.

go/zodb/storage/demo/demo.go

@@ -43,17 +43,67 @@ type Storage struct {
 	base zodb.IStorageDriver
 	δ    zodb.IStorageDriver
 
-	baseAt0 zodb.Tid
+	baseAt0    zodb.Tid
+	baseWatchq <-chan zodb.Event
+	δWatchq    <-chan zodb.Event // nil if demo is opened with watchq=nil
+	watchq     chan<- zodb.Event // user requested to deliver events here
+
+	watchWG     *xsync.WorkGroup
+	watchCancel func()
 }
 
 // baseMutatedError is reported when Storage.base is detected to change.
 type baseMutatedError struct {
 	baseAt0  zodb.Tid
-	baseHead zodb.Tid
+//	baseHead zodb.Tid
+	baseEvent zodb.Event
 }
 
 func (e *baseMutatedError) Error() string {
-	return fmt.Sprintf("base.head mutated from @%s to @%s", e.baseAt0, e.baseHead)
+//	return fmt.Sprintf("base.head mutated from @%s to @%s", e.baseAt0, e.baseHead)
+	return fmt.Sprintf("base mutated from @%s: %s", e.baseAt0, e.baseEvent)
+}
+
+// watcher detects base mutation and proxies δ events to user.
+// it runs as separate goroutine.
+func (d *Storage) watcher(ctx context.Context) error {
+	if d.watchq != nil {
+		defer close(d.watchq)
+	}
+
+	for {
+		select {
+		// Close requests to stop watching
+		case <-ctx.Done():
+			return ctx.Err()
+
+		// event on base -> base mutated + shutdown
+		case event, ok := <-d.baseWatchq:
+			if !ok {
+				// base closed
+				d.baseWatchq = nil
+				continue
+			}
+
+			edown := &baseMutatedError{d.baseAt0, event}
+			ev := &zodb.EventError{edown} // XXX + context
+			if d.watchq != nil {
+				d.watchq <- ev
+			}
+			// XXX d.shutdown(edown)
+			return edown
+
+		// event on δ -> proxy to user
+		case event, ok := <-d.δWatchq:
+			if !ok {
+				// δ closed
+				d.δWatchq = nil
+				continue
+			}
+
+			d.watchq <- event // !nil becase d.δWatchq != nil
+		}
+	}
 }
 
 
@@ -65,10 +115,11 @@ func (d *Storage) Close() (err error) {
 		}
 	}()
 
-	err1 := d.δ.Close()
-	err2 := d.base.Close()
-	// XXX close base watcher
-	return xerr.Merge(err1, err2)
+	errδ    := d.δ.Close()
+	errBase := d.base.Close()
+	d.watchCancel()
+	errWatch := d.watchWG.Wait()
+	return xerr.Merge(errWatch, errδ, errBase)
 }
 
 // Sync implements zodb.IStorageDriver .
@@ -92,7 +143,7 @@ func (d *Storage) Sync(ctx context.Context) (_ zodb.Tid, err error) {
 			return err
 		}
 		if baseHead != d.baseAt0 {
-			return &baseMutatedError{d.baseAt0, baseHead}
+			return &baseMutatedError{d.baseAt0, fmt.Errorf("to @%s", baseHead)}
 		}
 		return nil
 	})
@@ -214,8 +265,14 @@ func openByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (_ zodb
 		}
 	}()
 
-	// open δ    - as requested
-	δ, δAt0, err := zodb.OpenDriver(ctx, δZURL, opt)
+	// open δ    - as requested  but with events going through us
+	δopt := *opt
+	var δWatchq chan zodb.Event
+	if δopt.Watchq != nil {
+		δWatchq = make(chan zodb.Event)
+		δopt.Watchq = δWatchq
+	}
+	δ, δAt0, err := zodb.OpenDriver(ctx, δZURL, &δopt)
 	if err != nil {
 		return nil, zodb.InvalidTid, err
 	}
@@ -226,6 +283,8 @@ func openByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (_ zodb
 		}
 	}()
 
+	// XXX read watchqs while we verify? (not to deadlock)
+
 	// verify that either
 	// - δ is all empty (just created), or
 	// - all δ transactions come strictly after base.
@@ -245,21 +304,29 @@ func openByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (_ zodb
 				return nil, zodb.InvalidTid, err
 			}
 			// there is a δ transaction ∈ [δAt0, baseAt0)
-			// TODO iδ.Stop()
+			// TODO iδ.Close()
 			return nil, zodb.InvalidTid, fmt.Errorf("base overlaps with δ: base.head=%s  δ.tail=%s", baseAt0, δtxni.Tid)
 		}
 
 		at0 = δAt0
 	}
 
-	// XXX listen on baseWatchq and shutdown storage if base changes.
 	d := &Storage{
 		base: base,
 		δ   : δ,
 
-		baseAt0: baseAt0,
+		baseAt0:    baseAt0,
+		baseWatchq: baseWatchq,
+		δWatchq:    δWatchq,
+		watchq:     opt.Watchq,
 	}
 
+	// spawn watcher to listen on baseWatchq and shutdown storage if base changes.
+	ctx, cancel := context.WithCancel(context.Background())
+	d.watchWG = xsync.NewWorkGroup(ctx)
+	d.watchCancel = cancel
+	d.watchWG.Go(d.watcher)
+
 	return d, at0, nil
 }