Lesson 26 · The commit porcelain

The commit porcelain

The friendly commit command glues the pieces together: turn the index into a tree, then commit that tree. Today you build that glue and produce a real commit straight from the staging area.

The goal

Commit the current index in one step by combining write-tree and commit-tree.

Start here - the target

TO DO

Scenario: Committing the index produces a commit

Givenan index staged with hello.txt, README.md, and src/main.go, the fixed identity and time, and the message Initial commit

WhenCommit(index, no parents, message) runs write-tree then commit-tree on the result

Thenthe tree it writes is b7c8f173c30a232f001cc4d77c259e4c99afbbd8

Andthe commit id it returns is f18e98d326b190d096879422d56d4ac5cf572db1

Background

This is the command a user actually runs: commit. Everything under it already exists, so the porcelain is pure glue. It calls write-tree to snapshot the index into a root tree, then commit-tree to wrap that tree in a commit with the given parents and message. The result is a commit id that captures the exact staged state.

Notice how the layers stack: blobs hold file contents, trees hold directory structure, a commit names one root tree plus metadata, and this porcelain drives all three from the staging area. Staging the three files and committing with no parent yields the same f18e98d3... we built by hand, now produced end to end from real files. What is still missing is memory: this commit is not yet recorded anywhere a branch can find it. Refs, next chapter, fix that.

Make it work

// write-tree the index, then commit-tree the resulting root tree
func (r *Repo) Commit(ix *Index, parents []string, ident, msg string) (string, error) {
  tree, err := r.WriteTreeFromIndex(ix)
  if err != nil { return "", err }
  return r.CommitTree(tree, parents, ident, msg)
}

CheckpointDONE

You can commit the staging area in one step. Commit and stop here. cat-file -p the commit and the tree it points at.