testing: only snapshot coverage during fuzzing

Only snapshot/reset coverage counters when we are actually fuzzing.
This prevents a race when running corpus/seed values during the testing
phase.

Fixes #50488

Change-Id: I7dd5a0353a296c0b13eede29ad9af7c78814fa2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/376554
Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/testdata/script/test_fuzz_test_race.txt

@@ -0,0 +1,38 @@
+# Test that when both race detection and coverage instrumentation are enabled,
+# and seed values are being executed, the race detector isn't mistakenly
+# triggered.
+
+[short] skip
+[!fuzz] skip
+
+# Test with coverage instrumentation enbaled (-fuzz) and race instrumentation
+# but without actually fuzzing the target (by using a non-matching pattern)
+go test -fuzz=xxx -race -v
+! stderr 'race detected during execution of test'
+
+# Test with just race instrumentation enabled
+go test -race -v
+! stderr 'race detected during execution of test'
+
+# Test with coverage and race instrumentation enabled, and a matching fuzz
+# pattern
+go test -fuzz=FuzzRace -race -v -fuzztime=200x
+! stderr 'race detected during execution of test'
+
+-- go.mod --
+module test
+
+-- race_test.go --
+package race
+
+import "testing"
+
+func FuzzRace(f *testing.F) {
+	for i := 0; i < 100; i++ {
+		f.Add(i)
+	}
+
+	f.Fuzz(func(t *testing.T, i int) {
+		t.Parallel()
+	})
+}

src/testing/fuzz.go

@@ -327,8 +327,10 @@ func (f *F) Fuzz(ff any) {
 			// we make sure it is called right before the tRunner function
 			// exits, regardless of whether it was executed cleanly, panicked,
 			// or if the fuzzFn called t.Fatal.
-			defer f.fuzzContext.deps.SnapshotCoverage()
+			if f.testContext.isFuzzing {
-			f.fuzzContext.deps.ResetCoverage()
+				defer f.fuzzContext.deps.SnapshotCoverage()
+				f.fuzzContext.deps.ResetCoverage()
+			}
 			fn.Call(args)
 		})
 		<-t.signal