Merge pull request #613 from tonistiigi/data-race

solver: fix data race

solver/jobs.go

@@ -444,6 +444,7 @@ func (j *Job) Discard() error {
 	j.pw.Close()
 
 	for k, st := range j.list.actives {
+		st.mu.Lock()
 		if _, ok := st.jobs[j]; ok {
 			delete(st.jobs, j)
 			j.list.deleteIfUnreferenced(k, st)
@@ -451,6 +452,7 @@ func (j *Job) Discard() error {
 		if _, ok := st.allPw[j.pw]; ok {
 			delete(st.allPw, j.pw)
 		}
+		st.mu.Unlock()
 	}
 	return nil
 }