Resolve committed module identity from Git objects

internal/releaseidentity/identity.go

@@ -75,22 +75,84 @@ func ResolveSelection(repositoryDir, moduleDir string) (Selection, error) {
 // FromCommitted derives identity from the selected module declaration at a
 // captured commit. The selected module must contain its own go.mod.
 func FromCommitted(ctx context.Context, repositoryDir, moduleDir, version, commit string) (Identity, error) {
-	selection, err := ResolveSelection(repositoryDir, moduleDir)
+	resolver, err := committedPaths(repositoryDir, commit)
 	if err != nil {
-		return Identity{}, err
+		return Identity{Version: version}, err
 	}
-	return FromCommittedSelection(ctx, selection, version, commit)
+	selection, err := selectCommittedDirectory(ctx, resolver, moduleDir)
+	if err != nil {
+		return Identity{Selection: selection, Version: version}, err
+	}
+	return fromCommittedSelection(ctx, selection, version, commit, resolver)
+}
+
+// ResolveCommittedSelection resolves directory selection without reading the
+// module declaration or requiring the directory to be materialized.
+func ResolveCommittedSelection(ctx context.Context, repositoryDir, moduleDir, commit string) (Selection, error) {
+	resolver, err := committedPaths(repositoryDir, commit)
+	if err != nil {
+		return Selection{}, err
+	}
+	return selectCommittedDirectory(ctx, resolver, moduleDir)
+}
+
+func committedPaths(repositoryDir, commit string) (git.PathResolver, error) {
+	repositoryAbs, err := filepath.Abs(repositoryDir)
+	if err != nil {
+		return git.PathResolver{}, err
+	}
+	repositoryAbs, err = filepath.EvalSymlinks(repositoryAbs)
+	if err != nil {
+		return git.PathResolver{}, err
+	}
+	return git.CommittedPaths(repositoryAbs, commit), nil
+}
+
+func selectCommittedDirectory(ctx context.Context, resolver git.PathResolver, moduleDir string) (Selection, error) {
+	selection := Selection{RepositoryDir: resolver.RepositoryDir}
+	if moduleDir == "" {
+		moduleDir = "."
+	}
+	if filepath.IsAbs(moduleDir) {
+		return selection, fmt.Errorf("module directory must be repository-relative: %q", moduleDir)
+	}
+	directory, err := resolver.ResolveDestination(ctx, "go.mod", filepath.ToSlash(moduleDir))
+	if err != nil {
+		return selection, fmt.Errorf("resolve committed module directory: %w", err)
+	}
+	if directory.Kind != git.DestinationDirectory {
+		return selection, fmt.Errorf("selected committed module directory %q resolves to %s at %q", moduleDir, directory.Kind, directory.Path)
+	}
+	selection.ModuleDir = filepath.Join(resolver.RepositoryDir, filepath.FromSlash(directory.Path))
+	selection.ModuleSubdir = directory.Path
+	return selection, nil
 }
 
 // FromCommittedSelection derives identity from a previously resolved module
 // selection and the selected module declaration at a captured commit.
 func FromCommittedSelection(ctx context.Context, selection Selection, version, commit string) (Identity, error) {
+	return fromCommittedSelection(ctx, selection, version, commit, git.CommittedPaths(selection.RepositoryDir, commit))
+}
+
+func fromCommittedSelection(ctx context.Context, selection Selection, version, commit string, resolver git.PathResolver) (Identity, error) {
+	identity := Identity{Selection: selection, Version: version}
 	if err := ctx.Err(); err != nil {
-		return Identity{}, fmt.Errorf("resolve committed release identity: %w", err)
+		return identity, fmt.Errorf("resolve committed release identity: %w", err)
 	}
-	declaration, err := git.ReadCommitFile(ctx, selection.RepositoryDir, commit, moduleFilePath(selection.ModuleSubdir))
+	declarationPath, err := resolver.ResolveDestination(ctx, "go.mod", moduleFilePath(selection.ModuleSubdir))
 	if err != nil {
-		return Identity{}, fmt.Errorf("read selected module declaration at %s: %w", commit, err)
+		return identity, fmt.Errorf("resolve selected module declaration at %s: %w", commit, err)
+	}
+	if declarationPath.Kind != git.DestinationEntry {
+		return identity, fmt.Errorf("selected module declaration resolves to %s at %q", declarationPath.Kind, declarationPath.Path)
+	}
+	entry, _, err := resolver.Lookup(ctx, declarationPath.Path)
+	if err != nil {
+		return identity, err
+	}
+	declaration, err := git.ReadBlob(ctx, selection.RepositoryDir, entry.ObjectID)
+	if err != nil {
+		return identity, fmt.Errorf("read selected module declaration at %s: %w", commit, err)
 	}
 	return Derive(selection, declaration, version)
 }