os: use small writes during console io

Fixes #3767

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/6523043

src/pkg/os/file_windows.go

@@ -258,8 +258,18 @@ func (f *File) writeConsole(b []byte) (n int, err error) {
 		f.lastbits = make([]byte, len(b))
 		copy(f.lastbits, b)
 	}
-	if len(runes) > 0 {
+	// syscall.WriteConsole seems to fail, if given large buffer.
-		uint16s := utf16.Encode(runes)
+	// So limit the buffer to 16000 characters. This number was
+	// discovered by experimenting with syscall.WriteConsole.
+	const maxWrite = 16000
+	for len(runes) > 0 {
+		m := len(runes)
+		if m > maxWrite {
+			m = maxWrite
+		}
+		chunk := runes[:m]
+		runes = runes[m:]
+		uint16s := utf16.Encode(chunk)
 		for len(uint16s) > 0 {
 			var written uint32
 			err = syscall.WriteConsole(f.fd, &uint16s[0], uint32(len(uint16s)), &written, nil)

src/pkg/os/os_test.go

@@ -6,6 +6,7 @@ package os_test
 
 import (
 	"bytes"
+	"flag"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -1066,3 +1067,31 @@ func TestDevNullFile(t *testing.T) {
 		t.Fatalf("wrong file size have %d want 0", fi.Size())
 	}
 }
+
+var testLargeWrite = flag.Bool("large_write", false, "run TestLargeWriteToConsole test that floods console with output")
+
+func TestLargeWriteToConsole(t *testing.T) {
+	if !*testLargeWrite {
+		t.Logf("skipping console-flooding test; enable with -large_write")
+		return
+	}
+	b := make([]byte, 32000)
+	for i := range b {
+		b[i] = '.'
+	}
+	b[len(b)-1] = '\n'
+	n, err := Stdout.Write(b)
+	if err != nil {
+		t.Fatalf("Write to os.Stdout failed: %v", err)
+	}
+	if n != len(b) {
+		t.Errorf("Write to os.Stdout should return %d; got %d", len(b), n)
+	}
+	n, err = Stderr.Write(b)
+	if err != nil {
+		t.Fatalf("Write to os.Stderr failed: %v", err)
+	}
+	if n != len(b) {
+		t.Errorf("Write to os.Stderr should return %d; got %d", len(b), n)
+	}
+}