/remotes/origin/*",
        },
        "[core]": {
            "repositoryformatversion": "0",
            "naked": "false",
            "ignorecase": "true",
            "filemode": "true",
            "logallrefupdates": "true",
            "precomposeunicode": "true",
        },
        "[branch "master"]": {
            "distant": "origin",
            "merge": "refs/heads/grasp",
        },
    },
)
Distant 'origin' URL: git@github.com:git/git.git
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The SSH transport protocol



Chunks

Let’s attempt to perceive what the server sent over the SSH connection. It seems like a collection of strains, every beginning with a hexadecimal string. These seem like hashes, and actually they virtually are, besides they’re 44 characters lengthy as a substitute of 40. It’s possible you’ll discover that the primary 4 hexadecimal characters principally comply with the sample of “003x” or “004x”, and the final (empty) line has “0000” as its first 4 characters. You’ll be able to test that these 4 characters encode the size of every line (together with the 4 characters firstly and the newline character on the finish) in hexadecimal. The “0000” line is particular; it signifies the top of the strains being despatched. git documentation calls prefixing every line with its size in hexadecimal the “pkt-line” format. I will refer to those strains as “chunks“.

We’ll see chunks later with binary knowledge as a substitute of textual content, so we’ll begin with a technique to learn a bit as bytes:

const CHUNK_LENGTH_DIGITS: usize = 4;

impl Transport {
  fn read_chunk(&mut self) -> io::End result<Possibility<Vec<u8>>> {
    // Chunks begin with 4 hexadecimal digits indicating their size,
    // together with the size digits
    let length_digits: [_; CHUNK_LENGTH_DIGITS] =
      read_bytes(&mut self.ssh_output)?;
    let chunk_length = length_digits.iter().try_fold(0, |worth, &byte|  char_value as usize)
    ).ok_or_else(|| {
      make_error(&format!("Invalid chunk size: {:?}", length_digits))
    })?;
    // The chunk "0000" signifies the top of a sequence of chunks
    if chunk_length == 0 {
      return Okay(None)
    }

    let chunk_length = chunk_length.checked_sub(CHUNK_LENGTH_DIGITS)
      .ok_or_else(|| {
        make_error(&format!("Chunk size too brief: {}", chunk_length))
      })?;
    let mut chunk = vec![0; chunk_length];
    self.ssh_output.read_exact(&mut chunk)?;
    Okay(Some(chunk))
  }
}
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Then we are able to learn a textual content chunk by changing the bytes to a string and eradicating the n on the finish:

impl Transport {
  fn read_text_chunk(&mut self) -> io::End result<Possibility<String>> {
    let chunk = self.read_chunk()?;
    let chunk = match chunk {
      Some(chunk) => chunk,
      _ => return Okay(None),
    };

    let mut text_chunk = String::from_utf8(chunk).map_err(|_| {
      make_error("Invalid textual content chunk")
    })?;
    // Textual content chunks ought to finish with a newline character, however do not must.
    // Take away it if it exists.
    if text_chunk.ends_with('n') {
      text_chunk.pop();
    }
    Okay(Some(text_chunk))
  }
}

fn foremost() -> io::End result<()> {
  // ...

  let mut transport = Transport::join(origin_url)?;
  // Print every textual content chunk the server sends again
  whereas let Some(chunk) = transport.read_text_chunk()? {
    println!("{:?}", chunk);
  }
  Okay(())
}
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This program reveals each parsed textual content chunk till the 0000 line which signifies the top of chunks. The chunks look equivalent to the SSH output with the 4 hexadecimal characters faraway from the beginning of every line. We are able to additionally see a 0 byte (u{0}) within the first textual content chunk that was hidden by my terminal.

"74cc1aa55f30ed76424a0e7226ab519aa6265061 HEADu{0}multi_ack thin-pack side-band side-band-64k ofs-delta shallow deepen-since deepen-not deepen-relative no-progress include-tag multi_ack_detailed allow-tip-sha1-in-want allow-reachable-sha1-in-want symref=HEAD:refs/heads/grasp filter object-format=sha1 agent=git/github-g2faa647c16c3"
"74cc1aa55f30ed76424a0e7226ab519aa6265061 refs/heads/foremost"
"4c53a8c20f8984adb226293a3ffd7b88c3f4ac1a refs/heads/maint"
"74cc1aa55f30ed76424a0e7226ab519aa6265061 refs/heads/grasp"
"d65ed663a79d75fb636a4602eca466dbd258082e refs/heads/subsequent"
"583a5781c12c1d6d557fae77552f6cee5b966f8d refs/heads/seen"
"b1b3e2657f1904c7c603ea4313382a24af0fd91f refs/heads/todo"
"f0d0fd3a5985d5e588da1e1d11c85fba0ae132f8 refs/pull/10/head"
"c8198f6c2c9fc529b25988dfaf5865bae5320cb5 refs/pull/10/merge"
...
"dcba104ffdcf2f27bc5058d8321e7a6c2fe8f27e refs/tags/v2.9.5"
"4d4165b80d6b91a255e2847583bd4df98b5d54e1 refs/tags/v2.9.5^{}"
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Refs

