[dev.fuzz] testing: convert seed corpus values where possible

The types provided in f.Fuzz will be viewed as the
canonical types for fuzzing. If the type is different
for a seed corpus entry, then the testing package
will attempt to convert it. If it can't convert it,
f.Fuzz will fail.

Currently, this allows converting types that may result
in precision loss or a semantically different value.
For example, an int(-1) can be converted to uint even
though the value could be math.MaxUint64. There is a
TODO to consider improving this in the future.

Updates golang/go#45593

Change-Id: I2e752119662f46b68445d42b1ffa46dd30e9faea
Reviewed-on: https://go-review.googlesource.com/c/go/+/325702
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/src/testing/fuzz.go b/src/testing/fuzz.go
index 9f0bb1ec50..b4c1ffcdd5 100644
--- a/src/testing/fuzz.go
+++ b/src/testing/fuzz.go
@@ -222,7 +222,7 @@ func (f *F) TempDir() string {
 
 // Add will add the arguments to the seed corpus for the fuzz target. This will
 // be a no-op if called after or within the Fuzz function. The args must match
-// those in the Fuzz function.
+// or be convertible to those in the Fuzz function.
 func (f *F) Add(args ...interface{}) {
 	var values []interface{}
 	for i := range args {
@@ -291,6 +291,15 @@ func (f *F) Fuzz(ff interface{}) {
 		types = append(types, t)
 	}
 
+	// Check the corpus provided by f.Add
+	for _, c := range f.corpus {
+		if err := f.fuzzContext.checkCorpus(c.Values, types); err != nil {
+			// TODO: Is there a way to save which line number is associated
+			// with the f.Add call that failed?
+			f.Fatal(err)
+		}
+	}
+
 	// Load seed corpus
 	c, err := f.fuzzContext.readCorpus(filepath.Join(corpusDir, f.name), types)
 	if err != nil {
@@ -470,6 +479,7 @@ type fuzzContext struct {
 	coordinateFuzzing func(time.Duration, int64, time.Duration, int64, int, []corpusEntry, []reflect.Type, string, string) error
 	runFuzzWorker     func(func(corpusEntry) error) error
 	readCorpus        func(string, []reflect.Type) ([]corpusEntry, error)
+	checkCorpus       func(vals []interface{}, types []reflect.Type) error
 	resetCoverage     func()
 	snapshotCoverage  func()
 }
@@ -487,6 +497,7 @@ func runFuzzTargets(deps testDeps, fuzzTargets []InternalFuzzTarget) (ran, ok bo
 	fctx := &fuzzContext{
 		importPath:       deps.ImportPath,
 		readCorpus:       deps.ReadCorpus,
+		checkCorpus:      deps.CheckCorpus,
 		resetCoverage:    deps.ResetCoverage,
 		snapshotCoverage: deps.SnapshotCoverage,
 	}
@@ -543,6 +554,7 @@ func runFuzzing(deps testDeps, fuzzTargets []InternalFuzzTarget) (ran, ok bool)
 	fctx := &fuzzContext{
 		importPath:       deps.ImportPath,
 		readCorpus:       deps.ReadCorpus,
+		checkCorpus:      deps.CheckCorpus,
 		resetCoverage:    deps.ResetCoverage,
 		snapshotCoverage: deps.SnapshotCoverage,
 	}