runtime.GOMAXPROCS: hack it to have it return the old value.

R=rsc
CC=golang-dev
https://golang.org/cl/1140041

src/pkg/runtime/extern.go

@@ -106,8 +106,10 @@ func LockOSThread()
 func UnlockOSThread()
 
 // GOMAXPROCS sets the maximum number of CPUs that can be executing
-// simultaneously.   This call will go away when the scheduler improves.
+// simultaneously and returns the previous setting.  If n < 1, it does not
-func GOMAXPROCS(n int)
+// change the current setting.
+// This call will go away when the scheduler improves.
+func GOMAXPROCS(n int) int
 
 // Cgocalls returns the number of cgo calls made by the current process.
 func Cgocalls() int64

src/pkg/runtime/proc.c

@@ -1136,13 +1136,15 @@ void
 }
 
 // delete when scheduler is stronger
-void
+int32
-·GOMAXPROCS(int32 n)
+gomaxprocsfunc(int32 n)
 {
-	if(n < 1)
+	int32 ret;
-		n = 1;
 
 	lock(&sched);
+	ret = sched.gomaxprocs;
+	if (n <= 0)
+		n = ret;
 	sched.gomaxprocs = n;
 	sched.mcpumax = n;
 	// handle fewer procs?
@@ -1152,11 +1154,12 @@ void
 		// we'll only get rescheduled once the
 		// number has come down.
 		gosched();
-		return;
+		return ret;
 	}
 	// handle more procs
 	matchmg();
 	unlock(&sched);
+	return ret;
 }
 
 void

src/pkg/runtime/runtime.h

@@ -569,6 +569,7 @@ float64	modf(float64 d, float64 *ip);
 void	semacquire(uint32*);
 void	semrelease(uint32*);
 String	signame(int32 sig);
+int32	gomaxprocsfunc(int32 n);
 
 
 void	mapassign(Hmap*, byte*, byte*);

src/pkg/runtime/runtime1.goc

@@ -9,3 +9,6 @@ func mal(n uint32) (ret *uint8) {
 	ret = mal(n);
 }
 
+func GOMAXPROCS(n int32) (ret int32) {
+	ret = gomaxprocsfunc(n);
+}