Wanting on the strains despatched by the server, we are able to see that every one lists a commit hash and a reputation (HEAD, refs/heads/foremost, and many others.). The primary line additionally has an extra string of capabilities, which we’ll focus on shortly. These commit-name combos are known as “refs” (brief for “references”) and inform the consumer which commits it may possibly fetch. They fall into a number of classes:

  • HEAD: that is the default commit to take a look at when doing a git clone (it is equivalent to refs/heads/foremost)
  • refs/heads/BRANCH_NAME: these are the branches on the distant repository
  • refs/tags/TAG_NAME: these are the tags on the distant (not fetched by default)
  • refs/pull/PULL_REQUEST_NUMBER/head and /merge: these are GitHub-specific, indicating the present commit of every pull request and the commit that merged it into the repository (if relevant)

Here is code to learn the refs and capabilities returned by the server:

use std::collections::HashSet;

struct Refs {
  capabilities: HashSet<String>,
  // Map of ref identify (e.g. "refs/heads/foremost") to commit hashes
  refs: HashMap<String, Hash>,
}

impl Transport {
  fn receive_refs(&mut self) -> io::End result<Refs> {
    // The primary chunk incorporates the HEAD ref and an inventory of capabilities.
    // Even when the repository is empty, capabilities are nonetheless wanted,
    // so a hash of all 0s is distributed.
    let head_chunk = match self.read_text_chunk()? {
      Some(chunk) => chunk,
      _ => return Err(make_error("No chunk obtained from server")),
    };

    let (head_ref, capabilities) = head_chunk.split_once('').ok_or_else(|| {
      make_error("Invalid capabilities chunk")
    })?;
    let capabilities = capabilities.cut up(' ').map(str::to_string).gather();
    let mut refs = HashMap::new();
    let mut add_ref = |chunk: &str| -> io::End result<()> {
      // Every subsequent chunk incorporates a ref (a commit hash and a reputation)
      let (hash, ref_name) = chunk.split_once(' ').ok_or_else(|| {
        make_error("Invalid ref chunk")
      })?;
      let hash = Hash::from_str(hash)?;
      refs.insert(ref_name.to_string(), hash);
      Okay(())
    };
    add_ref(head_ref)?;
    whereas let Some(chunk) = self.read_text_chunk()? {
      add_ref(&chunk)?;
    }
    Okay(Refs { capabilities, refs })
  }
}

fn foremost() -> io::End result<()> {
  // ...

  let Refs { capabilities, refs } = transport.receive_refs()?;
  println!("Capabilities: {:?}", capabilities);
  for (ref_name, hash) in refs {
    println!("Ref {} has hash {}", ref_name, hash);
  }
  Okay(())
}
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Operating this program prints the capabilities and refs the server despatched again. Word that the order is randomized since we’re iterating over a HashSet and a HashMap.

Capabilities: {"deepen-since", "symref=HEAD:refs/heads/grasp", "object-format=sha1", "allow-reachable-sha1-in-want", "include-tag", "shallow", "thin-pack", "allow-tip-sha1-in-want", "side-band-64k", "deepen-not", "filter", "agent=git/github-g2faa647c16c3", "side-band", "multi_ack_detailed", "deepen-relative", "ofs-delta", "no-progress", "multi_ack"}
Ref refs/pull/531/head has hash 1572444361982199fdab9c6f6b7e94383717b6c9
Ref refs/pull/983/merge has hash d217f9ec363d5ed88a37ab15a72fad6b4d90acf1
Ref refs/pull/891/head has hash 7d7e794ab7286db0aea88c6e1eab881fc5d188f7
Ref refs/tags/v2.14.1^{} has hash 4d7268b888d7bb6d675340ec676e4239739d0f6d
...
Ref refs/tags/v1.2.3 has hash 51f2164fdc92913c3d1c6d199409b43cb9b6649f
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Capabilities

Each the server and consumer talk “capabilities” they help. This enables them every to implement new git options whereas remaining backwards-compatible with older shoppers and servers. For instance, the ofs-delta functionality signifies that the server can ship (or the consumer can perceive) “offset delta” objects in packfiles.

