.

wcfs/internal/zdata/δftail.go

@@ -158,6 +158,8 @@ type ΔFtail struct {
 type _ΔFileTail struct {
 	root zodb.Oid      // .blktab as of @head
 	vδE  []_ΔFileEpoch // epochs (changes to ZBigFile object itself)  ; nil if not yet rebuilt
+
+	rebuildJob *_RebuildJob // !nil if vδE rebuild is currently in-progress
 }
 
 // _ΔFileEpoch represent a change to ZBigFile object.
@@ -177,6 +179,12 @@ type zblkTrack struct {
 	inroot map[zodb.Oid]setI64  // {} root -> {}blk
 }
 
+// _RebuildJob represents currently in-progress vδE rebuilding job.
+type _RebuildJob struct {
+	ready chan struct{} // closed when job completes
+	err   error
+}
+
 
 // ΔF represents a change in files space.
 type ΔF struct {
@@ -299,10 +307,11 @@ func (δFtail *ΔFtail) rebuildAll() (err error) {
 	δZtail := δBtail.ΔZtail()
 	db     := δBtail.DB()
 	for foid := range δFtail.trackNew {
-		δFtail.trackNew.Del(foid)	// XXX -> to after vδEBuild
+		δFtail.trackNew.Del(foid)
 		δftail := δFtail.byFile[foid]
 		δftail.vδE, err = vδEBuild(foid, δZtail, db)
 		if err != nil {
+			δFtail.trackNew.Add(foid)
 			return err
 		}
 	}
@@ -315,23 +324,54 @@ func (δFtail *ΔFtail) rebuildAll() (err error) {
 // it returns corresponding δftail for convenience. XXX
 // the only case when vδE actually needs to be rebuilt is when the file just started to be tracked.
 //
-// XXX naming -> vδEForFile ?
+// XXX naming -> vδEForFile ?  vδEBuildIfNeeded?
 func (δFtail *ΔFtail) rebuild1IfNeeded(foid zodb.Oid) (vδE []_ΔFileEpoch, headRoot zodb.Oid, err error) {
 	δFtail.mu.Lock() // TODO verify that there is no in-progress writers
 	defer δFtail.mu.Unlock()
 
 	δftail := δFtail.byFile[foid]
-	if δftail.vδE == nil {
-		δFtail.trackNew.Del(foid)
-		δBtail := δFtail.δBtail
-		// XXX indicate in-progress rebuild via job
+	root := δftail.root
+	vδE = δftail.vδE
+	if vδE != nil {
+		return vδE, root, nil
+	}
+
+	// vδE needs to be built
+	job := δftail.rebuildJob
+
+	// rebuild is currently in-progress -> wait for corrsponding job to complete
+	if job != nil {
 		δFtail.mu.Unlock()
-		vδE, err = vδEBuild(foid, δBtail.ΔZtail(), δBtail.DB())
-		δFtail.mu.Lock()
+		<-job.ready
+		if job.err == nil {
+			δFtail.mu.Lock()
+			vδE = δftail.vδE
+		}
+		return vδE, root, job.err
+	}
+
+	// we become responsible to build vδE
+	// do the build without the lock
+	job = &_RebuildJob{ready: make(chan struct{})}
+	δftail.rebuildJob = job
+	δFtail.trackNew.Del(foid)
+	δBtail := δFtail.δBtail
+
+	δFtail.mu.Unlock()
+	vδE, err = vδEBuild(foid, δBtail.ΔZtail(), δBtail.DB())
+	δFtail.mu.Lock()
+
+	if err == nil {
 		δftail.vδE = vδE
+	} else {
+		δFtail.trackNew.Add(foid)
 	}
 
-	return δftail.vδE, δftail.root, err
+	δftail.rebuildJob = nil
+	job.err = err
+	close(job.ready)
+
+	return vδE, root, err
 }
 
 // Update updates δFtail given raw ZODB changes.