gh-stack

I use this tool to help managed stacked pull requests on Github, which are notoriously difficult to manage manually. Here are a few examples:

• https://unhashable.com/stacked-pull-requests-keeping-github-diffs-small
• https://stackoverflow.com/questions/26619478/are-dependent-pull-requests-in-github-possible
• https://gist.github.com/Jlevyd15/66743bab4982838932dda4f13f2bd02a

This tool assumes that:

• All PRs in a single "stack" all have a unique identifier in their title (I typically use a Jira ticket number for this).
• All PRs in the stack live in a single GitHub repository.
• All remote branches that these PRs represent have local branches named identically.

It then looks for all PRs containing this containing this identifier and builds a dependency graph in memory. This can technically support a "branched stack" instead of a single chain, but I haven't really tried the latter style. With this graph built up, the tool can:

• Add a markdown table to the PR description (idempotently) of each PR in the stack describing all PRs in the stack.
• Log a simple list of all PRs in the stack (+ dependencies) to stdout.
• Automatically update the stack + push after making local changes.

• Installation
• Usage
  • Examples
• Strategy
• Disclaimer

Installation

Building from source is the only option at the moment:

# Install Rust
$ curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

# Configure `PATH`
$ export PATH="$HOME/.cargo/bin:$PATH"

# Install `gh-stack`
$ cargo install gh-stack

Usage

$ export GHSTACK_OAUTH_TOKEN='<personal access token>'

$ gh-stack

USAGE:
    gh-stack <SUBCOMMAND>

FLAGS:
    -h, --help    Prints help information

SUBCOMMANDS:
    annotate      Annotate the descriptions of all PRs in a stack with metadata about all PRs in the stack
    autorebase    Rebuild a stack based on changes to local branches and mirror these changes up to the remote
    log           Print a list of all pull requests in a stack to STDOUT
    rebase        Print a bash script to STDOUT that can rebase/update the stack (with a little help)

# Idempotently add a markdown table summarizing the stack
# to the description of each PR in the stack.
$ gh-stack annotate 'stack-identifier'

# Same as above, but precede the markdown table with the 
# contents of `filename.txt`.
$ gh-stack annotate 'stack-identifier' -p filename.txt

# Print a description of the stack to stdout.
$ gh-stack log 'stack-identifier'

# Automatically update the entire stack, both locally and remotely.
# WARNING: This operation modifies local branches and force-pushes.
$ gh-stack autorebase 'stack-identifier' -C /path/to/repo

# Emit a bash script that can update a stack in the case of conflicts.
# WARNING: This script could potentially cause destructive behavior.
$ gh-stack rebase 'stack-identifier'

Examples

This is a quick overview of the ways this tool could be used in practice.

1. Write some code, create local commits/branches:

   $ git checkout -b first
   # Write code
   $ git add -A; git commit -m 'first'
   
   $ git checkout -b second
   # Write code
   $ git add -A; git commit -m 'second #1'
   # Write code
   $ git add -A; git commit -m 'second #2'
   
   $ git checkout -b third
   # Write code
   $ git add -A; git commit -m 'third'
   

2. Your Git tree now looks like:

   * 42315c4 U - (third) third
   |
   * 6db2c28 U - (second) second #2
   |
   * 5746a83 U - second #1
   |
   * e845ded U - (first) first
   |
   * 8031011 U - initial commit
   

3. Push each branch:

   $ git push origin first:first second:second third:third
     * [new branch]      first -> first
     * [new branch]      second -> second
     * [new branch]      third -> third
   

4. Create a PR for each new branch (starting at first), and:

   • Ensure that all the PRs have a common identifier in their title (I'll use [EXAMPLE-17399] here). This identifier (currently) is required to be unique across all GitHub repositories accessible to you (including all public repositories).
   • Set the base for each PR to the branch preceding it. Here, first's PR is set to merge into master, second's PR is set to merge into first, and third's PR is set to merge into second.

5. Log all PRs in the stack:

   $ gh-stack log 'EXAMPLE-13799'
    #1: [EXAMPLE-13799] PR for branch `first` (Base)
    #2: [EXAMPLE-13799] PR for branch `second` (Merges into #1)
    #3: [EXAMPLE-13799] PR for branch `third` (Merges into #2)
   

6. Annotate all PRs with information about the stack:

   $ gh-stack annotate 'EXAMPLE-13799'
    1: [EXAMPLE-13799] PR for branch `first`
    2: [EXAMPLE-13799] PR for branch `second`
    3: [EXAMPLE-13799] PR for branch `third`
    Going to update these PRs ☝️  Type 'yes' to continue: yes
    Done!
   

