os: cache Getwd result as hint for next time

Avoids the dot-dot-based algorithm on repeated calls
when the directory hasn't changed.

R=golang-dev, iant, bradfitz
CC=golang-dev
https://golang.org/cl/7340043

src/pkg/os/getwd.go

@@ -5,9 +5,15 @@
 package os
 
 import (
+	"sync"
 	"syscall"
 )
 
+var getwdCache struct {
+	sync.Mutex
+	dir string
+}
+
 // Getwd returns a rooted path name corresponding to the
 // current directory.  If the current directory can be
 // reached via multiple paths (due to symbolic links),
@@ -35,6 +41,17 @@ func Getwd() (pwd string, err error) {
 		}
 	}
 
+	// Apply same kludge but to cached dir instead of $PWD.
+	getwdCache.Lock()
+	pwd = getwdCache.dir
+	getwdCache.Unlock()
+	if len(pwd) > 0 {
+		d, err := Stat(pwd)
+		if err == nil && SameFile(dot, d) {
+			return pwd, nil
+		}
+	}
+
 	// Root is a special case because it has no parent
 	// and ends in a slash.
 	root, err := Stat("/")
@@ -88,5 +105,11 @@ func Getwd() (pwd string, err error) {
 		// Set up for next round.
 		dot = pd
 	}
+
+	// Save answer as hint to avoid the expensive path next time.
+	getwdCache.Lock()
+	getwdCache.dir = pwd
+	getwdCache.Unlock()
+
 	return pwd, nil
 }