testing: harmonize handling of prefix-matched benchmarks

If you have BenchmarkX1 with sub-benchmark Y
and you have BenchmarkX2 with no sub-benchmarks,
then

	go test -bench=X/Y

runs BenchmarkX1 once with b.N=1 (to find out about Y)
and then not again, because it has sub-benchmarks,
but arguably also because we're interested in Y.

In contrast, it runs BenchmarkX2 in full, even though clearly
that is not relevant to the match X/Y. We do have to run X2
once with b.N=1 to probe for having X2/Y, but we should not
run it with larger b.N.

Fixes #20589.

Change-Id: Ib86907e844f34dcaac6cd05757f57db1019201d0
Reviewed-on: https://go-review.googlesource.com/46031
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

1 files changed, 8 insertions, 3 deletions

diff --git a/src/testing/benchmark.go b/src/testing/benchmark.go
index 18a46d93bf..8b7f5cebaf 100644
--- a/src/testing/benchmark.go
+++ b/src/testing/benchmark.go
@@ -405,7 +405,7 @@ func runBenchmarks(importPath string, matchString func(pat, str string) (bool, e
 	}
 	var bs []InternalBenchmark
 	for _, Benchmark := range benchmarks {
-		if _, matched := ctx.match.fullName(nil, Benchmark.Name); matched {
+		if _, matched, _ := ctx.match.fullName(nil, Benchmark.Name); matched {
 			bs = append(bs, Benchmark)
 			benchName := benchmarkName(Benchmark.Name, maxprocs)
 			if l := len(benchName) + ctx.extLen + 1; l > ctx.maxLen {
@@ -492,9 +492,9 @@ func (b *B) Run(name string, f func(b *B)) bool {
 	benchmarkLock.Unlock()
 	defer benchmarkLock.Lock()
 
-	benchName, ok := b.name, true
+	benchName, ok, partial := b.name, true, false
 	if b.context != nil {
-		benchName, ok = b.context.match.fullName(&b.common, name)
+		benchName, ok, partial = b.context.match.fullName(&b.common, name)
 	}
 	if !ok {
 		return true
@@ -513,6 +513,11 @@ func (b *B) Run(name string, f func(b *B)) bool {
 		benchTime:  b.benchTime,
 		context:    b.context,
 	}
+	if partial {
+		// Partial name match, like -bench=X/Y matching BenchmarkX.
+		// Only process sub-benchmarks, if any.
+		atomic.StoreInt32(&sub.hasSub, 1)
+	}
 	if sub.run1() {
 		sub.run()
 	}