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BACKPORTING

The official Bitcoin ABC guide to backporting code from Bitcoin Core. When searching for items to backport, especially be on the lookout for bug fixes, code cleanup, and important refactors, as these help improve Bitcoin ABC despite consensus-level differences between eCash and Bitcoin Core.

Identifying commits

  1. Check out a copy of a Satoshi Bitcoin client somewhere on your machine.
  2. Identify the subsystem you’d like to work on.
  3. Tag the fork commit as fork-commit. Bitcoin ABC was forked from Bitcoin Core at commit 964a185cc83af34587194a6ecda3ed9cf6b49263. > git tag -a fork-commit 964a185 -m 'Where the fun started'
  4. git log --topo-order --graph fork-commit..HEAD -- <file or folder of interest>
  5. Find the commit, and if applicable, the associated merge commit that are worth backporting. The merge commit may indicate that there were other commits associated with this change that you will need to backport. E.g:
commit d083bd9b9c5249f21b8b7e4abd7aee48a25806b1
Merge: b3eb0d648 279fde58e
Author: Wladimir J. van der Laan <[email protected]>
Date:   Wed Jun 21 14:26:10 2017 +0200

    Merge #10533: [tests] Use cookie auth instead of rpcuser and rpcpassword

    279fde5 Check for rpcuser/rpcpassword first then for cookie (Andrew Chow)
    3ec5ad8 Add test for rpcuser/rpcpassword (Andrew Chow)
    c53c983 Replace cookie auth in tests (Andrew Chow)

    Tree-SHA512: 21efb84c87080a895cac8a7fe4766738c34eebe9686c7d10af1bf91ed4ae422e2d5dbbebffd00d34744eb6bb2d0195ea3aca86deebf085bbdeeb1d8b474241ed

If you saw that commit c53c983 was a good idea to backport, this merge commit would indicate that there are two other commits that are associated with this PR and likely to be needed.

When trying to find a patch worth backporting, it’s generally a good idea to backport significant refactors or bugfixes. This will help clean up the code in the ABC repository, fix bugs, and make future backports significantly easier. Backports are easiest done in topological order of commits.

Backporting

Before you begin backporting commits, you will need to add an additional remote to your Bitcoin ABC repo. For Bitcoin Core, this repository would be added as:

git remote add core [email protected]:bitcoin/bitcoin.git
git fetch core

(Assuming you have github ssh auth setup. The second command is required to obtain refs for cherry-picking.)

Once you have identified your commit, or commits on question to backport you have two choices:

  1. Backport each diff individually
  2. Squash the commits together for backporting.

In either case, you will find there are likely merge conflicts.

Backport each diff individually

  1. git checkout -b <name-of-branch>
  2. git cherry-pick <commit-of-interest>
  3. Run git status and fix conflicts.
  4. git add -u && git cherry-pick --continue
  5. Run git show side-by-side with git show <commit-of-interest> and verify that the changes are reasonable. If there are additional changes caused by the merge conflict
  6. Run the build, and the rpc test suite and verify completion.
  7. arc diff and at the bottom of the summary note: Backport of core#XXXXX1. the PR# can be obtained by searching on github for the commit you are backporting. If you are backporting a commit which depends on another commit from the same PR, note that you are backporting Part 1 of X. Additionally, if it is the second or more of a series of commits in a backport, note which other phabricator revision this diff depends on by typing Depends on DXXXX at the bottom of the summary.

Squash the commits together for backporting

  1. git checkout -b PRXXXXX
  2. Perform steps 2 through 6 from “Backporting each diff individually” in repetition for each diff in a Bitcoin Core pull request.
  3. git rebase -i origin/master and squash the commits together.
  4. arc diff and at the bottom of the summary note: Backport of core#XXXXX1. The title of the diff should be: [Backport] <Description of the included changes>

[1]: besides core, the prefixes core-gui and secp256k1 will also include a hyperlink to the appropriate pull request in the summary automatically, as long as they are in the format [prefix]#[pr_number]. No spaces, all lowercase.

Backporting from secp256k1, Electrum and Electron Cash

The Bitcoin ABC monorepo includes subprojects that are stored in subdirectories of the main repository. To backport commits for these subprojects, you can add the corresponding additional remotes and use the -Xsubtree option for git cherry-pick.

For instance to backport Electron-Cash commits, you can do:

git remote add electroncash [email protected]:Electron-Cash/Electron-Cash.git
git fetch electroncash
git cherry-pick -Xsubtree=electrum <commit hash>

You can also add git aliases to your ~/.gitconfig file so that you don’t need to remember the -Xsubtree option:

[alias]
    cpsecp256k1 = cherry-pick -Xsubtree=src/secp256k1
    cpelectrum = cherry-pick -Xsubtree=electrum

This allows you to then backport commits using the following shortcuts:

git cpelectrum <commit hash>
git cpsecp256k1 <commit hash>