The server sends the listing of its capabilities and the consumer requests a subset of them to allow. This fashion, each the server and consumer help all of the enabled capabilities.

git additionally makes use of the capabilities to ship miscellaneous info (e.g. symref=HEAD:refs/heads/grasp signifies that grasp is the default department).

For now, we’ll solely request the ofs-delta functionality (if the server helps it). The final publish (half 2) has an in-depth dialogue of offset deltas, however the gist is that they make for smaller packfiles than hash deltas (that are at all times supported). Simply because the server sends its capabilities in its first ref chunk, the consumer requests capabilities in its first “need” chunk, which we’ll focus on subsequent.



Needs

As soon as the server has marketed the accessible refs, the consumer chooses which of them it needs by responding with their hashes. For instance, working git pull origin foremost, the consumer would solely request the commit for ref refs/heads/foremost. The server sends solely the requested commit objects and the commit, tree, and blob objects it (not directly) references.

Wished refs are despatched as textual content chunks beginning with need. The identical format (prefixed by hexadecimal size) is used when sending chunks to the server as when receiving chunks. The one distinction is that they’re written to the SSH enter reasonably than learn from the SSH output.

Here is a Rust implementation. Word that we are able to ship an empty chunk (transport.write_text_chunk(None)) identical to we obtain an empty chunk on the finish of the refs.

// git reserves chunk lengths 65521 to 65535
const MAX_CHUNK_LENGTH: usize = 65520;

impl Transport {
  fn write_text_chunk(&mut self, chunk: Possibility<&str>) -> io::End result<()> {
    let chunk_length = match chunk {
      // Contains the 4 hexadecimal digits firstly and the n on the finish
      Some(chunk) => CHUNK_LENGTH_DIGITS + chunk.len() + 1,
      _ => 0,
    };
    if chunk_length >= MAX_CHUNK_LENGTH {
      return Err(make_error("Chunk is simply too massive"))
    }

    write!(self.ssh_input, "{:04x}", chunk_length)?;
    if let Some(chunk) = chunk {
      write!(self.ssh_input, "{}n", chunk)?;
    }
    Okay(())
  }
}
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To request a hash, we ship a textual content chunk beginning with need. The primary need, like the primary ref chunk, may also embody capabilities that the consumer requests.

impl Transport {
  fn send_wants(&mut self, hashes: &[Hash], capabilities: &[&str])
    -> io::End result<()>
  {
    let mut first_want = true;
    for hash in hashes {
      println!("Requesting {}", hash);
      let mut chunk = format!("need {}", hash);
      if first_want {
        // Solely the primary need ought to listing capabilities
        for functionality in capabilities {
          chunk.push(' ');
          chunk += functionality;
        }
      }
      self.write_text_chunk(Some(&chunk))?;
      first_want = false;
    }
    self.write_text_chunk(None)
  }
}
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Placing this all collectively, we are able to now inform the server which refs to ship. We’ll fetch all of the branches (i.e. refs beginning with refs/heads/).

const BRANCH_REF_PREFIX: &str = "refs/heads/";
const REQUESTED_CAPABILITIES: &[&str] = &["ofs-delta"];

impl Transport {
  fn fetch(&mut self) -> io::End result<()> {
    let Refs { capabilities, refs } = self.receive_refs()?;
    // Request all of the capabilities that we would like and the server helps
    let use_capabilities: Vec<_> = REQUESTED_CAPABILITIES.iter()
      .copied()
      .filter(|&functionality| capabilities.incorporates(functionality))
      .gather();
    // Request all refs similar to branches
    // (not tags, pull requests, and many others.)
    let branch_refs: Vec<_> = refs.iter()
      .filter_map(|(ref_name, &hash)|  (department, hash))
      )
      .gather();
    let needs: Vec<_> = branch_refs.iter().map(|&(_, hash)| hash).gather();
    self.send_wants(&needs, &use_capabilities)?;

    // TODO: there's one other negotiation with the server about which objects
    // the consumer already has, however for now we'll faux it has none.
    // We'll implement this later (see "Haves").
    self.write_text_chunk(Some("carried out"))?;
    self.read_text_chunk()?;

    // TODO: obtain the objects the server sends again
    Okay(())
  }
}

fn foremost() -> io::End result<()> {
  // ...

  transport.fetch()
}
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Operating this program reveals that 6 branches (foremost, maint, grasp, subsequent, seen, and todo) had been requested. Since foremost and grasp are interchangeable, one of many commits is requested twice (that is pointless however allowed).

