godoc: limit concurrency to local disk filesystem

Not for Go 1.2.  Still needs a flag.

Linux at least (and likely other OSes) don't like you doing
a few hundred readdirs at once and spawing as many threads.

R=golang-dev, adg, jeremyjackins
CC=golang-dev
https://golang.org/cl/30620043

cmd/godoc/main.go

@@ -46,6 +46,7 @@ import (
 	"code.google.com/p/go.tools/godoc"
 	"code.google.com/p/go.tools/godoc/static"
 	"code.google.com/p/go.tools/godoc/vfs"
+	"code.google.com/p/go.tools/godoc/vfs/gatefs"
 	"code.google.com/p/go.tools/godoc/vfs/mapfs"
 	"code.google.com/p/go.tools/godoc/vfs/zipfs"
 )
@@ -162,10 +163,13 @@ func main() {
 		usage()
 	}
 
+	var fsGate chan bool
+	fsGate = make(chan bool, 20)
+
 	// Determine file system to use.
 	if *zipfile == "" {
 		// use file system of underlying OS
-		fs.Bind("/", vfs.OS(*goroot), "/", vfs.BindReplace)
+		fs.Bind("/", gatefs.New(vfs.OS(*goroot), fsGate), "/", vfs.BindReplace)
 	} else {
 		// use file system specified via .zip file (path separator must be '/')
 		rc, err := zip.OpenReader(*zipfile)
@@ -183,7 +187,7 @@ func main() {
 
 	// Bind $GOPATH trees into Go root.
 	for _, p := range filepath.SplitList(build.Default.GOPATH) {
-		fs.Bind("/src/pkg", vfs.OS(p), "/src", vfs.BindAfter)
+		fs.Bind("/src/pkg", gatefs.New(vfs.OS(p), fsGate), "/src", vfs.BindAfter)
 	}
 
 	httpMode := *httpAddr != ""

godoc/vfs/gatefs/gatefs.go

@@ -0,0 +1,89 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package gatefs provides an implementation of the FileSystem
+// interface that wraps another FileSystem and limits its concurrency.
+package gatefs
+
+import (
+	"fmt"
+	"os"
+
+	"code.google.com/p/go.tools/godoc/vfs"
+)
+
+// New returns a new FileSystem that delegates to fs.
+// If gateCh is non-nil and buffered, it's used as a gate
+// to limit concurrency on calls to fs.
+func New(fs vfs.FileSystem, gateCh chan bool) vfs.FileSystem {
+	if cap(gateCh) == 0 {
+		return fs
+	}
+	return gatefs{fs, gate(gateCh)}
+}
+
+type gate chan bool
+
+func (g gate) enter() { g <- true }
+func (g gate) leave() { <-g }
+
+type gatefs struct {
+	fs vfs.FileSystem
+	gate
+}
+
+func (fs gatefs) String() string {
+	return fmt.Sprintf("gated(%s, %d)", fs.fs.String(), cap(fs.gate))
+}
+
+func (fs gatefs) Open(p string) (vfs.ReadSeekCloser, error) {
+	fs.enter()
+	defer fs.leave()
+	rsc, err := fs.fs.Open(p)
+	if err != nil {
+		return nil, err
+	}
+	return gatef{rsc, fs.gate}, nil
+}
+
+func (fs gatefs) Lstat(p string) (os.FileInfo, error) {
+	fs.enter()
+	defer fs.leave()
+	return fs.fs.Lstat(p)
+}
+
+func (fs gatefs) Stat(p string) (os.FileInfo, error) {
+	fs.enter()
+	defer fs.leave()
+	return fs.fs.Stat(p)
+}
+
+func (fs gatefs) ReadDir(p string) ([]os.FileInfo, error) {
+	fs.enter()
+	defer fs.leave()
+	return fs.fs.ReadDir(p)
+}
+
+type gatef struct {
+	rsc vfs.ReadSeekCloser
+	gate
+}
+
+func (f gatef) Read(p []byte) (n int, err error) {
+	f.enter()
+	defer f.leave()
+	return f.rsc.Read(p)
+}
+
+func (f gatef) Seek(offset int64, whence int) (ret int64, err error) {
+	f.enter()
+	defer f.leave()
+	return f.rsc.Seek(offset, whence)
+}
+
+func (f gatef) Close() error {
+	f.enter()
+	defer f.leave()
+	return f.rsc.Close()
+}