cmd/dist: support using cross-compiled std test binaries for slow builders

We want the builders to be able to cross-compile test binaries for a
few of the super slow builders that require either slow hardware or
slow full CPU emulation.

Updates golang/go#31217

Change-Id: I8d33b18efaf788f6f131354b2917ac9738ca975e
Reviewed-on: https://go-review.googlesource.com/c/go/+/178399
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -12,6 +12,7 @@ import (
 	"log"
 	"os"
 	"os/exec"
+	"path"
 	"path/filepath"
 	"reflect"
 	"regexp"
@@ -277,8 +278,17 @@ func (t *tester) tags() string {
 	return "-tags="
 }
 
+// timeoutDuration converts the provided number of seconds into a
+// time.Duration, scaled by the t.timeoutScale factor.
+func (t *tester) timeoutDuration(sec int) time.Duration {
+	return time.Duration(sec) * time.Second * time.Duration(t.timeoutScale)
+}
+
+// timeout returns the "-timeout=" string argument to "go test" given
+// the number of seconds of timeout. It scales it by the
+// t.timeoutScale factor.
 func (t *tester) timeout(sec int) string {
-	return "-timeout=" + fmt.Sprint(time.Duration(sec)*time.Second*time.Duration(t.timeoutScale))
+	return "-timeout=" + t.timeoutDuration(sec).String()
 }
 
 // ranGoTest and stdMatches are state closed over by the stdlib
@@ -319,6 +329,11 @@ func (t *tester) registerStdTest(pkg string) {
 					break
 				}
 			}
+			// Special case for our slow cross-compiled
+			// qemu builders:
+			if t.shouldUsePrecompiledStdTest() {
+				return t.runPrecompiledStdTest(t.timeoutDuration(timeoutSec))
+			}
 			args := []string{
 				"test",
 				short(),
@@ -1416,6 +1431,60 @@ func (t *tester) makeGOROOTUnwritable() {
 	}
 }
 
+// shouldUsePrecompiledStdTest reports whether "dist test" should use
+// a pre-compiled go test binary on disk rather than running "go test"
+// and compiling it again. This is used by our slow qemu-based builder
+// that do full processor emulation where we cross-compile the
+// make.bash step as well as pre-compile each std test binary.
+//
+// This only reports true if dist is run with an single go_test:foo
+// argument (as the build coordinator does with our slow qemu-based
+// builders), we're in a builder environment ("GO_BUILDER_NAME" is set),
+// and the pre-built test binary exists.
+func (t *tester) shouldUsePrecompiledStdTest() bool {
+	bin := t.prebuiltGoPackageTestBinary()
+	if bin == "" {
+		return false
+	}
+	_, err := os.Stat(bin)
+	return err == nil
+}
+
+// prebuiltGoPackageTestBinary returns the path where we'd expect
+// the pre-built go test binary to be on disk when dist test is run with
+// a single argument.
+// It returns an empty string if a pre-built binary should not be used.
+func (t *tester) prebuiltGoPackageTestBinary() string {
+	if len(stdMatches) != 1 || t.race || t.compileOnly || os.Getenv("GO_BUILDER_NAME") == "" {
+		return ""
+	}
+	pkg := stdMatches[0]
+	return filepath.Join(os.Getenv("GOROOT"), "src", pkg, path.Base(pkg)+".test")
+}
+
+// runPrecompiledStdTest runs the pre-compiled standard library package test binary.
+// See shouldUsePrecompiledStdTest above; it must return true for this to be called.
+func (t *tester) runPrecompiledStdTest(timeout time.Duration) error {
+	bin := t.prebuiltGoPackageTestBinary()
+	fmt.Fprintf(os.Stderr, "# %s: using pre-built %s...\n", stdMatches[0], bin)
+	cmd := exec.Command(bin, "-test.short", "-test.timeout="+timeout.String())
+	cmd.Dir = filepath.Dir(bin)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+	// And start a timer to kill the process if it doesn't kill
+	// itself in the prescribed timeout.
+	const backupKillFactor = 1.05 // add 5%
+	timer := time.AfterFunc(time.Duration(float64(timeout)*backupKillFactor), func() {
+		fmt.Fprintf(os.Stderr, "# %s: timeout running %s; killing...\n", stdMatches[0], bin)
+		cmd.Process.Kill()
+	})
+	defer timer.Stop()
+	return cmd.Wait()
+}
+
 // raceDetectorSupported is a copy of the function
 // cmd/internal/sys.RaceDetectorSupported, which can't be used here
 // because cmd/dist has to be buildable by Go 1.4.