Find runfiles in directories that are themselves runfiles

When a target has a runfile that is contained in a directory that is
itself one of its runfiles, the runfile will be shadowed by the
SymlinkEntry for the directory. While this still allows to resolve the
file in the runfiles symlink tree, a manifest-based lookup will fail.

This PR extends the lookup logic to also find runfiles contained within
directories that are themselves runfiles. It does so by searching the
manifest not only for the exact provided rlocation path, but also for
all path prefixes. If a prefix is looked up successfully, the
corresponding suffix is resolved relative to the looked up path.

See bazelbuild/bazel#14336 for more context.

manifest.go

@@ -18,6 +18,7 @@ import (
 	"bufio"
 	"fmt"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 )
@@ -60,13 +61,22 @@ func (f ManifestFile) parse() (manifest, error) {
 
 func (m manifest) path(s string) (string, error) {
 	r, ok := m[s]
-	if !ok {
-		return "", os.ErrNotExist
-	}
-	if r == "" {
+	if ok && r == "" {
 		return "", ErrEmpty
 	}
-	return r, nil
+	if ok {
+		return r, nil
+	}
+	// If path references a runfile that lies under a directory that itself is a
+	// runfile, then only the directory is listed in the manifest. Look up all
+	// prefixes of path in the manifest.
+	for prefix := s; prefix != ""; prefix, _ = path.Split(prefix) {
+		prefix = strings.TrimSuffix(prefix, "/")
+		if prefixMatch, ok := m[prefix]; ok {
+			return prefixMatch + strings.TrimPrefix(s, prefix), nil
+		}
+	}
+	return "", os.ErrNotExist
 }
 
 const manifestFileVar = "RUNFILES_MANIFEST_FILE"

runfiles_test.go

@@ -101,3 +101,30 @@ func TestRunfiles_empty(t *testing.T) {
 		t.Errorf("Path for empty file: got error %q, want something that wraps %q", got, want)
 	}
 }
+
+func TestRunfiles_manifestWithDir(t *testing.T) {
+	dir := t.TempDir()
+	manifest := filepath.Join(dir, "manifest")
+	if err := os.WriteFile(manifest, []byte("foo/dir path/to/foo/dir\n"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	r, err := runfiles.New(runfiles.ManifestFile(manifest))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for rlocation, want := range map[string]string{
+			"foo/dir":                    "path/to/foo/dir",
+			"foo/dir/file":               "path/to/foo/dir/file",
+			"foo/dir/deeply/nested/file": "path/to/foo/dir/deeply/nested/file",
+		} {
+		t.Run(rlocation, func(t *testing.T) {
+			got, err := r.Path(rlocation)
+			if err != nil {
+				t.Fatalf("Path failed: got unexpected error %q", err)
+			}
+			if got != want {
+				t.Errorf("Path failed: got %q, want %q", got, want)
+			}
+		})
+	}
+}