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

6 files changed, 218 insertions, 6 deletions

diff --git a/pkg/p9/client_file.go b/pkg/p9/client_file.go
index 7abc82e1b..28396b0ea 100644
--- a/pkg/p9/client_file.go
+++ b/pkg/p9/client_file.go
@@ -121,6 +121,22 @@ func (c *clientFile) WalkGetAttr(components []string) ([]QID, File, AttrMask, At
 	return rwalkgetattr.QIDs, c.client.newFile(FID(fid)), rwalkgetattr.Valid, rwalkgetattr.Attr, nil
 }
 
+func (c *clientFile) MultiGetAttr(names []string) ([]FullStat, error) {
+	if atomic.LoadUint32(&c.closed) != 0 {
+		return nil, unix.EBADF
+	}
+
+	if !versionSupportsTmultiGetAttr(c.client.version) {
+		return DefaultMultiGetAttr(c, names)
+	}
+
+	rmultigetattr := Rmultigetattr{}
+	if err := c.client.sendRecv(&Tmultigetattr{FID: c.fid, Names: names}, &rmultigetattr); err != nil {
+		return nil, err
+	}
+	return rmultigetattr.Stats, nil
+}
+
 // StatFS implements File.StatFS.
 func (c *clientFile) StatFS() (FSStat, error) {
 	if atomic.LoadUint32(&c.closed) != 0 {
diff --git a/pkg/p9/file.go b/pkg/p9/file.go
index c59c6a65b..97e0231d6 100644
--- a/pkg/p9/file.go
+++ b/pkg/p9/file.go
@@ -15,6 +15,8 @@
 package p9
 
 import (
+	"errors"
+
 	"golang.org/x/sys/unix"
 	"gvisor.dev/gvisor/pkg/fd"
 )
@@ -72,6 +74,15 @@ type File interface {
 	// On the server, WalkGetAttr has a read concurrency guarantee.
 	WalkGetAttr([]string) ([]QID, File, AttrMask, Attr, error)
 
+	// MultiGetAttr batches up multiple calls to GetAttr(). names is a list of
+	// path components similar to Walk(). If the first component name is empty,
+	// the current file is stat'd and included in the results. If the walk reaches
+	// a file that doesn't exist or not a directory, MultiGetAttr returns the
+	// partial result with no error.
+	//
+	// On the server, MultiGetAttr has a read concurrency guarantee.
+	MultiGetAttr(names []string) ([]FullStat, error)
+
 	// StatFS returns information about the file system associated with
 	// this file.
 	//
@@ -306,6 +317,53 @@ func (DisallowClientCalls) SetAttrClose(SetAttrMask, SetAttr) error {
 type DisallowServerCalls struct{}
 
