syscall: reduce redundant getwd tracking in Plan 9

In Plan 9, each M is implemented as a separate OS process with
its own working directory.  To keep the wd consistent across
goroutines (or rescheduling of the same goroutine), CL 6350
introduced a Fixwd procedure which checks using getwd and calls
chdir if necessary before any syscall operating on a pathname.

This wd checking will not be necessary if the pathname is absolute
(starts with '/' or '#').  Getwd is a fairly expensive operation
in Plan 9 (implemented by opening "." and calling Fd2path on the
file descriptor).  Eliminating the redundant getwd calls can
significantly reduce overhead for common operations like
"dist test --list" which perform many syscalls on absolute pathnames.

Updates #9428.

Change-Id: I13fd9380779de27b0ac2f2b488229778d6839255
Reviewed-on: https://go-review.googlesource.com/97675Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>

src/syscall/pwd_plan9.go

@@ -39,6 +39,15 @@ func fixwdLocked() {
 	}
 }
 
+func fixwd(paths ...string) {
+	for _, path := range paths {
+		if path != "" && path[0] != '/' && path[0] != '#' {
+			Fixwd()
+			return
+		}
+	}
+}
+
 // goroutine-specific getwd
 func getwd() (wd string, err error) {
 	fd, err := open(".", O_RDONLY)
@@ -66,6 +75,7 @@ func Getwd() (wd string, err error) {
 }
 
 func Chdir(path string) error {
+	fixwd(path)
 	wdmu.Lock()
 	defer wdmu.Unlock()
 

src/syscall/syscall_plan9.go

@@ -244,7 +244,7 @@ func Await(w *Waitmsg) (err error) {
 }
 
 func Unmount(name, old string) (err error) {
-	Fixwd()
+	fixwd(name, old)
 	oldp, err := BytePtrFromString(old)
 	if err != nil {
 		return err
@@ -326,43 +326,43 @@ func Getgroups() (gids []int, err error) {
 
 //sys	open(path string, mode int) (fd int, err error)
 func Open(path string, mode int) (fd int, err error) {
-	Fixwd()
+	fixwd(path)
 	return open(path, mode)
 }
 
 //sys	create(path string, mode int, perm uint32) (fd int, err error)
 func Create(path string, mode int, perm uint32) (fd int, err error) {
-	Fixwd()
+	fixwd(path)
 	return create(path, mode, perm)
 }
 
 //sys	remove(path string) (err error)
 func Remove(path string) error {
-	Fixwd()
+	fixwd(path)
 	return remove(path)
 }
 
 //sys	stat(path string, edir []byte) (n int, err error)
 func Stat(path string, edir []byte) (n int, err error) {
-	Fixwd()
+	fixwd(path)
 	return stat(path, edir)
 }
 
 //sys	bind(name string, old string, flag int) (err error)
 func Bind(name string, old string, flag int) (err error) {
-	Fixwd()
+	fixwd(name, old)
 	return bind(name, old, flag)
 }
 
 //sys	mount(fd int, afd int, old string, flag int, aname string) (err error)
 func Mount(fd int, afd int, old string, flag int, aname string) (err error) {
-	Fixwd()
+	fixwd(old)
 	return mount(fd, afd, old, flag, aname)
 }
 
 //sys	wstat(path string, edir []byte) (err error)
 func Wstat(path string, edir []byte) (err error) {
-	Fixwd()
+	fixwd(path)
 	return wstat(path, edir)
 }