cmd/go: cache results of HTTP requests done during meta tag discovery

Previously, running

  $ go get -u -v golang.org/x/tools/cmd/godoc

would results in dozens of HTTP requests for

  https://golang.org/x/tools?go-get=1

once per package under x/tools.

Now it caches the results. We still end up doing one HTTP request for
all the packages under x/tools, but this reduces the total number of
HTTP requests in ~half.

This also moves the singleflight package back into an internal
package. singleflight was originally elsewhere as a package, then got
copied into "net" (without its tests). But now that we have internal,
put it in its own package, and restore its test.

Fixes #9249

Change-Id: Ieb5cf04fc4d0a0c188cb957efdc7ea3068c34e3f
Reviewed-on: https://go-review.googlesource.com/8727
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/src/net/lookup.go b/src/net/lookup.go
index be4b0c2df6..5adcd8bb68 100644
--- a/src/net/lookup.go
+++ b/src/net/lookup.go
@@ -4,7 +4,10 @@
 
 package net
 
-import "time"
+import (
+	"internal/singleflight"
+	"time"
+)
 
 // protocols contains minimal mappings between internet protocol
 // names and numbers for platforms that don't have a complete list of
@@ -39,7 +42,7 @@ func LookupIP(host string) (ips []IP, err error) {
 	return
 }
 
-var lookupGroup singleflight
+var lookupGroup singleflight.Group
 
 // lookupIPMerge wraps lookupIP, but makes sure that for any given
 // host, only one lookup is in-flight at a time. The returned memory
@@ -98,7 +101,7 @@ func lookupIPDeadline(host string, deadline time.Time) (addrs []IPAddr, err erro
 		return nil, errTimeout
 
 	case r := <-ch:
-		return lookupIPReturn(r.v, r.err, r.shared)
+		return lookupIPReturn(r.Val, r.Err, r.Shared)
 	}
 }