cmd/go/internal/modfetch: fix GOSUMDB test failures

Use cfg.GOSUMDB consistently instead of re-resolving it from the environment.

Set cfg.GOSUMDB to 'off' explicitly in coderepo_test, since it may
include modules that cannot be fetched using a released version of the
'go' command.

Fixes #31964

Change-Id: I17cae9e0c6aa1168ba534e6da4e3652800ac81e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/176538
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modfetch/coderepo_test.go

@@ -16,6 +16,7 @@ import (
 	"testing"
 	"time"
 
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/modfetch/codehost"
 )
 
@@ -26,6 +27,12 @@ func TestMain(m *testing.M) {
 func testMain(m *testing.M) int {
 	SetProxy("direct")
 
+	// The sum database is populated using a released version of the go command,
+	// but this test may include fixes for additional modules that previously
+	// could not be fetched. Since this test isn't executing any of the resolved
+	// code, bypass the sum database.
+	cfg.GOSUMDB = "off"
+
 	dir, err := ioutil.TempDir("", "gitrepo-test-")
 	if err != nil {
 		log.Fatal(err)

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

@@ -60,7 +60,7 @@ 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.Getenv("GOSUMDB"))
+	key := strings.Fields(cfg.GOSUMDB)
 	if len(key) >= 1 {
 		if k := knownGOSUMDB[key[0]]; k != "" {
 			key[0] = k