runtime: avoid race with forcegc helper

While we are here, give the gc helper a real function name
that will appear in stack traces.

LGTM=rlh
R=rlh
CC=dvyukov, golang-codereviews
https://golang.org/cl/133470043

src/pkg/runtime/proc.c

@@ -2846,9 +2846,12 @@ sysmon(void)
 		lastgc = runtime·atomicload64(&mstats.last_gc);
 		if(lastgc != 0 && unixnow - lastgc > forcegcperiod && runtime·atomicload(&runtime·forcegc.idle)) {
 			runtime·lock(&runtime·forcegc.lock);
-			runtime·forcegc.idle = 0;
-			runtime·forcegc.g->schedlink = nil;
-			injectglist(runtime·forcegc.g);
+			if(runtime·forcegc.g != nil) {
+				// Goroutine may be started but has not initialized g yet.
+				runtime·forcegc.idle = 0;
+				runtime·forcegc.g->schedlink = nil;
+				injectglist(runtime·forcegc.g);
+			}
 			runtime·unlock(&runtime·forcegc.lock);
 		}
 

src/pkg/runtime/proc.go

@@ -31,23 +31,25 @@ var parkunlock_c byte
 
 // start forcegc helper goroutine
 func init() {
-	go func() {
-		forcegc.g = getg()
-		forcegc.g.issystem = true
-		for {
-			lock(&forcegc.lock)
-			if forcegc.idle != 0 {
-				gothrow("forcegc: phase error")
-			}
-			atomicstore(&forcegc.idle, 1)
-			goparkunlock(&forcegc.lock, "force gc (idle)")
-			// this goroutine is explicitly resumed by sysmon
-			if debug.gctrace > 0 {
-				println("GC forced")
-			}
-			gogc(1)
+	go forcegchelper()
+}
+
+func forcegchelper() {
+	forcegc.g = getg()
+	forcegc.g.issystem = true
+	for {
+		lock(&forcegc.lock)
+		if forcegc.idle != 0 {
+			gothrow("forcegc: phase error")
 		}
-	}()
+		atomicstore(&forcegc.idle, 1)
+		goparkunlock(&forcegc.lock, "force gc (idle)")
+		// this goroutine is explicitly resumed by sysmon
+		if debug.gctrace > 0 {
+			println("GC forced")
+		}
+		gogc(1)
+	}
 }
 
 // Gosched yields the processor, allowing other goroutines to run.  It does not