   This (idempotently) adds a table like this to the description of every PR in the stack:

7. Make changes to a branch that rewrites commits in some way (amend, remove a commit, combine commits):

   $ git checkout first
   # Do some work
   $ git add -A; git commit --amend -m 'amended first'
   

   History has now diverged, and this will cause conflicts with dependent PRs when first is (force-)pushed.

   * e7cb9c6 U - (HEAD -> first) amended first
   |
   | * 42315c4 N - (origin/third, third) third
   | |
   | * 6db2c28 N - (origin/second, second) second #2
   | |
   | * 5746a83 N - second #1
   | |
   | * e845ded N - (origin/first) first
   |/
   |
   * 8031011 U - (origin/master, master) initial commit
   

8. Use the autorebase subcommand to fix this inconsistency (it requires a path to a local checkout of the repository):

   $ gh-stack autorebase --repo /tmp/test EXAMPLE-13799
   Checking out Commit { id: 803101159653bf4bf92bf098e577abc436458b17, summary: "initial commit" }
   
   Working on PR: "first"
   Cherry-picking: Commit { id: e7cb9c6cdb03374a6c533cbf1fc23a7d611a73c7, summary: "amended first" }
   
   Working on PR: "second"
   Cherry-picking: Commit { id: 5746a83aed004d0867d52d40efc9bd800b5b7499, summary: "second #1" }
   Cherry-picking: Commit { id: 6db2c2817dfed244d5fbd8cbb9b8095965ac9a05, summary: "second #2" }
   
   Working on PR: "third"
   Cherry-picking: Commit { id: 42315c46b42044ebc4b57a995a75b97699f4855a, summary: "third" }
   
   ["b45e5838a93b33411a5f0c9f726bc1987bc71ff5:refs/heads/first", "93170d2199ed9c2ae30d1e7492947acf477fb035:refs/heads/second", "a85a1931c44c3138d993128591af2cad2ef6c68d:refs/heads/third"]
   Going to push these refspecs ☝️  Type 'yes' to continue: yes
   Enumerating objects: 12, done.
   Counting objects: 100% (12/12), done.
   Delta compression using up to 8 threads
   Compressing objects: 100% (8/8), done.
   Writing objects: 100% (11/11), 907 bytes | 453.00 KiB/s, done.
   Total 11 (delta 3), reused 0 (delta 0)
   remote: Resolving deltas: 100% (3/3), done.
   To github.com:timothyandrew/test.git
    + e845ded...b45e583 b45e5838a93b33411a5f0c9f726bc1987bc71ff5 -> first (forced update)
    + 6db2c28...93170d2 93170d2199ed9c2ae30d1e7492947acf477fb035 -> second (forced update)
    + 42315c4...a85a193 a85a1931c44c3138d993128591af2cad2ef6c68d -> third (forced update)
   
   Updating local branches so they point to the new stack.
   
     + Branch first now points to b45e5838a93b33411a5f0c9f726bc1987bc71ff5
     + Branch second now points to 93170d2199ed9c2ae30d1e7492947acf477fb035
     + Branch third now points to a85a1931c44c3138d993128591af2cad2ef6c68d
   All done!
   

   • This restores local history to a flat list and pushes the tip of each branch up to update the PRs themselves.

     * a85a193 N - (HEAD, origin/third, third) third
     |
     * 93170d2 N - (origin/second, second) second #2
     |
     * 61f64b6 N - second #1
     |
     * b45e583 N - (origin/first, first) amended first
     |
     * 8031011 U - (origin/master, master) initial commit
     

   • If conflicts are encountered, autorebase will pause and allow you to fix the conflicts before resuming.

Strategy

This is a quick summary of the strategy the autorebase subcommand uses:

1. Find the merge_base between the local branch of the first PR in the stack and the branch it merges into (usually develop). This forms the boundary for the initial cherry-pick. This is a heuristic and is not suitable for all situations, especially when changes have already been pushed or PRs are merged directly on GitHub. Accept an explicit boundary for the initial cherry-pick to avoid ambiguity here.
2. Check out the commit/ref that the first PR in the stack merges into (usually develop). We're going to cherry-pick the entire stack onto this commit.
3. Cherry-pick all commits from the first PR (stopping at the cherry-pick boundary calculated in 1.) onto HEAD.
4. Move the local branch for the first PR so it points at HEAD.
5. The remote tracking branch for the first PR becomes the next cherry-pick boundary.
6. Repeat steps 3-5 for each subsequent PR until all PRs have been cherry-picked over.
7. Push all refs at once by passing multiple refspecs to a single invocation of git push -f.

Disclaimer

Use at your own risk (and make sure your git repository is backed up), especially because:

• This tool works for my specific use case, but has not been extensively tested.
• I've been writing Rust for all of 3 weeks at this point.
• The autorebase command is in an experimental state; there are possibly edge cases I haven't considered.