Requesting b1b3e2657f1904c7c603ea4313382a24af0fd91f
Requesting 583a5781c12c1d6d557fae77552f6cee5b966f8d
Requesting 74cc1aa55f30ed76424a0e7226ab519aa6265061
Requesting 74cc1aa55f30ed76424a0e7226ab519aa6265061
Requesting d65ed663a79d75fb636a4602eca466dbd258082e
Requesting 4c53a8c20f8984adb226293a3ffd7b88c3f4ac1a
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Packfiles make a triumphant return

As soon as the server is aware of what objects the consumer wants, it should ship them. There are doubtlessly 1000’s of commits, timber, and blobs, so it is vital to encode them compactly. When you learn the final publish (half 2), you will see it is a major use case for packfiles.

So the server builds a packfile containing all of the objects and sends it to the consumer over the SSH connection. git might unpack the objects from this packfile, however as we noticed within the final publish, it leaves them packed by default to save lots of space for storing.

We’ll do the identical, making a temp.pack file within the packfile listing. Because the packfile contents are despatched to the SSH output, we are able to merely copy the output to a file:

const TEMP_PACK_FILE: &str = ".git/objects/pack/temp.pack";

impl Transport {
  fn fetch(&mut self) -> io::End result<()> {
    // ...

    let mut pack_file = File::create(TEMP_PACK_FILE)?;
    io::copy(&mut self.ssh_output, &mut pack_file)?;
    Okay(())
  }
}

fn foremost() -> io::End result<()> {
  // ...

  transport.fetch()
}
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Operating this program efficiently downloads the pack file!

$ mkdir git
$ cd git
$ git init # create an empty git repository to check fetching all of the objects
Initialized empty Git repository
$ git distant add origin git@github.com:git/git.git
$ cargo run
$ file .git/objects/pack/temp.pack
.git/objects/pack/temp.pack: Git pack, model 2, 324311 objects
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Saving refs

Now we’ve got all of the objects we’d like, however sadly attempting to make use of them in a git command nonetheless would not work:

$ git log origin/foremost
deadly: ambiguous argument 'origin/foremost': unknown revision or path not within the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
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It is because we have not up to date the “distant refs” that we obtained from the server. For instance, the server instructed us that foremost is presently at commit 74cc1aa55f30ed76424a0e7226ab519aa6265061:

74cc1aa55f30ed76424a0e7226ab519aa6265061 refs/heads/foremost
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So we have to retailer this distant ref within the native repository.

Again in publish 1, we noticed that the native branches (and refs generally) are saved within the .git/refs listing. Every distant (e.g. origin) has its personal subdirectory in .git/refs with all of the refs fetched from the distant.

Right here is code to create the ref recordsdata in the course of the fetch:

use std::fs;
use std::path::Path;

const REMOTE_ORIGIN_REFS_DIRECTORY: &str = ".git/refs/remotes/origin";

fn save_remote_ref(department: &str, hash: Hash) -> io::End result<()> {
  let origin_ref_path = Path::new(REMOTE_ORIGIN_REFS_DIRECTORY).be a part of(department);
  // Create .git/refs/remotes and .../origin if they do not exist.
  // Additionally, if the department consists of "https://style-tricks.com/" (e.g. "characteristic/abc"), the trail can be
  // .../characteristic/abc, so the "characteristic" listing should even be created.
  fs::create_dir_all(origin_ref_path.guardian().unwrap())?;
  let mut origin_ref_file = File::create(origin_ref_path)?;
  write!(origin_ref_file, "{}n", hash)
}

impl Transport {
  fn fetch(&mut self) -> io::End result<()> {
    // ...

    for (department, hash) in branch_refs {
      save_remote_ref(department, hash)?;
    }
    Okay(())
  }
}
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Now, working our program information the distant refs:

$ ls -R .git/refs/remotes
origin

.git/refs/remotes/origin:
foremost   maint  grasp subsequent   seen   todo
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Let’s strive working the git log once more. This time, it fails with a unique error: it is aware of that origin/foremost is commit 74cc1aa55f30ed76424a0e7226ab519aa6265061, however it may possibly’t learn that object.

$ git log origin/foremost
deadly: unhealthy object origin/foremost
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git cannot discover the thing as a result of we’re lacking a .idx file for the temp.pack file we created. We’ll repair this subsequent.



Constructing an index file

As we noticed within the final publish, scanning by a packfile is sluggish, so git depends upon a corresponding “pack index” file to find objects within the packfile. The index file acts like a HashMap<Hash, u64>, making it quick to search for the place an object is situated within the corresponding packfile. It may be generated from the packfile by decompressing (and un-deltifying, if needed) every object within the pack and computing its hash. The server would not ship it as a result of it would not comprise any further info, so we have to construct it ourselves.

