Clean up vfs.FilesystemImpl methods that operate on parent directories.

- Make FilesystemImpl methods that operate on parent directories require
  !rp.Done() (i.e. there is at least one path component to resolve) as
  precondition and postcondition (in cases where they do not finish path
  resolution due to mount boundary / absolute symlink), and require that they
  do not need to follow the last path component (the file being created /
  deleted) as a symlink. Check for these in VFS.

- Add FilesystemImpl.GetParentDentryAt(), which is required to obtain the old
  parent directory for VFS.RenameAt(). (Passing the Dentry to be renamed
  instead has the wrong semantics if the file named by the old path is a mount
  point since the Dentry will be on the wrong Mount.)

- Update memfs to implement these methods correctly (?), including RenameAt.

- Change fspath.Parse() to allow empty paths (to simplify implementation of
  AT_EMPTY_PATH).

- Change vfs.PathOperation to take a fspath.Path instead of a raw pathname;
  non-test callers will need to fspath.Parse() pathnames themselves anyway in
  order to detect absolute paths and select PathOperation.Start accordingly.

PiperOrigin-RevId: 286934941

3 files changed, 22 insertions, 29 deletions

diff --git a/pkg/fspath/BUILD b/pkg/fspath/BUILD
index 0c5f50397..ca540363c 100644
--- a/pkg/fspath/BUILD
+++ b/pkg/fspath/BUILD
@@ -14,7 +14,6 @@ go_library(
         "fspath.go",
     ],
     importpath = "gvisor.dev/gvisor/pkg/fspath",
-    deps = ["//pkg/syserror"],
 )
 
 go_test(
@@ -25,5 +24,4 @@ go_test(
         "fspath_test.go",
     ],
     embed = [":fspath"],
-    deps = ["//pkg/syserror"],
 )
diff --git a/pkg/fspath/fspath.go b/pkg/fspath/fspath.go
index f68752560..9fb3fee24 100644
--- a/pkg/fspath/fspath.go
+++ b/pkg/fspath/fspath.go
@@ -18,19 +18,17 @@ package fspath
 
 import (
 	"strings"
-
-	"gvisor.dev/gvisor/pkg/syserror"
 )
 
 const pathSep = '/'
 
-// Parse parses a pathname as described by path_resolution(7).
-func Parse(pathname string) (Path, error) {
+// Parse parses a pathname as described by path_resolution(7), except that
+// empty pathnames will be parsed successfully to a Path for which
+// Path.Absolute == Path.Dir == Path.HasComponents() == false. (This is
+// necessary to support AT_EMPTY_PATH.)
+func Parse(pathname string) Path {
 	if len(pathname) == 0 {
-		// "... POSIX decrees that an empty pathname must not be resolved
-		// successfully. Linux returns ENOENT in this case." -
-		// path_resolution(7)
-		return Path{}, syserror.ENOENT
+		return Path{}
 	}
 	// Skip leading path separators.
 	i := 0
@@ -41,7 +39,7 @@ func Parse(pathname string) (Path, error) {
 			return Path{
 				Absolute: true,
 				Dir:      true,
-			}, nil
+			}
 		}
 	}
 	// Skip trailing path separators. This is required by Iterator.Next. This
@@ -64,7 +62,7 @@ func Parse(pathname string) (Path, error) {
 		},
 		Absolute: i != 0,
 		Dir:      j != len(pathname)-1,
-	}, nil
+	}
 }
 
 // Path contains the information contained in a pathname string.
@@ -111,6 +109,12 @@ func (p Path) String() string {
 	return b.String()
 }
 
+// HasComponents returns true if p contains a non-zero number of path
+// components.
+func (p Path) HasComponents() bool {
+	return p.Begin.Ok()
+}
+
 // An Iterator represents either a path component in a Path or a terminal
 // iterator indicating that the end of the path has been reached.
 //
diff --git a/pkg/fspath/fspath_test.go b/pkg/fspath/fspath_test.go
index 215b35622..d5e9a549a 100644
--- a/pkg/fspath/fspath_test.go
+++ b/pkg/fspath/fspath_test.go
@@ -18,15 +18,10 @@ import (
 	"reflect"
 	"strings"
 	"testing"
-
-	"gvisor.dev/gvisor/pkg/syserror"
 )
 
 func TestParseIteratorPartialPathnames(t *testing.T) {
-	path, err := Parse("/foo//bar///baz////")
-	if err != nil {
-		t.Fatalf("Parse failed: %v", err)
-	}
+	path := Parse("/foo//bar///baz////")
 	// Parse strips leading slashes, and records their presence as
 	// Path.Absolute.
 	if !path.Absolute {
@@ -71,6 +66,12 @@ func TestParse(t *testing.T) {
 	}
 	tests := []testCase{
 		{
+			pathname: "",
+			relpath:  []string{},
+			abs:      false,
+			dir:      false,
+		},
+		{
 			pathname: "/",
 			relpath:  []string{},
 			abs:      true,
@@ -113,10 +114,7 @@ func TestParse(t *testing.T) {
 
 	for _, test := range tests {
 		t.Run(test.pathname, func(t *testing.T) {
-			p, err := Parse(test.pathname)
-			if err != nil {
-				t.Fatalf("failed to parse pathname %q: %v", test.pathname, err)
-			}
+			p := Parse(test.pathname)
 			t.Logf("pathname %q => path %q", test.pathname, p)
 			if p.Absolute != test.abs {
 				t.Errorf("path absoluteness: got %v, wanted %v", p.Absolute, test.abs)
@@ -134,10 +132,3 @@ func TestParse(t *testing.T) {
 		})
 	}
 }
-
-func TestParseEmptyPathname(t *testing.T) {
-	p, err := Parse("")
-	if err != syserror.ENOENT {
-		t.Errorf("parsing empty pathname: got (%v, %v), wanted (<unspecified>, ENOENT)", p, err)
-	}
-}