runtime: fix empty heap dump bug on windows.

Fixes #8119.

LGTM=khr, rsc
R=alex.brainman, khr, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/93640044

src/pkg/runtime/debug/heapdump_test.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package debug
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TestWriteHeapDumpNonempty(t *testing.T) {
+	f, err := ioutil.TempFile("", "heapdumptest")
+	if err != nil {
+		t.Fatalf("TempFile failed: %v", err)
+	}
+	defer os.Remove(f.Name())
+	defer f.Close()
+	WriteHeapDump(f.Fd())
+	fi, err := f.Stat()
+	if err != nil {
+		t.Fatalf("Stat failed: %v", err)
+	}
+	const minSize = 1
+	if size := fi.Size(); size < minSize {
+		t.Fatalf("Heap dump size %d bytes, expected at least %d bytes", size, minSize)
+	}
+}

src/pkg/runtime/os_plan9.c

@@ -394,9 +394,9 @@ runtime·read(int32 fd, void *buf, int32 nbytes)
 }
 
 int32
-runtime·write(int32 fd, void *buf, int32 nbytes)
+runtime·write(uintptr fd, void *buf, int32 nbytes)
 {
-	return runtime·pwrite(fd, buf, nbytes, -1LL);
+	return runtime·pwrite((int32)fd, buf, nbytes, -1LL);
 }
 
 uintptr

src/pkg/runtime/os_solaris.c

@@ -570,7 +570,7 @@ runtime·usleep(uint32 us)
 }
 
 int32
-runtime·write(int32 fd, void* buf, int32 nbyte)
+runtime·write(uintptr fd, void* buf, int32 nbyte)
 {
 	return runtime·sysvicall6(libc·write, 3, (uintptr)fd, (uintptr)buf, (uintptr)nbyte);
 }

src/pkg/runtime/os_windows.c

@@ -166,7 +166,7 @@ runtime·exit(int32 code)
 }
 
 int32
-runtime·write(int32 fd, void *buf, int32 n)
+runtime·write(uintptr fd, void *buf, int32 n)
 {
 	void *handle;
 	uint32 written;
@@ -180,7 +180,9 @@ runtime·write(int32 fd, void *buf, int32 n)
 		handle = runtime·stdcall(runtime·GetStdHandle, 1, (uintptr)-12);
 		break;
 	default:
-		return -1;
+		// assume fd is real windows handle.
+		handle = (void*)fd;
+		break;
 	}
 	runtime·stdcall(runtime·WriteFile, 5, handle, buf, (uintptr)n, &written, (uintptr)0);
 	return written;

src/pkg/runtime/runtime.h

@@ -838,7 +838,7 @@ int32	runtime·gotraceback(bool *crash);
 void	runtime·goroutineheader(G*);
 int32	runtime·open(int8*, int32, int32);
 int32	runtime·read(int32, void*, int32);
-int32	runtime·write(int32, void*, int32);
+int32	runtime·write(uintptr, void*, int32); // use uintptr to accommodate windows.
 int32	runtime·close(int32);
 int32	runtime·mincore(void*, uintptr, byte*);
 void	runtime·jmpdefer(FuncVal*, void*);