compress/flate: make compression level 0 consistent

Tests for determinism was not working as intended since io.Copybuffer
uses the io.WriterTo if available.

This exposed that level 0 (no compression) changed output
based on the number of writes and buffers given to the
writer.

Previously, Write would emit a new raw block (BTYPE=00) for
every non-empty call to Write.

This CL fixes it such that a raw block is only emitted upon
the following conditions:
 	* A full window is obtained (every 65535 bytes)
 	* Flush is called
 	* Close is called

Change-Id: I807f866d97e2db7820f11febab30a96266a6cbf1
Reviewed-on: https://go-review.googlesource.com/31174
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/compress/flate/deflate.go

@@ -521,10 +521,10 @@ func (d *compressor) fillStore(b []byte) int {
 }
 
 func (d *compressor) store() {
-	if d.windowEnd > 0 {
+	if d.windowEnd > 0 && (d.windowEnd == maxStoreBlockSize || d.sync) {
 		d.err = d.writeStoredBlock(d.window[:d.windowEnd])
+		d.windowEnd = 0
 	}
-	d.windowEnd = 0
 }
 
 // storeHuff compresses and stores the currently added data

src/compress/flate/writer_test.go

@@ -75,7 +75,7 @@ func TestWriteError(t *testing.T) {
 			if err != nil {
 				t.Fatalf("NewWriter: level %d: %v", l, err)
 			}
-			n, err := io.CopyBuffer(w, bytes.NewBuffer(in), copyBuffer)
+			n, err := io.CopyBuffer(w, struct{ io.Reader }{bytes.NewBuffer(in)}, copyBuffer)
 			if err == nil {
 				t.Fatalf("Level %d: Expected an error, writer was %#v", l, ew)
 			}
@@ -142,7 +142,7 @@ func testDeterministic(i int, t *testing.T) {
 	}
 	// Use a very small prime sized buffer.
 	cbuf := make([]byte, 787)
-	_, err = io.CopyBuffer(w, br, cbuf)
+	_, err = io.CopyBuffer(w, struct{ io.Reader }{br}, cbuf)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -157,7 +157,7 @@ func testDeterministic(i int, t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	_, err = io.CopyBuffer(w2, br2, cbuf)
+	_, err = io.CopyBuffer(w2, struct{ io.Reader }{br2}, cbuf)
 	if err != nil {
 		t.Fatal(err)
 	}