internal/fsys: follow root symlink in fsys.Walk

If fsys.Walk is called with a root directory that is a symlink, follow
the symlink when doing the walk. This allows for users setting their
current directory to a symlink to a module.

Fixes #50807

Change-Id: Ie65a7cb804b87dea632ea6c758c20adcfa62fcd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/448360
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/fsys/fsys.go

@@ -474,19 +474,23 @@ func IsDirWithGoFiles(dir string) (bool, error) {
 
 // walk recursively descends path, calling walkFn. Copied, with some
 // modifications from path/filepath.walk.
-func walk(path string, info fs.FileInfo, walkFn filepath.WalkFunc) error {
+// Walk follows the root if it's a symlink, but reports the original paths,
+// so it calls walk with both the resolvedPath (which is the path with the root resolved)
+// and path (which is the path reported to the walkFn).
+func walk(path, resolvedPath string, info fs.FileInfo, walkFn filepath.WalkFunc) error {
 	if err := walkFn(path, info, nil); err != nil || !info.IsDir() {
 		return err
 	}
 
-	fis, err := ReadDir(path)
+	fis, err := ReadDir(resolvedPath)
 	if err != nil {
 		return walkFn(path, info, err)
 	}
 
 	for _, fi := range fis {
 		filename := filepath.Join(path, fi.Name())
-		if err := walk(filename, fi, walkFn); err != nil {
+		resolvedFilename := filepath.Join(resolvedPath, fi.Name())
+		if err := walk(filename, resolvedFilename, fi, walkFn); err != nil {
 			if !fi.IsDir() || err != filepath.SkipDir {
 				return err
 			}
@@ -503,7 +507,23 @@ func Walk(root string, walkFn filepath.WalkFunc) error {
 	if err != nil {
 		err = walkFn(root, nil, err)
 	} else {
-		err = walk(root, info, walkFn)
+		resolved := root
+		if info.Mode()&os.ModeSymlink != 0 {
+			// Walk follows root if it's a symlink (but does not follow other symlinks).
+			if op, ok := OverlayPath(root); ok {
+				resolved = op
+			}
+			resolved, err = os.Readlink(resolved)
+			if err != nil {
+				return err
+			}
+			// Re-stat to get the info for the resolved file.
+			info, err = Lstat(resolved)
+			if err != nil {
+				return err
+			}
+		}
+		err = walk(root, resolved, info, walkFn)
 	}
 	if err == filepath.SkipDir {
 		return nil

src/cmd/go/internal/fsys/fsys_test.go

@@ -844,8 +844,8 @@ func TestWalkSymlink(t *testing.T) {
 		{"control", "dir", []string{"dir", "dir" + string(filepath.Separator) + "file"}},
 		// ensure Walk doesn't walk into the directory pointed to by the symlink
 		// (because it's supposed to use Lstat instead of Stat).
-		{"symlink_to_dir", "symlink", []string{"symlink"}},
-		{"overlay_to_symlink_to_dir", "overlay_symlink", []string{"overlay_symlink"}},
+		{"symlink_to_dir", "symlink", []string{"symlink", "symlink" + string(filepath.Separator) + "file"}},
+		{"overlay_to_symlink_to_dir", "overlay_symlink", []string{"overlay_symlink", "overlay_symlink" + string(filepath.Separator) + "file"}},
 	}
 
 	for _, tc := range testCases {

src/cmd/go/testdata/script/list_symlink_dotdotdot.txt

@@ -0,0 +1,20 @@
+[!symlink] skip
+
+symlink $WORK/gopath/src/sym -> $WORK/gopath/src/tree
+symlink $WORK/gopath/src/tree/squirrel -> $WORK/gopath/src/dir2 # this symlink should not be followed
+cd sym
+go list ./...
+cmp stdout $WORK/gopath/src/want_list.txt
+-- tree/go.mod --
+module example.com/tree
+
+go 1.20
+-- tree/tree.go --
+package tree
+-- tree/branch/branch.go --
+package branch
+-- dir2/squirrel.go --
+package squirrel
+-- want_list.txt --
+example.com/tree
+example.com/tree/branch