pkg/runtime: Fix semasleep on Plan 9

With the timed semacquire patch
(kernel-tsemacquire) for Plan 9,
we can now properly do a timed
wait for the semaphore, in
semasleep.

R=golang-dev, rsc, rminnich, ality, r
CC=0intro, golang-dev, john, mirtchovski
https://golang.org/cl/6197046

src/pkg/runtime/os_plan9.h

@@ -13,6 +13,7 @@ int32	runtime·brk_(void*);
 int32	runtime·sleep(int32 ms);
 int32	runtime·rfork(int32 flags, void *stk, M *m, G *g, void (*fn)(void));
 int32	runtime·plan9_semacquire(uint32 *addr, int32 block);
+int32	runtime·plan9_tsemacquire(uint32 *addr, int32 ms);
 int32 	runtime·plan9_semrelease(uint32 *addr, int32 count);
 int32	runtime·notify(void (*fn)(void*, byte*));
 int32	runtime·noted(int32);

src/pkg/runtime/sys_plan9_386.s

@@ -49,6 +49,11 @@ TEXT runtime·plan9_semacquire(SB),7,$0
 	INT	$64
 	RET
 
+TEXT runtime·plan9_tsemacquire(SB),7,$0
+	MOVL	$52, AX
+	INT	$64
+	RET
+
 TEXT runtime·notify(SB),7,$0
 	MOVL	$28, AX
 	INT	$64

src/pkg/runtime/thread_plan9.c

@@ -43,7 +43,7 @@ static int32
 getpid(void)
 {
 	byte b[20], *c;
-	int32 fd, n;
+	int32 fd;
 
 	runtime·memclr(b, sizeof(b));
 	fd = runtime·open((byte*)"#c/pid", 0);
@@ -276,36 +276,18 @@ runtime·semasleep(int64 ns)
 	int32 ms;
 
 	if(ns >= 0) {
-		// TODO: Plan 9 needs a new system call, tsemacquire.
-		// The kernel implementation is the same as semacquire
-		// except with a tsleep and check for timeout.
-		// It would be great if the implementation returned the
-		// value that was added to the semaphore, so that on
-		// timeout the return value would be 0, on success 1.
-		// Then the error string does not have to be parsed
-		// to detect timeout.
-		//
-		// If a negative time indicates no timeout, then
-		// semacquire can be implemented (in the kernel)
-		// as tsemacquire(p, v, -1).
-		runtime·throw("semasleep: timed sleep not implemented on Plan 9");
-
-		/*
-		if(ns < 0)
-			ms = -1;
-		else if(ns/1000 > 0x7fffffffll)
+		if(ns/1000000 > 0x7fffffffll)
 			ms = 0x7fffffff;
 		else
-			ms = ns/1000;
-		ret = runtime·plan9_tsemacquire(&m->waitsemacount, 1, ms);
+			ms = ns/1000000;
+		ret = runtime·plan9_tsemacquire(&m->waitsemacount, ms);
 		if(ret == 1)
 			return 0;  // success
 		return -1;  // timeout or interrupted
-		*/
 	}
 
 	while(runtime·plan9_semacquire(&m->waitsemacount, 1) < 0) {
-		/* interrupted; try again */
+		/* interrupted; try again (c.f. lock_sema.c) */
 	}
 	return 0;  // success
 }