Remove filesystem structure from vfs.Dentry.

This change:

- Drastically simplifies the synchronization model: filesystem structure is
  both implementation-defined and implementation-synchronized.

- Allows implementations of vfs.DentryImpl to use implementation-specific
  dentry types, reducing casts during path traversal.

- Doesn't require dentries representing non-directory files to waste space on a
  map of children.

- Allows dentry revalidation and mount lookup to be correctly ordered (fixed
  FIXME in fsimpl/gofer/filesystem.go).

- Removes the need to have two separate maps in gofer.dentry
  (dentry.vfsd.children and dentry.negativeChildren) for positive and negative
  lookups respectively.

//pkg/sentry/fsimpl/tmpfs/benchmark_test.go:
name                        old time/op  new time/op  delta
VFS2TmpfsStat/1-112          172ns ± 4%   165ns ± 3%   -4.08%  (p=0.002 n=9+9)
VFS2TmpfsStat/2-112          199ns ± 3%   195ns ±10%     ~     (p=0.132 n=8+9)
VFS2TmpfsStat/3-112          230ns ± 2%   216ns ± 2%   -6.15%  (p=0.000 n=8+8)
VFS2TmpfsStat/8-112          390ns ± 2%   358ns ± 4%   -8.33%  (p=0.000 n=9+8)
VFS2TmpfsStat/64-112        2.20µs ± 3%  2.01µs ± 3%   -8.48%  (p=0.000 n=10+8)
VFS2TmpfsStat/100-112       3.42µs ± 9%  3.08µs ± 2%   -9.82%  (p=0.000 n=9+8)
VFS2TmpfsMountStat/1-112     278ns ± 1%   286ns ±15%     ~     (p=0.712 n=8+10)
VFS2TmpfsMountStat/2-112     311ns ± 4%   298ns ± 2%   -4.27%  (p=0.000 n=9+8)
VFS2TmpfsMountStat/3-112     339ns ± 3%   330ns ± 9%     ~     (p=0.070 n=8+9)
VFS2TmpfsMountStat/8-112     503ns ± 3%   466ns ± 3%   -7.38%  (p=0.000 n=8+8)
VFS2TmpfsMountStat/64-112   2.53µs ±16%  2.17µs ± 7%  -14.19%  (p=0.000 n=10+9)
VFS2TmpfsMountStat/100-112  3.60µs ± 4%  3.30µs ± 8%   -8.33%  (p=0.001 n=8+9)

Updates #1035

PiperOrigin-RevId: 307655892

1 files changed, 8 insertions, 8 deletions

diff --git a/pkg/sentry/fsimpl/proc/tasks_test.go b/pkg/sentry/fsimpl/proc/tasks_test.go
index d0f97c137..19abb5034 100644
--- a/pkg/sentry/fsimpl/proc/tasks_test.go
+++ b/pkg/sentry/fsimpl/proc/tasks_test.go
@@ -415,36 +415,36 @@ func iterateDir(ctx context.Context, t *testing.T, s *testutil.System, fd *vfs.F
 		if d.Name == "." || d.Name == ".." {
 			continue
 		}
-		childPath := path.Join(fd.MappedName(ctx), d.Name)
+		absPath := path.Join(fd.MappedName(ctx), d.Name)
 		if d.Type == linux.DT_LNK {
 			link, err := s.VFS.ReadlinkAt(
 				ctx,
 				auth.CredentialsFromContext(ctx),
-				&vfs.PathOperation{Root: s.Root, Start: s.Root, Path: fspath.Parse(childPath)},
+				&vfs.PathOperation{Root: s.Root, Start: s.Root, Path: fspath.Parse(absPath)},
 			)
 			if err != nil {
-				t.Errorf("vfsfs.ReadlinkAt(%v) failed: %v", childPath, err)
+				t.Errorf("vfsfs.ReadlinkAt(%v) failed: %v", absPath, err)
 			} else {
-				t.Logf("Skipping symlink: /proc%s => %s", childPath, link)
+				t.Logf("Skipping symlink: %s => %s", absPath, link)
 			}
 			continue
 		}
 
-		t.Logf("Opening: /proc%s", childPath)
+		t.Logf("Opening: %s", absPath)
 		child, err := s.VFS.OpenAt(
 			ctx,
 			auth.CredentialsFromContext(ctx),
-			&vfs.PathOperation{Root: s.Root, Start: s.Root, Path: fspath.Parse(childPath)},
+			&vfs.PathOperation{Root: s.Root, Start: s.Root, Path: fspath.Parse(absPath)},
 			&vfs.OpenOptions{},
 		)
 		if err != nil {
-			t.Errorf("vfsfs.OpenAt(%v) failed: %v", childPath, err)
+			t.Errorf("vfsfs.OpenAt(%v) failed: %v", absPath, err)
 			continue
 		}
 		defer child.DecRef()
 		stat, err := child.Stat(ctx, vfs.StatOptions{})
 		if err != nil {
-			t.Errorf("Stat(%v) failed: %v", childPath, err)
+			t.Errorf("Stat(%v) failed: %v", absPath, err)
 		}
 		if got := linux.FileMode(stat.Mode).DirentType(); got != d.Type {
 			t.Errorf("wrong file mode, stat: %v, dirent: %v", got, d.Type)