We’ll use the code we wrote final time to learn objects out of packfiles, with one foremost modification. Earlier than, we solely needed to unpack a single object, so if the thing was a HashDelta or OffsetDelta, we needed to unpack its base object, and its base object’s base object, and many others. till we discovered an undeltified object. If we use this strategy for all of the objects within the packfile, we could recompute every base object many instances. For instance, if each objects B and C are deltified with base object A, then unpacking the objects will unpack A 3 instances (when computing every of A, B, and C). And for HashDeltas that confer with base objects inside the packfile, we won’t even discover the bottom object by hash as a result of we have not created a pack index but! So I’ve modified the code to recollect the objects unpacked from the packfile thus far by each hash and offset. See the supply code for the total particulars.

First we’ll learn the non permanent packfile (see the final publish for an in depth dialogue of the packfile format):

// Creates a short lived pack index for the non permanent packfile
// and returns the packfile's checksum
fn build_pack_index() -> io::End result<Hash> {
  let mut pack_file = File::open(TEMP_PACK_FILE)?;
  let magic = read_bytes(&mut pack_file)?;
  if magic != *b"PACK" {
    return Err(make_error("Invalid packfile"))
  }

  let model = read_u32(&mut pack_file)?;
  if model != 2 {
    return Err(make_error("Surprising packfile model"))
  }

  let total_objects = read_u32(&mut pack_file)?;
  // Cache the unpacked objects by offset and hash
  let mut object_cache = PackObjectCache::default();
  // Depend what number of objects have a hash beginning with every byte
  let mut first_byte_objects = [0u32; 1 << u8::BITS];
  // Retailer the place every hash is situated within the packfile
  // (the sorted model of that is the index)
  let mut object_offsets = Vec::with_capacity(total_objects as usize);
  // Unpack every object
  for _ in 0..total_objects {
    let offset = get_offset(&mut pack_file)?;
    let object = read_pack_object(&mut pack_file, offset, &mut object_cache)?;
    let object_hash = object.hash();
    first_byte_objects[object_hash.0[0] as usize] += 1;
    let offset = u32::try_from(offset).map_err(|_| {
      make_error("Packfile is simply too massive")
    })?;
    object_offsets.push((object_hash, offset));
  }
  let pack_checksum = read_hash(&mut pack_file)?;
  assert!(at_end_of_stream(&mut pack_file)?);

  // TODO: produce index file

  Okay(pack_checksum)
}
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Though the final publish mentioned version-2 index recordsdata, we’ll make a version-1 one for simplicity. git can nonetheless perceive them; the one restriction is that they will solely characterize offsets that slot in a u32 (therefore the test above). Here is the implementation:

const TEMP_INDEX_FILE: &str = ".git/objects/pack/idx.pack";

fn build_pack_index() -> io::End result<Hash> {
  // ...

  let mut index_file = File::create(TEMP_INDEX_FILE)?;
  let mut cumulative_objects = 0;
  for objects in first_byte_objects {
    cumulative_objects += objects;
    // The quantity (u32) of hashes with first byte <= 0, 1, ..., 255
    index_file.write_all(&cumulative_objects.to_be_bytes())?;
  }
  // Every hash and its offset (u32) within the pack file,
  // sorted for environment friendly lookup
  object_offsets.kind();
  for (hash, offset) in object_offsets {
    index_file.write_all(&offset.to_be_bytes())?;
    index_file.write_all(&hash.0)?;
  }
  // A SHA-1 checksum of the pack file
  index_file.write_all(&pack_checksum.0)?;
  // TODO: this must be a SHA-1 hash of the contents of the index file.
  // However git would not test it when studying the index file, so we'll skip it.
  index_file.write_all(&[0; HASH_BYTES])?;
  Okay(pack_checksum)
}
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And at last, we rename the non permanent pack and index recordsdata with the pack checksum, like git does:

impl Transport {
  fn fetch(&mut self) -> io::End result<()> {
    // ...

    let pack_hash = build_pack_index()?;
    // Rename the packfile to, e.g.
    // pack-bda11b853cfa9131a39b2e3e55f15bb7f7485450.pack
    let pack_file_name = Path::new(PACKS_DIRECTORY)
      .be a part of(format!("pack-{}{}", pack_hash, PACK_FILE_SUFFIX));
    fs::rename(TEMP_PACK_FILE, pack_file_name)?;
    // Rename the index file to, e.g.
    // pack-bda11b853cfa9131a39b2e3e55f15bb7f7485450.idx
    let index_file_name = Path::new(PACKS_DIRECTORY)
      .be a part of(format!("pack-{}{}", pack_hash, INDEX_FILE_SUFFIX));
    fs::rename(TEMP_INDEX_FILE, index_file_name)?;

    // ...
  }
}
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If we do one other fetch, we generate an index file and our git log lastly works! When you’re attempting this at residence, be certain that to run in launch mode, or in any other case it will likely be a lot too sluggish! (The code might most likely be sped up considerably by utilizing BufReaders and BufWriters with these recordsdata and the SSH output.)

