Add File.ReadFrom test on a SectionReader

This tests ReadFrom's determining concurrency from the estimated input
size. go test -cover -integration, before:

coverage: 77.3% of statements

After:

coverage: 79.2% of statements

client_integration_test.go

@@ -164,7 +164,7 @@ func netPipe(t testing.TB) (io.ReadWriteCloser, io.ReadWriteCloser) {
 	return c1, r.Conn
 }
 
-func testClientGoSvr(t testing.TB, readonly bool, delay time.Duration) (*Client, *exec.Cmd) {
+func testClientGoSvr(t testing.TB, readonly bool, delay time.Duration, opts ...ClientOption) (*Client, *exec.Cmd) {
 	c1, c2 := netPipe(t)
 
 	options := []ServerOption{WithDebug(os.Stderr)}
@@ -183,7 +183,7 @@ func testClientGoSvr(t testing.TB, readonly bool, delay time.Duration) (*Client,
 		wr = newDelayedWriter(wr, delay)
 	}
 
-	client, err := NewClientPipe(c2, wr)
+	client, err := NewClientPipe(c2, wr, opts...)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -194,13 +194,13 @@ func testClientGoSvr(t testing.TB, readonly bool, delay time.Duration) (*Client,
 
 // testClient returns a *Client connected to a locally running sftp-server
 // the *exec.Cmd returned must be defer Wait'd.
-func testClient(t testing.TB, readonly bool, delay time.Duration) (*Client, *exec.Cmd) {
+func testClient(t testing.TB, readonly bool, delay time.Duration, opts ...ClientOption) (*Client, *exec.Cmd) {
 	if !*testIntegration {
 		t.Skip("skipping integration test")
 	}
 
 	if *testServerImpl {
-		return testClientGoSvr(t, readonly, delay)
+		return testClientGoSvr(t, readonly, delay, opts...)
 	}
 
 	cmd := exec.Command(*testSftp, "-e", "-R", "-l", debuglevel) // log to stderr, read only
@@ -228,7 +228,7 @@ func testClient(t testing.TB, readonly bool, delay time.Duration) (*Client, *exe
 		t.Skipf("could not start sftp-server process: %v", err)
 	}
 
-	sftp, err := NewClientPipe(pr, pw)
+	sftp, err := NewClientPipe(pr, pw, opts...)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1489,6 +1489,49 @@ func TestClientReadFrom(t *testing.T) {
 	}
 }
 
+func TestClientReadFromSectionReader(t *testing.T) {
+	const (
+		packetSize = 1024
+		bufsize    = 4 * packetSize
+		skip       = 100
+		filesize   = bufsize - skip
+	)
+
+	sftp, cmd := testClient(t, READWRITE, NODELAY, MaxPacketChecked(packetSize), UseConcurrentWrites(true))
+	defer cmd.Wait()
+	defer sftp.Close()
+
+	d, err := ioutil.TempDir("", "sftptest-readfrom-sectionreader")
+	if err != nil {
+		t.Fatal("cannot create temp dir:", err)
+	}
+	defer os.RemoveAll(d)
+
+	buf := bytes.NewReader(make([]byte, bufsize))
+	r := io.NewSectionReader(buf, 0, bufsize)
+
+	// Skip some bytes so that r.Size is no longer accurate.
+	// ReadFrom uses it to determine concurrency,
+	// but should not rely on it to determine how many bytes to send.
+	r.Seek(skip, io.SeekStart)
+
+	f := path.Join(d, "writeTest")
+	w, err := sftp.Create(f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer w.Close()
+
+	n, err := w.ReadFrom(r)
+	assert.EqualValues(t, filesize, n)
+
+	fi, err := os.Stat(f)
+	if err != nil {
+		t.Fatal("unexpected error:", err)
+	}
+	assert.EqualValues(t, filesize, fi.Size())
+}
+
 // Issue #145 in github
 // Deadlock in ReadFrom when network drops after 1 good packet.
 // Deadlock would occur anytime desiredInFlight-inFlight==2 and 2 errors