runtime: increase assumed stack size in externalthreadhandler

On Windows, externalthreadhandler currently sets the assumed stack
size for the profiler thread and the ctrlhandler threads to 8KB. The
actual stack size is determined by the SizeOfStackReserve field in the
binary set by the linker, which is currently at least 64KB (and
typically 128KB).

It turns out the profiler thread is running within a few words of the
8KB-(stack guard) bound set by externalthreadhandler. If it overflows
this bound, morestack crashes unceremoniously with an access
violation, which we then fail to handle, causing the whole process to
exit without explanation.

To avoid this problem and give us some breathing room, increase the
assumed stack size in externalthreadhandler to 32KB (there's some
unknown amount of stack already in use, so it's not safe to increase
this all the way to the reserve size).

We also document the relationships between externalthreadhandler and
SizeOfStackReserve to make this more obvious in the future.

Change-Id: I2f9f9c0892076d78e09827022ff0f2bedd9680a9
Reviewed-on: https://go-review.googlesource.com/18304
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/link/internal/ld/pe.go

@@ -1217,12 +1217,17 @@ func Asmbpe() {
 	// larger size, as verified with VMMap.
 
 	// Go code would be OK with 64k stacks, but we need larger stacks for cgo.
-	// That default stack reserve size affects only the main thread,
-	// for other threads we specify stack size in runtime explicitly
+	//
+	// The default stack reserve size affects only the main
+	// thread, ctrlhandler thread, and profileloop thread. For
+	// these, it must be greater than the stack size assumed by
+	// externalthreadhandler.
+	//
+	// For other threads we specify stack size in runtime explicitly
 	// (runtime knows whether cgo is enabled or not).
-	// If you change stack reserve sizes here,
-	// change STACKSIZE in runtime/cgo/gcc_windows_{386,amd64}.c and correspondent
-	// CreateThread parameter in runtime.newosproc as well.
+	// For these, the reserve must match STACKSIZE in
+	// runtime/cgo/gcc_windows_{386,amd64}.c and the correspondent
+	// CreateThread parameter in runtime.newosproc.
 	if !iscgo {
 		oh64.SizeOfStackReserve = 0x00020000
 		oh.SizeOfStackReserve = 0x00020000

src/runtime/sys_windows_386.s

@@ -206,7 +206,8 @@ TEXT runtime·externalthreadhandler(SB),NOSPLIT,$0
 	CALL	runtime·memclr(SB)	// smashes AX,BX,CX
 	LEAL	g__size(SP), BX
 	MOVL	BX, g_m(SP)
-	LEAL	-8192(SP), CX
+
+	LEAL	-32768(SP), CX		// must be less than SizeOfStackReserve set by linker
 	MOVL	CX, (g_stack+stack_lo)(SP)
 	ADDL	$const__StackGuard, CX
 	MOVL	CX, g_stackguard0(SP)

src/runtime/sys_windows_amd64.s

@@ -243,7 +243,7 @@ TEXT runtime·externalthreadhandler(SB),NOSPLIT,$0
 	LEAQ	g__size(SP), BX
 	MOVQ	BX, g_m(SP)
 
-	LEAQ	-8192(SP), CX
+	LEAQ	-32768(SP), CX		// must be less than SizeOfStackReserve set by linker
 	MOVQ	CX, (g_stack+stack_lo)(SP)
 	ADDQ	$const__StackGuard, CX
 	MOVQ	CX, g_stackguard0(SP)