From e7d3b49a20907659e066567a17609526e55ae317 Mon Sep 17 00:00:00 2001
From: Klaus Post <klauspost@gmail.com>
Date: Tue, 15 Dec 2020 11:25:36 -0800
Subject: [PATCH] metacache: Make very small requests transient (#11109)

---
 cmd/erasure-server-sets.go                       |  2 +-
 ...e-server-sets.go => metacache-server-pool.go} | 16 ++++++++++++++--
 cmd/metacache-set.go                             | 14 ++++++++++----
 3 files changed, 25 insertions(+), 7 deletions(-)
 rename cmd/{metacache-server-sets.go => metacache-server-pool.go} (92%)

diff --git a/cmd/erasure-server-sets.go b/cmd/erasure-server-sets.go
index dcba6c9ce..740acbbf5 100644
--- a/cmd/erasure-server-sets.go
+++ b/cmd/erasure-server-sets.go
@@ -707,7 +707,7 @@ func (z *erasureServerPools) ListObjectVersions(ctx context.Context, bucket, pre
 	// more objects matching the prefix.
 	ri := logger.GetReqInfo(ctx)
 	if ri != nil && strings.Contains(ri.UserAgent, `1.0 Veeam/1.0 Backup`) && strings.HasSuffix(prefix, ".blk") {
-		opts.singleObject = true
+		opts.discardResult = true
 		opts.Transient = true
 	}
 
diff --git a/cmd/metacache-server-sets.go b/cmd/metacache-server-pool.go
similarity index 92%
rename from cmd/metacache-server-sets.go
rename to cmd/metacache-server-pool.go
index 332972435..5932d82ec 100644
--- a/cmd/metacache-server-sets.go
+++ b/cmd/metacache-server-pool.go
@@ -84,11 +84,21 @@ func (z *erasureServerPools) listPath(ctx context.Context, o listPathOptions) (e
 		o.ID = mustGetUUID()
 	}
 	o.BaseDir = baseDirFromPrefix(o.Prefix)
-	if o.singleObject {
+	if o.discardResult {
 		// Override for single object.
 		o.BaseDir = o.Prefix
 	}
 
+	// For very small recursive listings, don't same cache.
+	// Attempts to avoid expensive listings to run for a long
+	// while when clients aren't interested in results.
+	// If the client DOES resume the listing a full cache
+	// will be generated due to the marker without ID and this check failing.
+	if o.Limit < 10 && o.Marker == "" && o.Create && o.Recursive {
+		o.discardResult = true
+		o.Transient = true
+	}
+
 	var cache metacache
 	// If we don't have a list id we must ask the server if it has a cache or create a new.
 	if o.Create {
@@ -206,7 +216,9 @@ func (z *erasureServerPools) listPath(ctx context.Context, o listPathOptions) (e
 	}
 	truncated := entries.len() > o.Limit || !allAtEOF
 	entries.truncate(o.Limit)
-	entries.listID = o.ID
+	if !o.discardResult {
+		entries.listID = o.ID
+	}
 	if !truncated {
 		return entries, io.EOF
 	}
diff --git a/cmd/metacache-set.go b/cmd/metacache-set.go
index b1174e474..e4c2d11e5 100644
--- a/cmd/metacache-set.go
+++ b/cmd/metacache-set.go
@@ -95,8 +95,9 @@ type listPathOptions struct {
 	// A transient result will never be returned from the cache so knowing the list id is required.
 	Transient bool
 
-	// singleObject will assume that prefix refers to an exact single object.
-	singleObject bool
+	// discardResult will not persist the cache to storage.
+	// When the initial results are returned listing will be canceled.
+	discardResult bool
 }
 
 func init() {
@@ -545,7 +546,7 @@ func (er *erasureObjects) listPath(ctx context.Context, o listPathOptions) (entr
 	o.debugf("listPath with options: %#v\n", o)
 
 	// See if we have the listing stored.
-	if !o.Create && !o.singleObject {
+	if !o.Create && !o.discardResult {
 		entries, err := er.streamMetadataParts(ctx, o)
 		if IsErr(err, []error{
 			nil,
@@ -627,6 +628,11 @@ func (er *erasureObjects) listPath(ctx context.Context, o listPathOptions) (entr
 		close(filterCh)
 	}
 
+	// Cancel listing on return if non-saved list.
+	if o.discardResult {
+		defer cancel()
+	}
+
 	go func() {
 		defer cancel()
 		// Save continuous updates
@@ -658,7 +664,7 @@ func (er *erasureObjects) listPath(ctx context.Context, o listPathOptions) (entr
 
 		// Write results to disk.
 		bw := newMetacacheBlockWriter(cacheCh, func(b *metacacheBlock) error {
-			if o.singleObject {
+			if o.discardResult {
 				// Don't save single object listings.
 				return nil
 			}