$ ls -lh .git/objects/pack
complete 394896
-rw-r--r--  1 csander  workers   7.4M Mar 19 15:56 pack-bda11b853cfa9131a39b2e3e55f15bb7f7485450.idx
-rw-r--r--  1 csander  workers   185M Mar 19 15:56 pack-bda11b853cfa9131a39b2e3e55f15bb7f7485450.pack
$ git log origin/foremost
commit 74cc1aa55f30ed76424a0e7226ab519aa6265061 (origin/grasp, origin/foremost)
Writer: Junio C Hamano <gitster@pobox.com>
Date:   Wed Mar 16 17:45:59 2022 -0700

    The twelfth batch

    Signed-off-by: Junio C Hamano <gitster@pobox.com>
...
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We are able to even use git present to indicate the diff of this commit, which requires studying commit, tree, and blob objects from the packfile:

$ git present origin/foremost
commit 74cc1aa55f30ed76424a0e7226ab519aa6265061
Writer: Junio C Hamano <gitster@pobox.com>
Date:   Wed Mar 16 17:45:59 2022 -0700

    The twelfth batch

    Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/RelNotes/2.36.0.txt b/Documentation/RelNotes/2.36.0.txt
index 6b2c6bfcc7..d67727baa1 100644
--- a/Documentation/RelNotes/2.36.0.txt
+++ b/Documentation/RelNotes/2.36.0.txt
@@ -70,6 +70,10 @@ UI, Workflows & Options
  * The extent of verbose output from the ort backend throughout internal merge
    has been aligned to that of the recursive backend.

+ * "git distant rename A B", relying on the variety of remote-tracking
+   refs concerned, takes very long time renaming them.  The command has been
+   taught to indicate progress bar whereas making the person wait.
+

 Efficiency, Inner Implementation, Growth Help and many others.

@@ -122,6 +126,12 @@ Efficiency, Inner Implementation, Growth Help and many others.
  * Makefile refactoring with a little bit of suffixes rule stripping to
    optimize the runtime overhead.

+ * "git stash drop" is reimplemented as an inner name to
+   reflog_delete() perform, as a substitute of invoking "git reflog delete"
+   by way of run_command() API.
+
+ * Depend string_list objects in size_t, not "unsigned int".
+

 Fixes since v2.35
 -----------------
@@ -299,6 +309,17 @@ Fixes since v2.35
    Changes have been made to accommodate these adjustments.
    (merge b0b70d54c4 fs/gpgsm-update later to maint).

+ * The untracked cache newly computed weren't written again to the
+   on-disk index file when there isn't a different change to the index,
+   which has been corrected.
+
+ * "git config -h" didn't describe the "--type" choice accurately.
+   (merge 5445124fad mf/fix-type-in-config-h later to maint).
+
+ * The way in which technology quantity v2 within the commit-graph recordsdata are
+   (not) dealt with has been corrected.
+   (merge 6dbf4b8172 ds/commit-graph-gen-v2-fixes later to maint).
+
  * Different code cleanup, docfix, construct repair, and many others.
    (merge cfc5cf428b jc/find-header later to maint).
    (merge 40e7cfdd46 jh/p4-fix-use-of-process-error-exception later to maint).
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Haves

Nice, we are able to clone an actual repository!

Now we could say there’s a slight change to the repository (e.g. one commit is pushed to foremost). If we do Transport::fetch() once more, we’ll obtain a brand new packfile with all of the objects now within the distant repository. This is able to work, however sadly we might find yourself with two copies of every object that was already within the repository!

We might undoubtedly wish to keep away from losing area storing duplicate objects. We might do that by figuring out the duplicate objects and making a brand new packfile with out them. However ideally the server would not have despatched them within the first place, as this makes the fetch unnecessarily sluggish.

