testing/fstest: avoid symlink-induced failures in tester

Do not require directory entry and Stat result to match for symlinks,
because they won't (Stat dereferences the symlink).

Fixes #44113.

Change-Id: Ifc6dbce5719906e2f42254a7172f1ef787464a9e
Reviewed-on: https://go-review.googlesource.com/c/go/+/290009
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

2 files changed, 49 insertions, 7 deletions

diff --git a/src/testing/fstest/testfs.go b/src/testing/fstest/testfs.go
index a7f8007333e..8fc8acaaf3b 100644
--- a/src/testing/fstest/testfs.go
+++ b/src/testing/fstest/testfs.go
@@ -403,9 +403,10 @@ func (t *fsTester) checkStat(path string, entry fs.DirEntry) {
 		return
 	}
 	fentry := formatEntry(entry)
-	finfo := formatInfoEntry(info)
-	if fentry != finfo {
-		t.errorf("%s: mismatch:\n\tentry = %s\n\tfile.Stat() = %s", path, fentry, finfo)
+	fientry := formatInfoEntry(info)
+	// Note: mismatch here is OK for symlink, because Open dereferences symlink.
+	if fentry != fientry && entry.Type()&fs.ModeSymlink == 0 {
+		t.errorf("%s: mismatch:\n\tentry = %s\n\tfile.Stat() = %s", path, fentry, fientry)
 	}
 
 	einfo, err := entry.Info()
@@ -413,12 +414,22 @@ func (t *fsTester) checkStat(path string, entry fs.DirEntry) {
 		t.errorf("%s: entry.Info: %v", path, err)
 		return
 	}
-	fentry = formatInfo(einfo)
-	finfo = formatInfo(info)
-	if fentry != finfo {
-		t.errorf("%s: mismatch:\n\tentry.Info() = %s\n\tfile.Stat() = %s\n", path, fentry, finfo)
+	finfo := formatInfo(info)
+	if entry.Type()&fs.ModeSymlink != 0 {
+		// For symlink, just check that entry.Info matches entry on common fields.
+		// Open deferences symlink, so info itself may differ.
+		feentry := formatInfoEntry(einfo)
+		if fentry != feentry {
+			t.errorf("%s: mismatch\n\tentry = %s\n\tentry.Info() = %s\n", path, fentry, feentry)
+		}
+	} else {
+		feinfo := formatInfo(einfo)
+		if feinfo != finfo {
+			t.errorf("%s: mismatch:\n\tentry.Info() = %s\n\tfile.Stat() = %s\n", path, feinfo, finfo)
+		}
 	}
 
+	// Stat should be the same as Open+Stat, even for symlinks.
 	info2, err := fs.Stat(t.fsys, path)
 	if err != nil {
 		t.errorf("%s: fs.Stat: %v", path, err)
diff --git a/src/testing/fstest/testfs_test.go b/src/testing/fstest/testfs_test.go
new file mode 100644
index 00000000000..5b8813c3431
--- /dev/null
+++ b/src/testing/fstest/testfs_test.go
@@ -0,0 +1,31 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fstest
+
+import (
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	tmp := t.TempDir()
+	tmpfs := os.DirFS(tmp)
+
+	if err := os.WriteFile(filepath.Join(tmp, "hello"), []byte("hello, world\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := os.Symlink(filepath.Join(tmp, "hello"), filepath.Join(tmp, "hello.link")); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := TestFS(tmpfs, "hello", "hello.link"); err != nil {
+		t.Fatal(err)
+	}
+}