io/fs: add Sub

Sub provides a convenient way to refer to a subdirectory
automatically in future operations, like Unix's chdir(2).

The CL also includes updates to fstest to check Sub implementations.

As part of updating fstest, I changed the meaning of TestFS's
expected list to introduce a special case: if you list no expected files,
that means the FS must be empty. In general it's OK not to list all
the expected files, but if you list none, that's almost certainly a
mistake - if your FS were broken and empty, you wouldn't find out.
Making no expected files mean "must be empty" makes the mistake
less likely - if your file system ever worked, then your test will keep
it working.

That change found a testing bug: embedtest was making exactly
that mistake.

Fixes #42322.

Change-Id: I63fd4aa866b30061a0e51ca9a1927e576d6ec41e
Reviewed-on: https://go-review.googlesource.com/c/go/+/274856
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

2 files changed, 52 insertions, 0 deletions

diff --git a/src/testing/fstest/mapfs.go b/src/testing/fstest/mapfs.go
index 10e56f5b3c..a5d4a23fac 100644
--- a/src/testing/fstest/mapfs.go
+++ b/src/testing/fstest/mapfs.go
@@ -132,6 +132,16 @@ func (fsys MapFS) Glob(pattern string) ([]string, error) {
 	return fs.Glob(fsOnly{fsys}, pattern)
 }
 
+type noSub struct {
+	MapFS
+}
+
+func (noSub) Sub() {} // not the fs.SubFS signature
+
+func (fsys MapFS) Sub(dir string) (fs.FS, error) {
+	return fs.Sub(noSub{fsys}, dir)
+}
+
 // A mapFileInfo implements fs.FileInfo and fs.DirEntry for a given map file.
 type mapFileInfo struct {
 	name string
diff --git a/src/testing/fstest/testfs.go b/src/testing/fstest/testfs.go
index 4912a271b2..2602bdf0cc 100644
--- a/src/testing/fstest/testfs.go
+++ b/src/testing/fstest/testfs.go
@@ -22,6 +22,8 @@ import (
 // It walks the entire tree of files in fsys,
 // opening and checking that each file behaves correctly.
 // It also checks that the file system contains at least the expected files.
+// As a special case, if no expected files are listed, fsys must be empty.
+// Otherwise, fsys must only contain at least the listed files: it can also contain others.
 //
 // If TestFS finds any misbehaviors, it returns an error reporting all of them.
 // The error text spans multiple lines, one per detected misbehavior.
@@ -33,6 +35,32 @@ import (
 //	}
 //
 func TestFS(fsys fs.FS, expected ...string) error {
+	if err := testFS(fsys, expected...); err != nil {
+		return err
+	}
+	for _, name := range expected {
+		if i := strings.Index(name, "/"); i >= 0 {
+			dir, dirSlash := name[:i], name[:i+1]
+			var subExpected []string
+			for _, name := range expected {
+				if strings.HasPrefix(name, dirSlash) {
+					subExpected = append(subExpected, name[len(dirSlash):])
+				}
+			}
+			sub, err := fs.Sub(fsys, dir)
+			if err != nil {
+				return err
+			}
+			if err := testFS(sub, subExpected...); err != nil {
+				return fmt.Errorf("testing fs.Sub(fsys, %s): %v", dir, err)
+			}
+			break // one sub-test is enough
+		}
+	}
+	return nil
+}
+
+func testFS(fsys fs.FS, expected ...string) error {
 	t := fsTester{fsys: fsys}
 	t.checkDir(".")
 	t.checkOpen(".")
@@ -43,6 +71,20 @@ func TestFS(fsys fs.FS, expected ...string) error {
 	for _, file := range t.files {
 		found[file] = true
 	}
+	delete(found, ".")
+	if len(expected) == 0 && len(found) > 0 {
+		var list []string
+		for k := range found {
+			if k != "." {
+				list = append(list, k)
+			}
+		}
+		sort.Strings(list)
+		if len(list) > 15 {
+			list = append(list[:10], "...")
+		}
+		t.errorf("expected empty file system but found files:\n%s", strings.Join(list, "\n"))
+	}
 	for _, name := range expected {
 		if !found[name] {
 			t.errorf("expected but not found: %s", name)