Support Bazel's test-sharding protocol.

Bazel can shard tests, but runner runs a lot of tests inside a single
“test”, as Go sees it. In order to shard within runner we implement
support for Bazel's environment variables[1] directly. This does mean
that the handful of other tests in runner are run in every shard.

[1]
https://docs.bazel.build/versions/1.1.0/test-encyclopedia.html#initial-conditions

Change-Id: Idaa5c6ae5225cd86951cd40f47b5f86f31664e04
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/51245
Reviewed-by: David Benjamin <davidben@google.com>

ssl/test/runner/runner.go

diff --git a/ssl/test/runner/runner.go b/ssl/test/runner/runner.go
index 370fa94..4c1c955 100644
--- a/ssl/test/runner/runner.go
+++ b/ssl/test/runner/runner.go
@@ -19309,8 +19309,22 @@
 		noneOfPattern = strings.Split(*skipTest, ";")
 	}
 
+	shardIndex, shardTotal, err := getSharding()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	if shardTotal > 0 {
+		fmt.Printf("This is shard %d of 0..%d (inclusive)\n", shardIndex, shardTotal-1)
+	}
+
 	var foundTest bool
 	for i := range testCases {
+		if shardTotal > 0 && i%shardTotal != shardIndex {
+			continue
+		}
+
 		matched, err := match(oneOfPatternIfAny, noneOfPattern, testCases[i].name)
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "Error matching pattern: %s\n", err)
@@ -19348,7 +19362,7 @@
 		}
 	}
 
-	if !foundTest {
+	if !foundTest && shardTotal == 0 {
 		fmt.Fprintf(os.Stderr, "No tests run\n")
 		os.Exit(1)
 	}

ssl/test/runner/sharding.go

diff --git a/ssl/test/runner/sharding.go b/ssl/test/runner/sharding.go
new file mode 100644
index 0000000..5061a6f
--- /dev/null
+++ b/ssl/test/runner/sharding.go
@@ -0,0 +1,77 @@
+// Copyright (c) 2022, Google Inc.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+package runner
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strconv"
+)
+
+const (
+	shardStatusFileEnv = "TEST_SHARD_STATUS_FILE"
+	shardTotalEnv      = "TEST_TOTAL_SHARDS"
+	shardIndexEnv      = "TEST_SHARD_INDEX"
+	shardPrefix        = "RUNNER_"
+)
+
+func init() {
+	// When run under `go test`, init() functions may be run twice if the
+	// test binary ends up forking and execing itself. Therefore we move
+	// the environment variables to names that don't interfere with Go's
+	// own support for sharding. If we recorded and erased them, then they
+	// wouldn't exist the second time the binary runs.
+	for _, key := range []string{shardStatusFileEnv, shardTotalEnv, shardIndexEnv} {
+		value := os.Getenv(key)
+		if len(value) > 0 {
+			os.Setenv(shardPrefix+key, value)
+			os.Setenv(key, "")
+		}
+	}
+}
+
+// getSharding returns the shard index and count, or zeros if sharding is not
+// enabled.
+func getSharding() (index, total int, err error) {
+	statusFile := os.Getenv(shardPrefix + shardStatusFileEnv)
+	totalNumStr := os.Getenv(shardPrefix + shardTotalEnv)
+	indexStr := os.Getenv(shardPrefix + shardIndexEnv)
+	if len(totalNumStr) == 0 || len(indexStr) == 0 {
+		return 0, 0, nil
+	}
+
+	totalNum, err := strconv.Atoi(totalNumStr)
+	if err != nil {
+		return 0, 0, fmt.Errorf("$%s is %q, but expected a number\n", shardTotalEnv, totalNumStr)
+	}
+
+	index, err = strconv.Atoi(indexStr)
+	if err != nil {
+		return 0, 0, fmt.Errorf("$%s is %q, but expected a number\n", shardIndexEnv, indexStr)
+	}
+
+	if index < 0 || index >= totalNum {
+		return 0, 0, fmt.Errorf("shard index/total of %d/%d is invalid\n", index, totalNum)
+	}
+
+	if len(statusFile) > 0 {
+		if err := ioutil.WriteFile(statusFile, nil, 0664); err != nil {
+			return 0, 0, err
+		}
+	}
+
+	return index, totalNum, nil
+}