os: make Open() O_APPEND flag work on windows

Fixes #1124.
Implementation is suggested by Skip.
Test is suggested by PeterGo.

R=r, PeterGo, rsc
CC=golang-dev, skip.tavakkolian
https://golang.org/cl/2256041

src/pkg/os/os_test.go

@@ -742,3 +742,33 @@ func TestWriteAt(t *testing.T) {
 		t.Fatalf("after write: have %q want %q", string(b), "hello, WORLD\n")
 	}
 }
+
+func writeFile(t *testing.T, fname string, flag int, text string) string {
+	f, err := Open(fname, flag, 0666)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	n, err := io.WriteString(f, text)
+	if err != nil {
+		t.Fatalf("WriteString: %d, %v", n, err)
+	}
+	f.Close()
+	data, err := ioutil.ReadFile(fname)
+	if err != nil {
+		t.Fatalf("ReadFile: %v", err)
+	}
+	return string(data)
+}
+
+func TestAppend(t *testing.T) {
+	const f = "append.txt"
+	defer Remove(f)
+	s := writeFile(t, f, O_CREAT|O_TRUNC|O_RDWR, "new")
+	if s != "new" {
+		t.Fatalf("writeFile: have %q want %q", s, "new")
+	}
+	s = writeFile(t, f, O_APPEND|O_RDWR, "|append")
+	if s != "new|append" {
+		t.Fatalf("writeFile: have %q want %q", s, "new|append")
+	}
+}

src/pkg/syscall/syscall_windows.go

@@ -184,6 +184,10 @@ func Open(path string, mode int, perm uint32) (fd int, errno int) {
 	if mode&O_CREAT != 0 {
 		access |= GENERIC_WRITE
 	}
+	if mode&O_APPEND != 0 {
+		access &^= GENERIC_WRITE
+		access |= FILE_APPEND_DATA
+	}
 	sharemode := uint32(FILE_SHARE_READ | FILE_SHARE_WRITE)
 	var createmode uint32
 	switch {

src/pkg/syscall/ztypes_windows_386.go

@@ -56,6 +56,8 @@ const (
 	GENERIC_EXECUTE = 0x20000000
 	GENERIC_ALL     = 0x10000000
 
+	FILE_APPEND_DATA = 0x00000004
+
 	FILE_SHARE_READ          = 0x00000001
 	FILE_SHARE_WRITE         = 0x00000002
 	FILE_SHARE_DELETE        = 0x00000004