runtime: use mcache0 if no P in profilealloc

A case that I missed in CL 205239: profilealloc can be called at
program startup if GOMAXPROCS is large enough.

Fixes #38474

Change-Id: I2f089fc6ec00c376680e1c0b8a2557b62789dd7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/228420
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/malloc.go

@@ -1207,7 +1207,16 @@ func reflect_unsafe_NewArray(typ *_type, n int) unsafe.Pointer {
 }
 
 func profilealloc(mp *m, x unsafe.Pointer, size uintptr) {
-	mp.p.ptr().mcache.next_sample = nextSample()
+	var c *mcache
+	if mp.p != 0 {
+		c = mp.p.ptr().mcache
+	} else {
+		c = mcache0
+		if c == nil {
+			throw("profilealloc called with no P")
+		}
+	}
+	c.next_sample = nextSample()
 	mProf_Malloc(x, size)
 }
 

src/runtime/proc_test.go

@@ -1037,3 +1037,16 @@ loop:
 		t.Errorf("netpollBreak did not interrupt netpoll: slept for: %v", dur)
 	}
 }
+
+// TestBigGOMAXPROCS tests that setting GOMAXPROCS to a large value
+// doesn't cause a crash at startup. See issue 38474.
+func TestBigGOMAXPROCS(t *testing.T) {
+	t.Parallel()
+	output := runTestProg(t, "testprog", "NonexistentTest", "GOMAXPROCS=1024")
+	if strings.Contains(output, "failed to create new OS thread") {
+		t.Skipf("failed to create 1024 threads")
+	}
+	if !strings.Contains(output, "unknown function: NonexistentTest") {
+		t.Errorf("output:\n%s\nwanted:\nunknown function: NonexistentTest", output)
+	}
+}