X wcfs: hook in δFtail.Forget

wcfs/wcfs.go

@@ -308,6 +308,9 @@ package main
 //    4.4) processing ZODB invalidations and serving file reads (see 7) are
 //      organized to be mutually exclusive.
 //
+//    5.5) similarly, processing ZODB invalidations and setting up watches (see
+//      7.2) are organized to be mutually exclusive.
+//
 // 5) after OS file cache was invalidated, we resync zhead to new database
 //    view corresponding to tid.
 //
@@ -481,7 +484,7 @@ import (
 	"sync"
 	"sync/atomic"
 	"syscall"
-//	"time"
+	"time"
 
 	log "github.com/golang/glog"
 
@@ -952,7 +955,19 @@ retry:
 		}
 	}
 
-	// XXX δFtail.ForgetPast(...)
+	// shrink δFtail not to grow indefinitely.
+	// cover history for at least 1 minute, but including all watches.
+	// No need to lock anything because we are holding zheadMu and
+	// setupWatch too runs with zheadMu locked.
+	revCut := zodb.TidFromTime(zhead.At().Time().Add(-1*time.Minute))
+	for wlink := range head.wlinkTab {
+		for _, w := range wlink.byfile {
+			if w.at < revCut {
+				revCut = w.at
+			}
+		}
+	}
+	bfdir.δFtail.ForgetPast(revCut)
 
 	// notify zhead.At waiters
 	for hw := range head.hwait {