internal/fuzz: don't deflake coverage found while fuzzing

Previously, the worker would attempt to deflake
an input that was reported to have caused new
coverage. The chances of a flake causing new
coverage seem pretty low to me, and even if it
was a flake that caused it, adding that input to
the cache doesn't seem like a bad thing. The
input is already going to be deflaked during
minimization anyway. If by some off-chance the
code is causing a lot of flaky coverage
increases, and the user doesn't want minimization
to occur, then setting -fuzzminimizetime=1x will
deflake in the way they want without minimizing.
This can be documented as needed.

This fixes a bug where the mem.header().count
could have been one too large if an unrecoverable
crash occured while deflaking an input that
caused code coverage.

Fixes #49047

Change-Id: Ibdf893d7a89a46dd700702afb09e35623615390e
Reviewed-on: https://go-review.googlesource.com/c/go/+/358094
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 15 insertions, 9 deletions

diff --git a/src/cmd/go/testdata/script/test_fuzz_mutator_repeat.txt b/src/cmd/go/testdata/script/test_fuzz_mutator_repeat.txt
index f2952c349b..15d7cb6b32 100644
--- a/src/cmd/go/testdata/script/test_fuzz_mutator_repeat.txt
+++ b/src/cmd/go/testdata/script/test_fuzz_mutator_repeat.txt
@@ -1,5 +1,3 @@
-skip  # Flaky: https://golang.org/issue/49047
-
 # TODO(jayconrod): support shared memory on more platforms.
 [!darwin] [!linux] [!windows] skip
 
@@ -9,11 +7,11 @@ skip  # Flaky: https://golang.org/issue/49047
 
 [short] skip
 
-# Start fuzzing. The worker crashes after ~100 iterations.
+# Start fuzzing. The worker crashes after 100 iterations.
 # The fuzz function writes the crashing input to "want" before exiting.
 # The fuzzing engine reconstructs the crashing input and saves it to testdata.
 ! exists want
-! go test -fuzz=. -parallel=1
+! go test -fuzz=. -parallel=1 -fuzztime=110x -fuzzminimizetime=10x -v
 stdout 'fuzzing process terminated unexpectedly'
 stdout 'Crash written to testdata'
 
@@ -46,12 +44,20 @@ func FuzzRepeat(f *testing.F) {
 		if i == 100 {
 			f, err := os.OpenFile("want", os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0666)
 			if err != nil {
-				// Couldn't create the file, probably because it already exists,
-				// and we're minimizing now. Return without crashing.
-				return
+				// Couldn't create the file. Return without crashing, and try
+				// again.
+				i--
+				t.Skip(err)
+			}
+			if _, err := f.Write(b); err != nil {
+				// We already created the file, so if we failed to write it
+				// there's not much we can do. The test will fail anyway, but
+				// at least make sure the error is logged to stdout.
+				t.Fatal(err)
+			}
+			if err := f.Close(); err != nil {
+				t.Fatal(err)
 			}
-			f.Write(b)
-			f.Close()
 			os.Exit(1) // crash without communicating
 		}