Use done channel to avoid goroutine leak

This deals with a memory leak, caused by goroutines, experienced when using the
s3 driver. Unfortunately, this section of the code leaks goroutines like a
sieve. There is probably some refactoring that could be done to avoid this but
instead, we have a done channel that will cause waiting goroutines to exit.

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

docs/storage/driver/s3/s3.go

@@ -310,6 +310,7 @@ func (d *driver) WriteStream(path string, offset int64, reader io.Reader) (total
 	var putErrChan chan error
 	parts := []s3.Part{}
 	var part s3.Part
+	done := make(chan struct{}) // stopgap to free up waiting goroutines
 
 	multi, err := d.Bucket.InitMulti(d.s3Path(path), d.getContentType(), getPermissions(), d.getOptions())
 	if err != nil {
@@ -344,6 +345,7 @@ func (d *driver) WriteStream(path string, offset int64, reader io.Reader) (total
 		}
 
 		d.putbuf(buf) // needs to be here to pick up new buf value
+		close(done)   // free up any waiting goroutines
 	}()
 
 	// Fills from 0 to total from current
@@ -407,7 +409,11 @@ func (d *driver) WriteStream(path string, offset int64, reader io.Reader) (total
 			// the underlying s3 library should handle it, it doesn't seem to
 			// be part of the shouldRetry function (see AdRoll/goamz/s3).
 			defer func() {
-				putErrChan <- nil // for some reason, we do this no matter what.
+				select {
+				case putErrChan <- nil: // for some reason, we do this no matter what.
+				case <-done:
+					return // ensure we don't leak the goroutine
+				}
 			}()
 
 			if bytesRead <= 0 {
@@ -449,7 +455,11 @@ func (d *driver) WriteStream(path string, offset int64, reader io.Reader) (total
 
 			if err != nil {
 				logrus.Errorf("error putting part, aborting: %v", err)
-				putErrChan <- err
+				select {
+				case putErrChan <- err:
+				case <-done:
+					return // don't leak the goroutine
+				}
 			}
 
 			// parts and partNumber are safe, because this function is the