cmd/go: work around occasional ETXTBSY running cgo

Fixes #3001.  (This time for sure!)

R=golang-dev, r, fullung
CC=golang-dev
https://golang.org/cl/5845044

src/cmd/go/build.go

@@ -21,6 +21,7 @@ import (
 	"runtime"
 	"strings"
 	"sync"
+	"time"
 )
 
 var cmdBuild = &Command{
@@ -1047,6 +1048,8 @@ func (b *builder) runOut(dir string, desc string, cmdargs ...interface{}) ([]byt
 		}
 	}
 
+	nbusy := 0
+	for {
 		var buf bytes.Buffer
 		cmd := exec.Command(cmdline[0], cmdline[1:]...)
 		cmd.Stdout = &buf
@@ -1054,7 +1057,57 @@ func (b *builder) runOut(dir string, desc string, cmdargs ...interface{}) ([]byt
 		cmd.Dir = dir
 		// TODO: cmd.Env
 		err := cmd.Run()
+
+		// cmd.Run will fail on Unix if some other process has the binary
+		// we want to run open for writing.  This can happen here because
+		// we build and install the cgo command and then run it.
+		// If another command was kicked off while we were writing the
+		// cgo binary, the child process for that command may be holding
+		// a reference to the fd, keeping us from running exec.
+		//
+		// But, you might reasonably wonder, how can this happen?
+		// The cgo fd, like all our fds, is close-on-exec, so that we need
+		// not worry about other processes inheriting the fd accidentally.
+		// The answer is that running a command is fork and exec.
+		// A child forked while the cgo fd is open inherits that fd.
+		// Until the child has called exec, it holds the fd open and the 
+		// kernel will not let us run cgo.  Even if the child were to close
+		// the fd explicitly, it would still be open from the time of the fork
+		// until the time of the explicit close, and the race would remain.
+		//
+		// On Unix systems, this results in ETXTBSY, which formats
+		// as "text file busy".  Rather than hard-code specific error cases,
+		// we just look for that string.  If this happens, sleep a little
+		// and try again.  We let this happen three times, with increasing
+		// sleep lengths: 100+200+400 ms = 0.7 seconds.
+		//
+		// An alternate solution might be to split the cmd.Run into
+		// separate cmd.Start and cmd.Wait, and then use an RWLock
+		// to make sure that copyFile only executes when no cmd.Start
+		// call is in progress.  However, cmd.Start (really syscall.forkExec)
+		// only guarantees that when it returns, the exec is committed to
+		// happen and succeed.  It uses a close-on-exec file descriptor
+		// itself to determine this, so we know that when cmd.Start returns,
+		// at least one close-on-exec file descriptor has been closed.
+		// However, we cannot be sure that all of them have been closed,
+		// so the program might still encounter ETXTBSY even with such
+		// an RWLock.  The race window would be smaller, perhaps, but not
+		// guaranteed to be gone.
+		//
+		// Sleeping when we observe the race seems to be the most reliable
+		// option we have.
+		//
+		// http://golang.org/issue/3001
+		//
+		if err != nil && nbusy < 3 && strings.Contains(err.Error(), "text file busy") {
+			time.Sleep(100 * time.Millisecond << uint(nbusy))
+			nbusy++
+			continue
+		}
+
 		return buf.Bytes(), err
+	}
+	panic("unreachable")
 }
 
 // mkdir makes the named directory.