cmd/go: accept GOSUMDB=sum.golang.google.cn

This CL makes the go command understand that
GOSUMDB=sum.golang.google.cn should connect
to that domain but expect to find a checksum database
signed by sum.golang.org there.

The host sum.golang.google.cn is not yet completely
configured; we hope it will be available in a few weeks.

Change-Id: Ie0fc4323f0c7084dda59bd3b45fc406717fa16d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/191137
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/cmd/go/alldocs.go

@@ -2743,9 +2743,11 @@
 // 	GOSUMDB="sum.golang.org+<publickey>"
 // 	GOSUMDB="sum.golang.org+<publickey> https://sum.golang.org"
 //
-// The go command knows the public key of sum.golang.org; use of any other
-// database requires giving the public key explicitly. The URL defaults to
-// "https://" followed by the database name.
+// The go command knows the public key of sum.golang.org, and also that the name
+// sum.golang.google.cn (available inside mainland China) connects to the
+// sum.golang.org checksum database; use of any other database requires giving
+// the public key explicitly.
+// The URL defaults to "https://" followed by the database name.
 //
 // GOSUMDB defaults to "sum.golang.org", the Go checksum database run by Google.
 // See https://sum.golang.org/privacy for the service's privacy policy.

src/cmd/go/internal/modfetch/fetch.go

@@ -701,9 +701,11 @@ to use and optionally its public key and URL, as in:
 	GOSUMDB="sum.golang.org+<publickey>"
 	GOSUMDB="sum.golang.org+<publickey> https://sum.golang.org"
 
-The go command knows the public key of sum.golang.org; use of any other
-database requires giving the public key explicitly. The URL defaults to
-"https://" followed by the database name.
+The go command knows the public key of sum.golang.org, and also that the name
+sum.golang.google.cn (available inside mainland China) connects to the
+sum.golang.org checksum database; use of any other database requires giving
+the public key explicitly.
+The URL defaults to "https://" followed by the database name.
 
 GOSUMDB defaults to "sum.golang.org", the Go checksum database run by Google.
 See https://sum.golang.org/privacy for the service's privacy policy.

src/cmd/go/internal/modfetch/sumdb.go

@@ -60,7 +60,17 @@ func dbDial() (dbName string, db *sumweb.Conn, err error) {
 	// $GOSUMDB can be "key" or "key url",
 	// and the key can be a full verifier key
 	// or a host on our list of known keys.
-	key := strings.Fields(cfg.GOSUMDB)
+
+	// Special case: sum.golang.google.cn
+	// is an alias, reachable inside mainland China,
+	// for sum.golang.org. If there are more
+	// of these we should add a map like knownGOSUMDB.
+	gosumdb := cfg.GOSUMDB
+	if gosumdb == "sum.golang.google.cn" {
+		gosumdb = "sum.golang.org https://sum.golang.google.cn"
+	}
+
+	key := strings.Fields(gosumdb)
 	if len(key) >= 1 {
 		if k := knownGOSUMDB[key[0]]; k != "" {
 			key[0] = k