cmd/dist: allow builders to control granularity of test/ directory sharding

Even with 10 shards on builders, it still takes about ~2.5 minutes per
shard (and getting slower all the time as the test/ directory grows).
I'm currently experimenting with massively sharding out testing on
Cloud Run (each dist test & normal TestFoo func all running in
parallel), and in such a setup, 2.5 minutes is an eternity. I'd like
to increase that dist test's sharding from 10 to more like 1,000.

Updates golang/go#31834

Change-Id: I8b02989727793b5b5b2013d67e1eb01ef4786e28
Reviewed-on: https://go-review.googlesource.com/c/go/+/175297Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -709,11 +709,14 @@ func (t *tester) registerTests() {
 	if goos != "android" && !t.iOS() {
 		// Only start multiple test dir shards on builders,
 		// where they get distributed to multiple machines.
-		// See issue 20141.
+		// See issues 20141 and 31834.
 		nShards := 1
 		if os.Getenv("GO_BUILDER_NAME") != "" {
 			nShards = 10
 		}
+		if n, err := strconv.Atoi(os.Getenv("GO_TEST_SHARDS")); err == nil {
+			nShards = n
+		}
 		for shard := 0; shard < nShards; shard++ {
 			shard := shard
 			t.tests = append(t.tests, distTest{

src/run.bash

@@ -3,6 +3,17 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
+# Environment variables that control run.bash:
+#
+# GO_TEST_SHARDS: number of "dist test" test shards that the
+# $GOROOT/test directory will be sliced up into for parallel
+# execution. Defaults to 1, unless GO_BUILDER_NAME is also specified,
+# in which case it defaults to 10.
+#
+# GO_BUILDER_NAME: the name of the Go builder that's running the tests.
+# Some tests are conditionally enabled or disabled based on the builder
+# name or the builder name being non-empty.
+
 set -e
 
 eval $(go env)