Add MultiGetAttr message to 9P

While using remote-validation, the vast majority of time spent during
FS operations is re-walking the path to check for modifications and
then closing the file given that in most cases it has not been
modified externally.

This change introduces a new 9P message called MultiGetAttr which bulks
query attributes of several files in one shot. The returned attributes are
then used to update cached dentries before they are walked. File attributes
are updated for files that still exist. Dentries that have been deleted are
removed from the cache. And negative cache entries are removed if a new
file/directory was created externally. Similarly, synthetic dentries are
replaced if a file/directory is created externally.

The bulk update needs to be carefull not to follow symlinks, cross mount
points, because the gofer doesn't know how to resolve symlinks and where
mounts points are located. It also doesn't walk to the parent ("..") to
avoid deadlocks.

Here are the results:

Workload        VFS1       VFS2     Change
bazel action     115s       70s	     28.8s
Stat/100      11,043us   7,623us      974us

Updates #1638

PiperOrigin-RevId: 369325957

2 files changed, 54 insertions, 13 deletions

diff --git a/runsc/fsgofer/fsgofer.go b/runsc/fsgofer/fsgofer.go
index e04ddda47..b81ede5ae 100644
--- a/runsc/fsgofer/fsgofer.go
+++ b/runsc/fsgofer/fsgofer.go
@@ -21,6 +21,7 @@
 package fsgofer
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"math"
@@ -58,9 +59,6 @@ var verityXattrs = map[string]struct{}{
 
 // join is equivalent to path.Join() but skips path.Clean() which is expensive.
 func join(parent, child string) string {
-	if child == "." || child == ".." {
-		panic(fmt.Sprintf("invalid child path %q", child))
-	}
 	return parent + "/" + child
 }
 
@@ -1226,3 +1224,56 @@ func (l *localFile) checkROMount() error {
 	}
 	return nil
 }
+
+func (l *localFile) MultiGetAttr(names []string) ([]p9.FullStat, error) {
+	stats := make([]p9.FullStat, 0, len(names))
+
+	if len(names) > 0 && names[0] == "" {
+		qid, valid, attr, err := l.GetAttr(p9.AttrMask{})
+		if err != nil {
+			return nil, err
+		}
+		stats = append(stats, p9.FullStat{
+			QID:   qid,
+			Valid: valid,
+			Attr:  attr,
+		})
+		names = names[1:]
+	}
+
+	parent := l.file.FD()
+	for _, name := range names {
+		child, err := unix.Openat(parent, name, openFlags|unix.O_PATH, 0)
+		if parent != l.file.FD() {
+			// Parent is no longer needed.
+			_ = unix.Close(parent)
+		}
+		if err != nil {
+			if errors.Is(err, unix.ENOENT) {
+				// No pont in continuing any further.
+				break
+			}
+			return nil, err
+		}
+
+		var stat unix.Stat_t
+		if err := unix.Fstat(child, &stat); err != nil {
+			_ = unix.Close(child)
+			return nil, err
+		}
+		valid, attr := l.fillAttr(&stat)
+		stats = append(stats, p9.FullStat{
+			QID:   l.attachPoint.makeQID(&stat),
+			Valid: valid,
+			Attr:  attr,
+		})
+		if (stat.Mode & unix.S_IFMT) != unix.S_IFDIR {
+			// Doesn't need to continue if entry is not a dir. Including symlinks
+			// that cannot be followed.
+			_ = unix.Close(child)
+			break
+		}
+		parent = child
+	}
+	return stats, nil
+}
diff --git a/runsc/fsgofer/fsgofer_test.go b/runsc/fsgofer/fsgofer_test.go
index d7e141476..77723827a 100644
--- a/runsc/fsgofer/fsgofer_test.go
+++ b/runsc/fsgofer/fsgofer_test.go
@@ -703,16 +703,6 @@ func TestWalkNotFound(t *testing.T) {
 	})
 }
 
-func TestWalkPanic(t *testing.T) {
-	runCustom(t, []uint32{unix.S_IFDIR}, allConfs, func(t *testing.T, s state) {
-		for _, name := range []string{".", ".."} {
-			assertPanic(t, func() {
-				s.file.Walk([]string{name})
-			})
-		}
-	})
-}
-
 func TestWalkDup(t *testing.T) {
 	runAll(t, func(t *testing.T, s state) {
 		_, dup, err := s.file.Walk([]string{})