To ensure that the server to know precisely which objects the packfile wants, the consumer wants to inform the server which of them it already has. After the need chunks are despatched within the transport protocol, the consumer informs the server of objects it already has by sending have chunks. The haves are terminated by a "carried out" chunk. The server responds with an ACK chunk if it acknowledges any of the consumer’s haves, or a NAK chunk in any other case. (See the multi_ack documentation for the extra difficult negotiation that git makes use of in apply.)

The consumer might inform the server each object it has, however there can simply be lots of of 1000’s, and so this could nonetheless take plenty of area even at 20 bytes every. git makes use of the truth that when the consumer receives objects from the server, it at all times will get precisely these which can be referenced by a number of commits. For instance, suppose there are commits C1, C2, and C3 with timber T1, T2, and T3, respectively, and some blobs:

C1 <-- C2 <-- C3
|      |      |
v      v      v
T1     T2     T3
|    /     /| 
v   v      v  v  v
B1  B2     B3 B4 B5
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When the consumer despatched need C2 earlier than, it obtained C1, C2, T1, T2, B1, B2, and B3 as a result of they C2 (not directly) references them. So if the consumer tells the server it has C2, the server is aware of it has all these objects, however not C3, T3, B4, or B5.

Subsequently, the consumer can simply say the most recent commit it has fetched on every distant department and the server will know precisely which of its objects the consumer already has. (git’s implementation additionally checks for commits from the consumer which can be on the distant with out the consumer’s data. For instance, the consumer pushed to distant A and another person then fetched and pushed to distant B. However we can’t fear about optimizing for that scenario.)

We are going to ship a have for the commit hash we’ve got recorded for every distant department:

use std::path::PathBuf;

impl Transport {
  // Sends haves for all refs beneath the given ref listing
  fn send_haves_dir(&mut self, ref_path: &mut PathBuf) -> io::End result<()> {
    let entries = fs::read_dir(&ref_path);
    if let Err(err) = &entries {
      if err.form() == ErrorKind::NotFound {
        // If .git/refs/remotes/origin would not exist, there are not any haves
        return Okay(())
      }
    }

    for entry in entries? {
      let entry = entry?;
      ref_path.push(entry.file_name());
      let entry_type = entry.file_type()?;
      if entry_type.is_dir() {
        // Discover subdirectories recursively (to search out refs containing "https://style-tricks.com/")
        self.send_haves_dir(ref_path)?;
      }
      else {
        let hash = fs::read_to_string(&ref_path)?;
        let hash = Hash::from_str(hash.trim_end())?;
        self.write_text_chunk(Some(&format!("have {}", hash)))?;
      }
      ref_path.pop();
    }
    Okay(())
  }

  fn send_haves(&mut self) -> io::End result<()> {
    fn valid_have_response(response: Possibility<&str>) -> bool {
      // Anticipate "ACK {HASH}" if acknowledged, "NAK" in any other case
      match response {
        Some("NAK") => true,
        Some(response) => {
          match response.strip_prefix("ACK ") {
            Some(hash) => Hash::from_str(hash).is_ok(),
            _ => false,
          }
        }
        _ => false,
      }
    }

    // Ship haves for all the newest commits we've got fetched
    self.send_haves_dir(&mut PathBuf::from(REMOTE_ORIGIN_REFS_DIRECTORY))?;
    self.write_text_chunk(Some("carried out"))?;
    let response = self.read_text_chunk()?;
    if !valid_have_response(response.as_deref()) {
      return Err(make_error("Invalid ACK/NAK"))
    }

    Okay(())
  }

  fn fetch(&mut self) -> io::End result<()> {
    // ...

    self.send_wants(&needs, &use_capabilities)?;

    self.send_haves()?;

    // ...
  }
}
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If we fetch the git repository once more with no new commits, the server sends an empty packfile as a result of the consumer already has all of the required objects:

$ cargo run
$ ls -lh .git/objects/pack
complete 410624
-rw-r--r--  1 csander  workers   1.0K Mar 19 17:29 pack-029d08823bd8a8eab510ad6ac75c823cfd3ed31e.idx
-rw-r--r--  1 csander  workers    32B Mar 19 17:29 pack-029d08823bd8a8eab510ad6ac75c823cfd3ed31e.pack
-rw-r--r--  1 csander  workers   7.4M Mar 19 16:38 pack-8641e8298f69b5dc78c3eb224dc508757f59a13f.idx
-rw-r--r--  1 csander  workers   185M Mar 19 16:37 pack-8641e8298f69b5dc78c3eb224dc508757f59a13f.pack
$ file .git/objects/pack/pack-029d08823bd8a8eab510ad6ac75c823cfd3ed31e.pack
.git/objects/pack/pack-029d08823bd8a8eab510ad6ac75c823cfd3ed31e.pack: Git pack, model 2, 0 objects
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Facet-band progress updates

It might take some time for the server to arrange and transmit a packfile, so it is useful to supply the person some progress updates. The protocol we have seen thus far would not permit for this, however there may be one more functionality, side-band-64k, to allow it.