 // Renamed implements File.Renamed.
-func (*clientFile) Renamed(File, string) {
+func (*DisallowServerCalls) Renamed(File, string) {
 	panic("Renamed should not be called on the client")
 }
+
+// DefaultMultiGetAttr implements File.MultiGetAttr() on top of File.
+func DefaultMultiGetAttr(start File, names []string) ([]FullStat, error) {
+	stats := make([]FullStat, 0, len(names))
+	parent := start
+	mask := AttrMaskAll()
+	for i, name := range names {
+		if len(name) == 0 && i == 0 {
+			qid, valid, attr, err := parent.GetAttr(mask)
+			if err != nil {
+				return nil, err
+			}
+			stats = append(stats, FullStat{
+				QID:   qid,
+				Valid: valid,
+				Attr:  attr,
+			})
+			continue
+		}
+		qids, child, valid, attr, err := parent.WalkGetAttr([]string{name})
+		if parent != start {
+			_ = parent.Close()
+		}
+		if err != nil {
+			if errors.Is(err, unix.ENOENT) {
+				return stats, nil
+			}
+			return nil, err
+		}
+		stats = append(stats, FullStat{
+			QID:   qids[0],
+			Valid: valid,
+			Attr:  attr,
+		})
+		if attr.Mode.FileType() != ModeDirectory {
+			// Doesn't need to continue if entry is not a dir. Including symlinks
+			// that cannot be followed.
+			_ = child.Close()
+			break
+		}
+		parent = child
+	}
+	if parent != start {
+		_ = parent.Close()
+	}
+	return stats, nil
+}
diff --git a/pkg/p9/handlers.go b/pkg/p9/handlers.go
index 58312d0cc..758e11b13 100644
--- a/pkg/p9/handlers.go
+++ b/pkg/p9/handlers.go
@@ -1421,3 +1421,31 @@ func (t *Tchannel) handle(cs *connState) message {
 	}
 	return rchannel
 }
+
+// handle implements handler.handle.
+func (t *Tmultigetattr) handle(cs *connState) message {
+	for i, name := range t.Names {
+		if len(name) == 0 && i == 0 {
+			// Empty name is allowed on the first entry to indicate that the current
+			// FID needs to be included in the result.
+			continue
+		}
+		if err := checkSafeName(name); err != nil {
+			return newErr(err)
+		}
+	}
+	ref, ok := cs.LookupFID(t.FID)
+	if !ok {
+		return newErr(unix.EBADF)
+	}
+	defer ref.DecRef()
+
+	var stats []FullStat
+	if err := ref.safelyRead(func() (err error) {
+		stats, err = ref.file.MultiGetAttr(t.Names)
+		return err
+	}); err != nil {
+		return newErr(err)
+	}
+	return &Rmultigetattr{Stats: stats}
+}
diff --git a/pkg/p9/messages.go b/pkg/p9/messages.go
index cf13cbb69..2ff4694c0 100644
--- a/pkg/p9/messages.go
+++ b/pkg/p9/messages.go
@@ -254,8 +254,8 @@ func (r *Rwalk) decode(b *buffer) {
 // encode implements encoder.encode.
 func (r *Rwalk) encode(b *buffer) {
 	b.Write16(uint16(len(r.QIDs)))
-	for _, q := range r.QIDs {
-		q.encode(b)
+	for i := range r.QIDs {
+		r.QIDs[i].encode(b)
 	}
 }
 
@@ -2243,8 +2243,8 @@ func (r *Rwalkgetattr) encode(b *buffer) {
 	r.Valid.encode(b)
 	r.Attr.encode(b)
 	b.Write16(uint16(len(r.QIDs)))
-	for _, q := range r.QIDs {
-		q.encode(b)
+	for i := range r.QIDs {
+		r.QIDs[i].encode(b)
 	}
 }
 
@@ -2552,6 +2552,80 @@ func (r *Rchannel) String() string {
 	return fmt.Sprintf("Rchannel{Offset: %d, Length: %d}", r.Offset, r.Length)
 }
 
+// Tmultigetattr is a multi-getattr request.
+type Tmultigetattr struct {
+	// FID is the FID to be walked.
+	FID FID
+
+	// Names are the set of names to be walked.
+	Names []string
+}
+
+// decode implements encoder.decode.
+func (t *Tmultigetattr) decode(b *buffer) {
+	t.FID = b.ReadFID()
+	n := b.Read16()
+	t.Names = t.Names[:0]
+	for i := 0; i < int(n); i++ {
+		t.Names = append(t.Names, b.ReadString())
+	}
+}
+
+// encode implements encoder.encode.
+func (t *Tmultigetattr) encode(b *buffer) {
+	b.WriteFID(t.FID)
+	b.Write16(uint16(len(t.Names)))
+	for _, name := range t.Names {
+		b.WriteString(name)
+	}
+}
+
+// Type implements message.Type.
+func (*Tmultigetattr) Type() MsgType {
+	return MsgTmultigetattr
+}
+
+// String implements fmt.Stringer.
+func (t *Tmultigetattr) String() string {
+	return fmt.Sprintf("Tmultigetattr{FID: %d, Names: %v}", t.FID, t.Names)
+}
+
+// Rmultigetattr is a multi-getattr response.
+type Rmultigetattr struct {
+	// Stats are the set of FullStat returned for each of the names in the
+	// request.
+	Stats []FullStat
+}
+
+// decode implements encoder.decode.
+func (r *Rmultigetattr) decode(b *buffer) {
+	n := b.Read16()
+	r.Stats = r.Stats[:0]
+	for i := 0; i < int(n); i++ {
+		var fs FullStat
+		fs.decode(b)
+		r.Stats = append(r.Stats, fs)
+	}
+}
+
+// encode implements encoder.encode.
+func (r *Rmultigetattr) encode(b *buffer) {
+	b.Write16(uint16(len(r.Stats)))
+	for i := range r.Stats {
+		r.Stats[i].encode(b)
+	}
+}
+
+// Type implements message.Type.
+func (*Rmultigetattr) Type() MsgType {
+	return MsgRmultigetattr
+}
+
+// String implements fmt.Stringer.
+func (r *Rmultigetattr) String() string {
+	return fmt.Sprintf("Rmultigetattr{Stats: %v}", r.Stats)
+}
+
 const maxCacheSize = 3
 
