crypto/rand: handle EAGAIN reads from /dev/urandom

Fixes #9205

Change-Id: Iacd608ba43332008984aa8ece17dcb5757f27b3f
Reviewed-on: https://go-review.googlesource.com/1611Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/crypto/rand/eagain.go

+// Copyright 2014 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.
+
+// +build darwin dragonfly freebsd linux nacl netbsd openbsd solaris
+
+package rand
+
+import (
+	"os"
+	"syscall"
+)
+
+func init() {
+	isEAGAIN = unixIsEAGAIN
+}
+
+// unixIsEAGAIN reports whether err is a syscall.EAGAIN wrapped in a PathError.
+// See golang.org/issue/9205
+func unixIsEAGAIN(err error) bool {
+	if pe, ok := err.(*os.PathError); ok {
+		if errno, ok := pe.Err.(syscall.Errno); ok && errno == syscall.EAGAIN {
+			return true
+		}
+	}
+	return false
+}

src/crypto/rand/rand_unix.go

@@ -58,12 +58,28 @@ func (r *devReader) Read(b []byte) (n int, err error) {
 		if runtime.GOOS == "plan9" {
 			r.f = f
 		} else {
-			r.f = bufio.NewReader(f)
+			r.f = bufio.NewReader(hideAgainReader{f})
 		}
 	}
 	return r.f.Read(b)
 }
 
+var isEAGAIN func(error) bool // set by eagain.go on unix systems
+
+// hideAgainReader masks EAGAIN reads from /dev/urandom.
+// See golang.org/issue/9205
+type hideAgainReader struct {
+	r io.Reader
+}
+
+func (hr hideAgainReader) Read(p []byte) (n int, err error) {
+	n, err = hr.r.Read(p)
+	if err != nil && isEAGAIN != nil && isEAGAIN(err) {
+		err = nil
+	}
+	return
+}
+
 // Alternate pseudo-random implementation for use on
 // systems without a reliable /dev/urandom.