pinmfs: mitigate slow mfs writes when it triggers

This mitigates slow mfs writes when the pinmfs daemon calls mfs.RootNode()

When writing lots of files to MFS, this call triggers a mfs directory cache
sync operations. The cache grows forever and is unbounded. The more files
added to a directory, the longer it takes. In the meantime, writing to mfs is
locked.

The pinmfs, even when no remote pinning services are configured, will trigger
this issue. When RootNode() takes more than 30 seconds, the issue will be triggered
continuously causing a write-deadlock onto MFS.

This commit does not fix the fact that if you write 1M items into an MFS
directory, the first time that the directory is traversed it will still have
to sync those 1M items. It does at least prevent writes from stalling after
about ~6000 items.

cmd/ipfs/kubo/pinmfs.go

@@ -90,34 +90,46 @@ func pinMFSOnChange(cctx pinMFSContext, configPollInterval time.Duration, node p
 		case <-cctx.Context().Done():
 			return
 		case <-tmo.C:
-			tmo.Reset(configPollInterval)
-		}
-
-		// reread the config, which may have changed in the meantime
-		cfg, err := cctx.GetConfig()
-		if err != nil {
-			mfslog.Errorf("pinning reading config (%v)", err)
-			continue
-		}
-		mfslog.Debugf("pinning loop is awake, %d remote services", len(cfg.Pinning.RemoteServices))
+			// reread the config, which may have changed in the meantime
+			cfg, err := cctx.GetConfig()
+			if err != nil {
+				mfslog.Errorf("pinning reading config (%v)", err)
+				continue
+			}
+			mfslog.Debugf("pinning loop is awake, %d remote services", len(cfg.Pinning.RemoteServices))
 
-		// get the most recent MFS root cid
-		rootNode, err := node.RootNode()
-		if err != nil {
-			mfslog.Errorf("pinning reading MFS root (%v)", err)
-			continue
+			// pin to all remote services in parallel
+			pinAllMFS(cctx.Context(), node, cfg, lastPins)
 		}
-
-		// pin to all remote services in parallel
-		pinAllMFS(cctx.Context(), node, cfg, rootNode.Cid(), lastPins)
+		// pinAllMFS may take long. Reset interval only when we are done doing it
+		// so that we are not pinning constantly.
+		tmo.Reset(configPollInterval)
 	}
 }
 
 // pinAllMFS pins on all remote services in parallel to overcome DoS attacks.
-func pinAllMFS(ctx context.Context, node pinMFSNode, cfg *config.Config, rootCid cid.Cid, lastPins map[string]lastPin) {
+func pinAllMFS(ctx context.Context, node pinMFSNode, cfg *config.Config, lastPins map[string]lastPin) {
 	ch := make(chan lastPin)
 	var started int
 
+	// Bail out to mitigate issue below when not needing to do anything.
+	if len(cfg.Pinning.RemoteServices) == 0 {
+		return
+	}
+
+	// get the most recent MFS root cid.
+	// Warning! This can be super expensive.
+	// See https://github.com/ipfs/boxo/pull/751
+	// and https://github.com/ipfs/kubo/issues/8694
+	// Reading an MFS-directory nodes can take minutes due to
+	// ever growing cache being synced to unixfs.
+	rootNode, err := node.RootNode()
+	if err != nil {
+		mfslog.Errorf("pinning reading MFS root (%v)", err)
+		return
+	}
+	rootCid := rootNode.Cid()
+
 	for svcName, svcConfig := range cfg.Pinning.RemoteServices {
 		if ctx.Err() != nil {
 			break