As a substitute of sending the packfile instantly on the SSH connection, the server breaks it up and sends each bit inside a bit. Between packfile chunks, the server can ship progress or error message chunks. The primary byte of every chunk signifies the kind of chunk (1 for packfile knowledge, 2 for progress message, or 3 for deadly error message). The rest of the chunk is both the subsequent piece of the packfile or a message to print. An empty chunk is distributed to terminate the side-band chunks.

Right here is the implementation:

const SIDE_BAND_CAPABILITY: &str = "side-band-64k";
const REQUESTED_CAPABILITIES: &[&str] = &["ofs-delta", SIDE_BAND_CAPABILITY];

impl Transport {
  fn receive_side_band_pack(&mut self, pack_file: &mut File) -> io::End result<()> {
    whereas let Some(chunk) = self.read_chunk()? {
      let (&chunk_type, chunk) = chunk.split_first().ok_or_else(|| {
        make_error("Lacking side-band chunk kind")
      })?;
      match chunk_type {
        // Packfile knowledge
        1 => pack_file.write_all(chunk)?,
        // Progress message; print to stderr
        2 => io::stderr().write_all(chunk)?,
        // Deadly fetch error message
        3 => {
          let err = format!("Fetch error: {}", String::from_utf8_lossy(chunk));
          return Err(make_error(&err))
        }
        _ => {
          let err = format!("Invalid side-band chunk kind {}", chunk_type);
          return Err(make_error(&err))
        }
      }
    }
    Okay(())
  }

  fn fetch(&mut self) -> io::End result<()> {
    // ...

    let mut pack_file = File::create(TEMP_PACK_FILE)?;
    // Examine whether or not we had been capable of allow side-band-64k
    if capabilities.incorporates(SIDE_BAND_CAPABILITY) {
      // The packfile is wrapped in side-band chunks
      self.receive_side_band_pack(&mut pack_file)?;
    }
    else {
      // The SSH stream has the packfile contents
      io::copy(&mut self.ssh_output, &mut pack_file)?;
    }

    // ...
  }
}
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If we now name Transport::fetch(), we see the server’s progress indicators:

Enumerating objects: 324311, carried out.
Whole 324311 (delta 0), reused 0 (delta 0), pack-reused 324311
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Right here, the server had already created a packfile with the required objects and is just sending it to us. If the git server must generate a brand new packfile, we’ll see further standing indicators, for instance:

Enumerating objects: 7605, carried out.
Counting objects: 100% (630/630), carried out.
Compressing objects: 100% (292/292), carried out.
Whole 7605 (delta 421), reused 448 (delta 333), pack-reused 6975
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Within the “Counting objects” part, git is figuring out which objects it would not have already got in packfiles (630 = 7605 – 6975). Within the “Compressing objects” part, git is creating deltified representations for a few of these new objects.

For lengthy fetches, you could have observed that these progress indicators replace periodically. When you’re questioning how that works, it is by printing the r (carriage return) character adopted by the brand new contents of the road. r this resets the terminal’s printing location to the beginning of the present line, however in contrast to n, would not advance to the subsequent line.



push protocol

We have coated all the main elements of a git fetch over the SSH transport. However how does a git push work? Maybe unsurprisingly, git reuses a lot of the SSH protocol for pushes. A lot is identical that I do not assume there’s a lot to be taught by implementing git push too.

The most important variations between fetch and push are:

  • The SSH command invokes git-receive-pack as a substitute of git-upload-pack
  • No have negotiation is required as a result of the consumer already is aware of which of its commits the server has (because it pushed them)
  • After receiving the listing of refs from the server, the consumer signifies which of them it needs to create (e.g. new department), replace (e.g. new commit on department), or delete (e.g. take away department)
  • The consumer sends the packfile of latest objects to the server



The tip

And that is a wrap on the git internals collection! We realized how a big portion of git works beneath the hood, from the .git listing to how repository historical past is saved in objects, from how packfiles mix and compress objects to how a git consumer and server talk to share a repository. Hopefully the subsequent time you run a git command you will have a newfound understanding and appreciation for the way it does its process.

Sorry these posts ended up being so lengthy; there are simply so many attention-grabbing items within the git puzzle! Please let me know if there are every other git matters you want me to cowl. I’ve a number of different (hopefully shorter!) posts I might like to write down on a wide range of matters, so keep tuned.

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