net: check /etc/hosts for modifications every 5 seconds, not 5 minutes

But also cache the previous parsed form and don't reread if the
size and modification time are both unchanged from before.

On systems with stable /etc/hosts this should result in more stat calls
but only a single parsing of /etc/hosts.

On systems with variable /etc/hosts files (like some Docker systems)
this should result in quicker adoption of changes.

Fixes #13340.

Change-Id: Iba93b204be73d6d903cd17c58038a4fcfd0952b9
Reviewed-on: https://go-review.googlesource.com/18258
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/hosts.go

@@ -9,7 +9,7 @@ import (
 	"time"
 )
 
-const cacheMaxAge = 5 * time.Minute
+const cacheMaxAge = 5 * time.Second
 
 func parseLiteralIP(addr string) string {
 	var ip IP
@@ -44,12 +44,23 @@ var hosts struct {
 
 	expire time.Time
 	path   string
+	mtime  time.Time
+	size   int64
 }
 
 func readHosts() {
 	now := time.Now()
 	hp := testHookHostsPath
-	if len(hosts.byName) == 0 || now.After(hosts.expire) || hosts.path != hp {
+
+	if now.Before(hosts.expire) && hosts.path == hp && len(hosts.byName) > 0 {
+		return
+	}
+	mtime, size, err := stat(hp)
+	if err == nil && hosts.path == hp && hosts.mtime.Equal(mtime) && hosts.size == size {
+		hosts.expire = now.Add(cacheMaxAge)
+		return
+	}
+
 	hs := make(map[string][]string)
 	is := make(map[string][]string)
 	var file *file
@@ -83,9 +94,10 @@ func readHosts() {
 	hosts.path = hp
 	hosts.byName = hs
 	hosts.byAddr = is
+	hosts.mtime = mtime
+	hosts.size = size
 	file.close()
 }
-}
 
 // lookupStaticHost looks up the addresses for the given host from /etc/hosts.
 func lookupStaticHost(host string) []string {

src/net/parse.go

@@ -10,6 +10,7 @@ package net
 import (
 	"io"
 	"os"
+	"time"
 	_ "unsafe" // For go:linkname
 )
 
@@ -71,6 +72,14 @@ func open(name string) (*file, error) {
 	return &file{fd, make([]byte, 0, os.Getpagesize()), false}, nil
 }
 
+func stat(name string) (mtime time.Time, size int64, err error) {
+	st, err := os.Stat(name)
+	if err != nil {
+		return time.Time{}, 0, err
+	}
+	return st.ModTime(), st.Size(), nil
+}
+
 // byteIndex is strings.IndexByte. It returns the index of the
 // first instance of c in s, or -1 if c is not present in s.
 // strings.IndexByte is implemented in  runtime/asm_$GOARCH.s