Create TEST_SHARD_STATUS_FILE when sharding tests (#3547)

Bazel claims to check that this file has been touched when sharding
tests. It currently doesn't, but that will change soon.

Author: fmeum

go/tools/builders/generate_test_main.go

@@ -146,6 +146,11 @@ func testsInShard() []testing.InternalTest {
 	if err != nil || totalShards <= 1 {
 		return allTests
 	}
+	file, err := os.Create(os.Getenv("TEST_SHARD_STATUS_FILE"))
+	if err != nil {
+		log.Fatalf("Failed to touch TEST_SHARD_STATUS_FILE: %v", err)
+	}
+	_ = file.Close()
 	shardIndex, err := strconv.Atoi(os.Getenv("TEST_SHARD_INDEX"))
 	if err != nil || shardIndex < 0 {
 		return allTests

tests/core/go_test/BUILD.bazel

@@ -244,6 +244,12 @@ go_test(
     ],
 )
 
+go_test(
+    name = "sharding_test",
+    srcs = ["sharding_test.go"],
+    shard_count = 2,
+)
+
 go_bazel_test(
     name = "env_inherit_test",
     srcs = ["env_inherit_test.go"],

tests/core/go_test/sharding_test.go

@@ -0,0 +1,13 @@
+package sharding_test
+
+import (
+	"log"
+	"os"
+	"testing"
+)
+
+func TestShardStatusFile(t *testing.T) {
+	if _, err := os.Stat(os.Getenv("TEST_SHARD_STATUS_FILE")); err != nil {
+		log.Fatalf("Expected Go test runner to create TEST_SHARD_STATUS_FILE: %v", err)
+	}
+}