Release lock during WriteStream for inmemory driver

While reading from the input in WriteStream, the inmemory driver can deadlock
if the reader is from the same instance. To fix this, the write lock is
released before reading into a local buffer. The lock is re-acquired to
finish the actual write.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

storagedriver/inmemory/driver.go

@@ -133,6 +133,11 @@ func (d *Driver) WriteStream(path string, offset int64, reader io.Reader) (nn in
 		return 0, fmt.Errorf("not a file")
 	}
 
+	// Unlock while we are reading from the source, in case we are reading
+	// from the same mfs instance. This can be fixed by a more granular
+	// locking model.
+	d.mutex.Unlock()
+	d.mutex.RLock() // Take the readlock to block other writers.
 	var buf bytes.Buffer
 
 	nn, err = buf.ReadFrom(reader)
@@ -142,9 +147,13 @@ func (d *Driver) WriteStream(path string, offset int64, reader io.Reader) (nn in
 		// backend. What is the correct return value? Really, the caller needs
 		// to know that the reader has been advanced and reattempting the
 		// operation is incorrect.
+		d.mutex.RUnlock()
+		d.mutex.Lock()
 		return nn, err
 	}
 
+	d.mutex.RUnlock()
+	d.mutex.Lock()
 	f.WriteAt(buf.Bytes(), offset)
 	return nn, err
 }