runtime: allow OutputDebugString to be sent to debugger

We mark DBG_PRINTEXCEPTION_C messages in VEH handler
as handled, thus preventing debugger from seeing them.
I don't see reason for doing that. The comment warns
of crashes, but I added test and don't see any crashes.

This is also simplify VEH handler before making
changes to fix issue 8006.

Update #8006

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/146800043

src/runtime/os_windows_386.c

@@ -24,8 +24,6 @@ runtime·dumpregs(Context *r)
 	runtime·printf("gs      %x\n", r->SegGs);
 }
 
-#define DBG_PRINTEXCEPTION_C 0x40010006
-
 // Called by sigtramp from Windows VEH handler.
 // Return value signals whether the exception has been handled (-1)
 // or should be made available to other handlers in the chain (0).
@@ -36,19 +34,6 @@ runtime·sighandler(ExceptionRecord *info, Context *r, G *gp)
 	uintptr *sp;
 	extern byte runtime·text[], runtime·etext[];
 
-	if(info->ExceptionCode == DBG_PRINTEXCEPTION_C) {
-		// This exception is intended to be caught by debuggers.
-		// There is a not-very-informational message like
-		// "Invalid parameter passed to C runtime function"
-		// sitting at info->ExceptionInformation[0] (a wchar_t*),
-		// with length info->ExceptionInformation[1].
-		// The default behavior is to ignore this exception,
-		// but somehow returning 0 here (meaning keep going)
-		// makes the program crash instead. Maybe Windows has no
-		// other handler registered? In any event, ignore it.
-		return -1;
-	}
-
 	// Only handle exception if executing instructions in Go binary
 	// (not Windows library code). 
 	if(r->Eip < (uint32)runtime·text || (uint32)runtime·etext < r->Eip)

src/runtime/os_windows_amd64.c

@@ -32,8 +32,6 @@ runtime·dumpregs(Context *r)
 	runtime·printf("gs      %X\n", (uint64)r->SegGs);
 }
 
-#define DBG_PRINTEXCEPTION_C 0x40010006
-
 // Called by sigtramp from Windows VEH handler.
 // Return value signals whether the exception has been handled (-1)
 // or should be made available to other handlers in the chain (0).
@@ -44,19 +42,6 @@ runtime·sighandler(ExceptionRecord *info, Context *r, G *gp)
 	uintptr *sp;
 	extern byte runtime·text[], runtime·etext[];
 
-	if(info->ExceptionCode == DBG_PRINTEXCEPTION_C) {
-		// This exception is intended to be caught by debuggers.
-		// There is a not-very-informational message like
-		// "Invalid parameter passed to C runtime function"
-		// sitting at info->ExceptionInformation[0] (a wchar_t*),
-		// with length info->ExceptionInformation[1].
-		// The default behavior is to ignore this exception,
-		// but somehow returning 0 here (meaning keep going)
-		// makes the program crash instead. Maybe Windows has no
-		// other handler registered? In any event, ignore it.
-		return -1;
-	}
-
 	// Only handle exception if executing instructions in Go binary
 	// (not Windows library code). 
 	if(r->Rip < (uint64)runtime·text || (uint64)runtime·etext < r->Rip)

src/runtime/syscall_windows_test.go

@@ -488,3 +488,9 @@ func TestRegisterClass(t *testing.T) {
 		t.Fatalf("UnregisterClass failed: %v", err)
 	}
 }
+
+func TestOutputDebugString(t *testing.T) {
+	d := GetDLL(t, "kernel32.dll")
+	p := syscall.StringToUTF16Ptr("testing OutputDebugString")
+	d.Proc("OutputDebugStringW").Call(uintptr(unsafe.Pointer(p)))
+}