cmd/dist: skip compiling 100 packages without benchmarks in race mode

The go_test_bench:* tests run:

    go test -short -race -run=^$ -benchtime=.1s -cpu=4 $PKG

... on each discovered package with any tests. (The same set used for
the "go_test:*" tests)

That set was 168 packages:

$ go tool dist test -list | grep go_test: | wc -l
168

But only 76 of those have a "func Benchmark", and running each
"go_test_bench:" test and compiling it in race mode, just to do
nothing took 1-2 seconds each.

So stop doing that and filter out the useless packages earlier. Now:

$ go tool dist test -list -race | grep go_test_bench:  | wc -l
76

Should save 90-180 seconds. (or maybe 45 seconds for trybots, since
they're sharded)

Updates #17104

Change-Id: I08ccb072a0dc0454ea425540ee8e74b59f83b773
Reviewed-on: https://go-review.googlesource.com/29153
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

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"log"
 	"os"
 	"os/exec"
@@ -361,10 +362,12 @@ func (t *tester) registerTests() {
 		}
 		if t.race {
 			for _, pkg := range pkgs {
+				if t.packageHasBenchmarks(pkg) {
 					t.registerRaceBenchTest(pkg)
 				}
 			}
 		}
+	}
 
 	if t.race {
 		return
@@ -1086,3 +1089,38 @@ var cgoPackages = []string{
 	"net",
 	"os/user",
 }
+
+var funcBenchmark = []byte("\nfunc Benchmark")
+
+// packageHasBenchmarks reports whether pkg has benchmarks.
+// On any error, it conservatively returns true.
+//
+// This exists just to eliminate work on the builders, since compiling
+// a test in race mode just to discover it has no benchmarks costs a
+// second or two per package, and this function returns false for
+// about 100 packages.
+func (t *tester) packageHasBenchmarks(pkg string) bool {
+	pkgDir := filepath.Join(t.goroot, "src", pkg)
+	d, err := os.Open(pkgDir)
+	if err != nil {
+		return true // conservatively
+	}
+	defer d.Close()
+	names, err := d.Readdirnames(-1)
+	if err != nil {
+		return true // conservatively
+	}
+	for _, name := range names {
+		if !strings.HasSuffix(name, "_test.go") {
+			continue
+		}
+		slurp, err := ioutil.ReadFile(filepath.Join(pkgDir, name))
+		if err != nil {
+			return true // conservatively
+		}
+		if bytes.Contains(slurp, funcBenchmark) {
+			return true
+		}
+	}
+	return false
+}