 // msgFactory is used to reduce allocations by caching messages for reuse.
@@ -2717,6 +2791,8 @@ func init() {
 	msgRegistry.register(MsgRallocate, func() message { return &Rallocate{} })
 	msgRegistry.register(MsgTsetattrclunk, func() message { return &Tsetattrclunk{} })
 	msgRegistry.register(MsgRsetattrclunk, func() message { return &Rsetattrclunk{} })
+	msgRegistry.register(MsgTmultigetattr, func() message { return &Tmultigetattr{} })
+	msgRegistry.register(MsgRmultigetattr, func() message { return &Rmultigetattr{} })
 	msgRegistry.register(MsgTchannel, func() message { return &Tchannel{} })
 	msgRegistry.register(MsgRchannel, func() message { return &Rchannel{} })
 }
diff --git a/pkg/p9/p9.go b/pkg/p9/p9.go
index 648cf4b49..3d452a0bd 100644
--- a/pkg/p9/p9.go
+++ b/pkg/p9/p9.go
@@ -402,6 +402,8 @@ const (
 	MsgRallocate     MsgType = 139
 	MsgTsetattrclunk MsgType = 140
 	MsgRsetattrclunk MsgType = 141
+	MsgTmultigetattr MsgType = 142
+	MsgRmultigetattr MsgType = 143
 	MsgTchannel      MsgType = 250
 	MsgRchannel      MsgType = 251
 )
@@ -1178,3 +1180,29 @@ func (a *AllocateMode) encode(b *buffer) {
 	}
 	b.Write32(mask)
 }
+
+// FullStat is used in the result of a MultiGetAttr call.
+type FullStat struct {
+	QID   QID
+	Valid AttrMask
+	Attr  Attr
+}
+
+// String implements fmt.Stringer.
+func (f *FullStat) String() string {
+	return fmt.Sprintf("FullStat{QID: %v, Valid: %v, Attr: %v}", f.QID, f.Valid, f.Attr)
+}
+
+// decode implements encoder.decode.
+func (f *FullStat) decode(b *buffer) {
+	f.QID.decode(b)
+	f.Valid.decode(b)
+	f.Attr.decode(b)
+}
+
+// encode implements encoder.encode.
+func (f *FullStat) encode(b *buffer) {
+	f.QID.encode(b)
+	f.Valid.encode(b)
+	f.Attr.encode(b)
+}
diff --git a/pkg/p9/version.go b/pkg/p9/version.go
index 8d7168ef5..950236162 100644
--- a/pkg/p9/version.go
+++ b/pkg/p9/version.go
@@ -26,7 +26,7 @@ const (
 	//
 	// Clients are expected to start requesting this version number and
 	// to continuously decrement it until a Tversion request succeeds.
-	highestSupportedVersion uint32 = 12
+	highestSupportedVersion uint32 = 13
 
 	// lowestSupportedVersion is the lowest supported version X in a
 	// version string of the format 9P2000.L.Google.X.
@@ -179,3 +179,9 @@ func versionSupportsListRemoveXattr(v uint32) bool {
 func versionSupportsTsetattrclunk(v uint32) bool {
 	return v >= 12
 }
+
+// versionSupportsTmultiGetAttr returns true if version v supports
+// the TmultiGetAttr message.
+func versionSupportsTmultiGetAttr(v uint32) bool {
+	return v >= 13
+}