net: update zoneCache on cache misses to cover appearing interfaces

performance differences are in measurement noise as per benchcmp:

benchmark                            old ns/op     new ns/op     delta
BenchmarkUDP6LinkLocalUnicast-12     5012          5009          -0.06%

Fixes #28535

Change-Id: Id022e2ed089ce8388a2398e755848ec94e77e653
Reviewed-on: https://go-review.googlesource.com/c/146941
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
Reviewed-by: Mikio Hara <mikioh.public.networking@gmail.com>

1 files changed, 30 insertions, 10 deletions

diff --git a/src/net/interface.go b/src/net/interface.go
index 375a4568e3..46b0400f2f 100644
--- a/src/net/interface.go
+++ b/src/net/interface.go
@@ -102,7 +102,7 @@ func Interfaces() ([]Interface, error) {
 		return nil, &OpError{Op: "route", Net: "ip+net", Source: nil, Addr: nil, Err: err}
 	}
 	if len(ift) != 0 {
-		zoneCache.update(ift)
+		zoneCache.update(ift, false)
 	}
 	return ift, nil
 }
@@ -159,7 +159,7 @@ func InterfaceByName(name string) (*Interface, error) {
 		return nil, &OpError{Op: "route", Net: "ip+net", Source: nil, Addr: nil, Err: err}
 	}
 	if len(ift) != 0 {
-		zoneCache.update(ift)
+		zoneCache.update(ift, false)
 	}
 	for _, ifi := range ift {
 		if name == ifi.Name {
@@ -187,18 +187,21 @@ var zoneCache = ipv6ZoneCache{
 	toName:  make(map[int]string),
 }
 
-func (zc *ipv6ZoneCache) update(ift []Interface) {
+// update refreshes the network interface information if the cache was last
+// updated more than 1 minute ago, or if force is set. It returns whether the
+// cache was updated.
+func (zc *ipv6ZoneCache) update(ift []Interface, force bool) (updated bool) {
 	zc.Lock()
 	defer zc.Unlock()
 	now := time.Now()
-	if zc.lastFetched.After(now.Add(-60 * time.Second)) {
-		return
+	if !force && zc.lastFetched.After(now.Add(-60*time.Second)) {
+		return false
 	}
 	zc.lastFetched = now
 	if len(ift) == 0 {
 		var err error
 		if ift, err = interfaceTable(0); err != nil {
-			return
+			return false
 		}
 	}
 	zc.toIndex = make(map[string]int, len(ift))
@@ -209,16 +212,25 @@ func (zc *ipv6ZoneCache) update(ift []Interface) {
 			zc.toName[ifi.Index] = ifi.Name
 		}
 	}
+	return true
 }
 
 func (zc *ipv6ZoneCache) name(index int) string {
 	if index == 0 {
 		return ""
 	}
-	zoneCache.update(nil)
+	updated := zoneCache.update(nil, false)
 	zoneCache.RLock()
-	defer zoneCache.RUnlock()
 	name, ok := zoneCache.toName[index]
+	zoneCache.RUnlock()
+	if !ok {
+		if !updated {
+			zoneCache.update(nil, true)
+			zoneCache.RLock()
+			name, ok = zoneCache.toName[index]
+			zoneCache.RUnlock()
+		}
+	}
 	if !ok {
 		name = uitoa(uint(index))
 	}
@@ -229,10 +241,18 @@ func (zc *ipv6ZoneCache) index(name string) int {
 	if name == "" {
 		return 0
 	}
-	zoneCache.update(nil)
+	updated := zoneCache.update(nil, false)
 	zoneCache.RLock()
-	defer zoneCache.RUnlock()
 	index, ok := zoneCache.toIndex[name]
+	zoneCache.RUnlock()
+	if !ok {
+		if !updated {
+			zoneCache.update(nil, true)
+			zoneCache.RLock()
+			index, ok = zoneCache.toIndex[name]
+			zoneCache.RUnlock()
+		}
+	}
 	if !ok {
 		index, _, _ = dtoi(name)
 	}