knotserver/git: compute changed files between commits and wire into workflow compiler

Signed-off-by: Anirudh Oppiliappan <[e-mail]>

knotserver/git/diff.go

···
 	return stdout.String(), &formatPatch[0], nil
 }
 
+// ChangedFilesBetween returns the list of files changed between oldSha and newSha.
+// If oldSha is the zero hash (initial push), all files in newSha are returned.
+func (g *GitRepo) ChangedFilesBetween(oldSha, newSha string) ([]string, error) {
+	newCommit, err := g.ResolveRevision(newSha)
+	if err != nil {
+		return nil, err
+	}
+
+	if plumbing.NewHash(oldSha) == plumbing.ZeroHash {
+		tree, err := newCommit.Tree()
+		if err != nil {
+			return nil, err
+		}
+		var files []string
+		tree.Files().ForEach(func(f *object.File) error {
+			files = append(files, f.Name)
+			return nil
+		})
+		return files, nil
+	}
+
+	oldCommit, err := g.ResolveRevision(oldSha)
+	if err != nil {
+		return nil, err
+	}
+
+	dt, err := g.DiffTree(oldCommit, newCommit)
+	if err != nil {
+		return nil, err
+	}
+
+	seen := make(map[string]struct{})
+	for _, f := range dt.Diff {
+		if f.OldName != "" {
+			seen[f.OldName] = struct{}{}
+		}
+		if f.NewName != "" {
+			seen[f.NewName] = struct{}{}
+		}
+	}
+
+	files := make([]string, 0, len(seen))
+	for name := range seen {
+		files = append(files, name)
+	}
+	return files, nil
+}
+
 func (g *GitRepo) ResolveRevision(revStr string) (*object.Commit, error) {
 	rev, err := g.r.ResolveRevision(plumbing.Revision(revStr))
 	if err != nil {

knotserver/internal.go

···
 		NewSha: line.NewSha.String(),
 	}
 
+	changedFiles, err := gr.ChangedFilesBetween(line.OldSha.String(), line.NewSha.String())
+	if err != nil {
+		return fmt.Errorf("getting changed files: %w", err)
+	}
+
 	compiler := workflow.Compiler{
 		Trigger: tangled.Pipeline_TriggerMetadata{
 			Kind: string(workflow.TriggerKindPush),
···
 				Repo: repoName,
 			},
 		},
+		ChangedFiles: changedFiles,
 	}
 
 	cp := compiler.Compile(compiler.Parse(pipeline))