.

go/zodb/storage/demo/demo.go

@@ -30,6 +30,7 @@ import (
 	"io"
 	"net/url"
 	"regexp"
+	"sync"
 
 	"lab.nexedi.com/kirr/go123/mem"
 	"lab.nexedi.com/kirr/go123/xerr"
@@ -54,6 +55,10 @@ type Storage struct {
 
 	watchWG     *xsync.WorkGroup
 	watchCancel func()
+
+	downOnce sync.Once
+	down     chan struct{} // ready when storage is down
+	downErr  error         // reason for shutdown
 }
 
 // baseMutatedError is reported when Storage.base is detected to change.
@@ -78,6 +83,7 @@ func (e *baseError) Error() string {
 func (e *baseError) Cause() error  { return e.err }
 func (e *baseError) Unwrap() error { return e.err }
 
+
 // watcher detects base mutation and proxies δ events to user watchq.
 // it runs as separate goroutine.
 func (d *Storage) watcher(ctx context.Context) error {
@@ -111,11 +117,11 @@ func (d *Storage) watcher(ctx context.Context) error {
 				edown = fmt.Errorf("base: unexpected event %T", event)
 			}
 
+			d.shutdown(edown)
 			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
@@ -132,6 +138,15 @@ func (d *Storage) watcher(ctx context.Context) error {
 	}
 }
 
+// shutdown marks Storage as no longer being operational due to reason.
+func (d *Storage) shutdown(reason error) {
+	d.downOnce.Do(func() {
+		d.downErr = reason
+		close(d.down)
+	})
+}
+
+var errClosed = errors.New("storage is closed")
 
 // Close implements zodb.IStorageDriver .
 func (d *Storage) Close() (err error) {
@@ -141,6 +156,7 @@ func (d *Storage) Close() (err error) {
 		}
 	}()
 
+	d.shutdown(errClosed)
 	errδ    := d.δ.Close()
 	errBase := d.base.Close()
 
@@ -200,38 +216,51 @@ func (d *Storage) Load(ctx context.Context, xid zodb.Xid) (_ *mem.Buf, _ zodb.Ti
 		}
 	}()
 
-	data, serial, err := d.δ.Load(ctx, xid)
-	if err == nil {
-		// object data is present in δ
-		return data, serial, nil
+	if ready(d.down) {
+		return nil, zodb.InvalidTid, d.downErr
 	}
 
-	useBase := false
-	inδ := false
 	var eNoData   *zodb.NoDataError
 	var eNoObject *zodb.NoObjectError
-	switch {
-	case errors.As(err, &eNoData):
-		if eNoData.DeletedAt != 0 {
-			// object deleted in δ -> whiteout
-			return data, serial, eNoData
-		} else {
-			// object present in δ but not yet created as of xid.at
+
+	inδ := false
+	if xid.At > d.baseAt0 {
+		data, serial, err := d.δ.Load(ctx, xid)
+		if err == nil {
+			// object data is present in δ
+			return data, serial, nil
+		}
+
+		useBase := false
+		switch {
+		case errors.As(err, &eNoData):
+			if eNoData.DeletedAt != 0 {
+				// object deleted in δ -> whiteout
+				return data, serial, eNoData
+			} else {
+				// object present in δ but not yet created as of xid.at
+				useBase = true
+				inδ = true
+			}
+
+		case errors.As(err, &eNoObject):
+			// object not created in δ
 			useBase = true
-			inδ = true
 		}
 
-	case errors.As(err, &eNoObject):
-		// object not created in δ
-		useBase = true
+		if !useBase {
+			return data, serial, err
+		}
 	}
 
-	if !useBase {
-		return data, serial, err
+	// cap .at in xid to .baseAt0 (we convert it back on error return, and
+	// it makes more robust wrt simultaneous base mutation).
+	xidBase := xid
+	if xid.At > d.baseAt0 {
+		xidBase.At = d.baseAt0
 	}
 
-	// XXX cap .at in xid to .baseAt0 ? (and convert back on error return)
-	data, serial, err = d.base.Load(ctx, xid)
+	data, serial, err := d.base.Load(ctx, xidBase)
 	if err == nil {
 		return data, serial, nil
 	}
@@ -350,6 +379,8 @@ func openByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (_ zodb
 		baseWatchq: baseWatchq,
 		δWatchq:    δWatchq,
 		watchq:     opt.Watchq,
+
+		down: make(chan struct{}),
 	}
 
 	// spawn watcher to listen on baseWatchq and shutdown storage if base changes.
@@ -364,3 +395,16 @@ func openByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (_ zodb
 func init() {
 	zodb.RegisterDriver("demo", openByURL)
 }
+
+
+// misc
+
+// ready returns whether c is ready.
+func ready(c <-chan struct{}) bool {
+	select {
+	case <-c:
+		return true
+	default:
+		return false
+	}
+}

go/zodb/storage/demo/demo_test.go

@@ -240,7 +240,7 @@ func TestWatchLoad_vs_BaseMutate(t *testing.T) {
 			t.Fatalf("after base mutate: load: unexpected error:\nhave: %s\nwant: %s",
 				err, errOk)
 		}
-		if !(data == nil && serial == 0) {
+		if !(data == nil && serial == zodb.InvalidTid) {
 			t.Fatalf("after base mutate: load: unexpected data=%v  serial=%v", data, serial)
 		}
 	})