ssh: defer channel window adjustment

Sending a window adjustment after every read is unnecessarily chatty,
especially with a series of small reads like with TTY interactions.

Copy OpenSSH's logic for deferring these, which seemingly hasn't changed
since 2007. Note that since channelWindowSize and c.maxIncomingPayload
are currently constants here, the two checks could be combined into a
single check for c.myWindow < 2 MiB - 96 KiB (with the current values
of the constants).

Fixes golang/go#57424.

Change-Id: Ifcef5be76fcc3f0b1a6dc396096bed9c50d64f21
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/459915
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Nicola Murino <nicola.murino@gmail.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Commit-Queue: Nicola Murino <nicola.murino@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

ssh/mempipe_test.go

@@ -13,9 +13,10 @@ import (
 // An in-memory packetConn. It is safe to call Close and writePacket
 // from different goroutines.
 type memTransport struct {
-	eof     bool
-	pending [][]byte
-	write   *memTransport
+	eof        bool
+	pending    [][]byte
+	write      *memTransport
+	writeCount uint64
 	sync.Mutex
 	*sync.Cond
 }
@@ -63,9 +64,16 @@ func (t *memTransport) writePacket(p []byte) error {
 	copy(c, p)
 	t.write.pending = append(t.write.pending, c)
 	t.write.Cond.Signal()
+	t.writeCount++
 	return nil
 }
 
+func (t *memTransport) getWriteCount() uint64 {
+	t.write.Lock()
+	defer t.write.Unlock()
+	return t.writeCount
+}
+
 func memPipe() (a, b packetConn) {
 	t1 := memTransport{}
 	t2 := memTransport{}
@@ -81,6 +89,9 @@ func TestMemPipe(t *testing.T) {
 	if err := a.writePacket([]byte{42}); err != nil {
 		t.Fatalf("writePacket: %v", err)
 	}
+	if wc := a.(*memTransport).getWriteCount(); wc != 1 {
+		t.Fatalf("got %v, want 1", wc)
+	}
 	if err := a.Close(); err != nil {
 		t.Fatal("Close: ", err)
 	}
@@ -95,6 +106,9 @@ func TestMemPipe(t *testing.T) {
 	if err != io.EOF {
 		t.Fatalf("got %v, %v, want EOF", p, err)
 	}
+	if wc := b.(*memTransport).getWriteCount(); wc != 0 {
+		t.Fatalf("got %v, want 0", wc)
+	}
 }
 
 func TestDoubleClose(t *testing.T) {