Lesson 30 · Committing moves the branch

Committing moves the branch

A commit should remember itself. Today the commit porcelain hooks into HEAD: it takes its parent from the current branch and, after writing the commit, advances that branch to point at it.

The goal

Make commit find its parent from HEAD and move the current branch forward.

Start here - the target

TO DO

Scenario: The first commit creates and advances the branch

Givena fresh repository with HEAD set to ref: refs/heads/main (an unborn branch), an index staged with hello.txt, README.md, and src/main.go, and the message Initial commit

WhenCommit(index, message) resolves HEAD for a parent (none, since unborn), writes the commit, and updates refs/heads/main to it

Thenthe commit id is f18e98d326b190d096879422d56d4ac5cf572db1 with no parent

Andrefs/heads/main now holds that id, so Resolve(HEAD) returns f18e98d326b190d096879422d56d4ac5cf572db1

Background

Until now a commit vanished the moment you made it: nothing pointed at it. This lesson wires the porcelain into refs so history accumulates. Two halves, both reading HEAD. First, the commit’s parent is whatever the current branch points at right now, found via Resolve("HEAD"); for an unborn branch that is empty, so the first commit has no parent. Second, after writing the commit, we advance the branch HEAD names to the new id, creating refs/heads/main if it did not exist.

The commit command no longer needs you to pass parents by hand; it finds them from HEAD. That is the everyday git commit behavior in miniature: commit, and your current branch moves to include it. The first commit produces the same f18e98d3... and leaves main pointing at it. With the branch remembering the tip, the next commit can build on it automatically.

Make it work

// parent = Resolve("HEAD") (empty when unborn); branch = HEAD's target name
func (r *Repo) Commit(ix *Index, ident, msg string) (string, error) {
  parent, _ := r.Resolve("HEAD")
  var parents []string
  if parent != "" { parents = []string{parent} }
  id, _ := r.CommitTree(tree, parents, ident, msg)
  r.UpdateRef(headTargetName, id) // move the current branch
  return id, nil
}

CheckpointDONE

Committing records itself on the current branch. Commit and stop here.