git: fix goroutine block while pushing a remote

remote.go

@@ -1020,7 +1020,12 @@ func pushHashes(
 	if err != nil {
 		return nil, err
 	}
-	done := make(chan error)
+
+	// Set buffer size to 1 so the error message can be written when
+	// ReceivePack fails. Otherwise the goroutine will be blocked writing
+	// to the channel.
+	done := make(chan error, 1)
+
 	go func() {
 		e := packfile.NewEncoder(wr, s, useRefDeltas)
 		if _, err := e.Encode(hs, config.Pack.Window); err != nil {
@@ -1033,6 +1038,8 @@ func pushHashes(
 
 	rs, err := sess.ReceivePack(ctx, req)
 	if err != nil {
+		// close the pipe to unlock encode write
+		_ = rd.Close()
 		return nil, err
 	}
 

remote_test.go

@@ -6,6 +6,8 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"runtime"
+	"time"
 
 	"gopkg.in/src-d/go-git.v4/config"
 	"gopkg.in/src-d/go-git.v4/plumbing"
@@ -448,10 +450,17 @@ func (s *RemoteSuite) TestPushContext(c *C) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 
+	numGoroutines := runtime.NumGoroutine()
+
 	err = r.PushContext(ctx, &PushOptions{
 		RefSpecs: []config.RefSpec{"refs/tags/*:refs/tags/*"},
 	})
 	c.Assert(err, NotNil)
+
+	// let the goroutine from pushHashes finish and check that the number of
+	// goroutines is the same as before
+	time.Sleep(100 * time.Millisecond)
+	c.Assert(runtime.NumGoroutine(), Equals, numGoroutines)
 }
 
 func (s *RemoteSuite) TestPushTags(c *C) {