build: add build comments to core packages

The go/build package already recognizes
system-specific file names like

        mycode_darwin.go
        mycode_darwin_386.go
        mycode_386.s

However, it is also common to write files that
apply to multiple architectures, so a recent CL added
to go/build the ability to process comments
listing a set of conditions for building.  For example:

        // +build darwin freebsd openbsd/386

says that this file should be compiled only on
OS X, FreeBSD, or 32-bit x86 OpenBSD systems.

These conventions are not yet documented
(hence this long CL description).

This CL adds build comments to the multi-system
files in the core library, a step toward making it
possible to use go/build to build them.

With this change go/build can handle crypto/rand,
exec, net, path/filepath, os/user, and time.

os and syscall need additional adjustments.

R=golang-dev, r, gri, r, gustavo
CC=golang-dev
https://golang.org/cl/5011046

src/pkg/crypto/rand/rand_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // Unix cryptographically secure pseudorandom number
 // generator.
 

src/pkg/exec/lp_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package exec
 
 import (

src/pkg/net/cgo_bsd.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd
+
 package net
 
 /*

src/pkg/net/cgo_stub.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build openbsd
+
 // Stub cgo routines for systems that do not use cgo to do network lookups.
 
 package net

src/pkg/net/cgo_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux
+
 package net
 
 /*

src/pkg/net/dnsclient_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // DNS client: see RFC 1035.
 // Has to be linked into package net for Dial.
 

src/pkg/net/dnsconfig.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // Read system DNS config from /etc/resolv.conf
 
 package net

src/pkg/net/fd.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package net
 
 import (

src/pkg/net/file.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package net
 
 import (

src/pkg/net/interface_bsd.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd openbsd
+
 // Network interface identification for BSD variants
 
 package net

src/pkg/net/interface_stub.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build plan9
+
 // Network interface identification
 
 package net

src/pkg/net/iprawsock_posix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd windows
+
 // (Raw) IP sockets
 
 package net

src/pkg/net/ipsock_posix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd windows
+
 package net
 
 import (

src/pkg/net/lookup_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package net
 
 import (

src/pkg/net/newpollserver.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package net
 
 import (

src/pkg/net/port.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // Read system port mappings from /etc/services
 
 package net

src/pkg/net/sendfile_stub.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd openbsd
+
 package net
 
 import (

src/pkg/net/sock.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd windows
+
 // Sockets
 
 package net

src/pkg/net/sock_bsd.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd
+
 // Sockets for BSD variants
 
 package net

src/pkg/net/tcpsock_posix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd windows
+
 // TCP sockets
 
 package net

src/pkg/net/udpsock_posix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd windows
+
 // UDP sockets
 
 package net

src/pkg/net/unixsock_posix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd windows
+
 // Unix domain sockets
 
 package net

src/pkg/os/dir_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package os
 
 import (

src/pkg/os/env_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // Unix environment variables.
 
 package os

src/pkg/os/error_posix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd windows
+
 package os
 
 import syscall "syscall"

src/pkg/os/exec_posix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd windows
+
 package os
 
 import (

src/pkg/os/exec_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package os
 
 import (

src/pkg/os/file_posix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd windows
+
 package os
 
 import (

src/pkg/os/file_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package os
 
 import (

src/pkg/os/path_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package os
 
 const (

src/pkg/os/str.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build plan9
+
 package os
 
 func itoa(val int) string { // do it here rather than with fmt to avoid dependency

src/pkg/os/sys_bsd.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd openbsd
+
 // os code shared between *BSD systems including OS X (Darwin)
 // and FreeBSD.
 

src/pkg/os/user/lookup_stubs.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build openbsd plan9 windows
+
 package user
 
 import (

src/pkg/os/user/lookup_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux
+
 package user
 
 import (

src/pkg/path/Makefile

@@ -9,6 +9,4 @@ GOFILES=\
 	match.go\
 	path.go\
 
-GOFILES+=$(GOFILES_$(GOOS))
-
 include ../../Make.pkg

src/pkg/path/filepath/path_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package filepath
 
 import "strings"

src/pkg/syscall/bpf_bsd.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd openbsd
+
 // Berkeley packet filter for BSD variants
 
 package syscall

src/pkg/syscall/exec_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // Fork, exec, wait, etc.
 
 package syscall

src/pkg/syscall/route_bsd.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd openbsd
+
 // Routing sockets and messages
 
 package syscall

src/pkg/syscall/sockcmsg_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // Socket control messages
 
 package syscall

src/pkg/syscall/syscall_bsd.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd openbsd
+
 // BSD system call wrappers shared by *BSD based systems
 // including OS X (Darwin) and FreeBSD.  Like the other
 // syscall_*.go files it is compiled as Go code but also

src/pkg/syscall/syscall_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package syscall
 
 import (

src/pkg/time/sys_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package time
 
 import (

src/pkg/time/zoneinfo_posix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd plan9
+
 package time
 
 import "sync"

src/pkg/time/zoneinfo_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // Parse "zoneinfo" time zone file.
 // This is a fairly standard file format used on OS X, Linux, BSD, Sun, and others.
 // See tzfile(5), http://en.wikipedia.org/wiki/Zoneinfo,