pkg/fuzzer: implement basic max signal rotation

Once in 15 minutes, drop 1000 elements of the pure max signal (that is,
max signal minus corpus signal).

It seems to have a positive effect on the total fuzzing performance.

1 files changed, 26 insertions, 6 deletions

diff --git a/pkg/fuzzer/cover.go b/pkg/fuzzer/cover.go
index 5af00f167..31d1fee1b 100644
--- a/pkg/fuzzer/cover.go
+++ b/pkg/fuzzer/cover.go
@@ -11,16 +11,19 @@ import (
 
 // Cover keeps track of the signal known to the fuzzer.
 type Cover struct {
-	mu        sync.RWMutex
-	maxSignal signal.Signal // max signal ever observed (including flakes)
-	newSignal signal.Signal // newly identified max signal
+	mu         sync.RWMutex
+	maxSignal  signal.Signal // max signal ever observed (including flakes)
+	newSignal  signal.Signal // newly identified max signal
+	dropSignal signal.Signal // the newly dropped max signal
 }
 
 // Signal that should no longer be chased after.
+// It is not returned in GrabSignalDelta().
 func (cover *Cover) AddMaxSignal(sign signal.Signal) {
 	cover.mu.Lock()
 	defer cover.mu.Unlock()
 	cover.maxSignal.Merge(sign)
+	cover.dropSignal.Subtract(sign)
 }
 
 func (cover *Cover) addRawMaxSignal(signal []uint32, prio uint8) signal.Signal {
@@ -32,21 +35,30 @@ func (cover *Cover) addRawMaxSignal(signal []uint32, prio uint8) signal.Signal {
 	}
 	cover.maxSignal.Merge(diff)
 	cover.newSignal.Merge(diff)
+	cover.dropSignal.Subtract(diff)
 	return diff
 }
 
+func (cover *Cover) pureMaxSignal(corpus signal.Signal) signal.Signal {
+	cover.mu.RLock()
+	defer cover.mu.RUnlock()
+	return corpus.Diff(cover.maxSignal)
+}
+
 func (cover *Cover) CopyMaxSignal() signal.Signal {
 	cover.mu.RLock()
 	defer cover.mu.RUnlock()
 	return cover.maxSignal.Copy()
 }
 
-func (cover *Cover) GrabNewSignal() signal.Signal {
+func (cover *Cover) GrabSignalDelta() (plus, minus signal.Signal) {
 	cover.mu.Lock()
 	defer cover.mu.Unlock()
-	sign := cover.newSignal
+	plus = cover.newSignal
 	cover.newSignal = nil
-	return sign
+	minus = cover.dropSignal
+	cover.dropSignal = nil
+	return
 }
 
 type CoverStats struct {
@@ -60,3 +72,11 @@ func (cover *Cover) Stats() CoverStats {
 		MaxSignal: len(cover.maxSignal),
 	}
 }
+
+func (cover *Cover) subtract(delta signal.Signal) {
+	cover.mu.Lock()
+	defer cover.mu.Unlock()
+	cover.maxSignal.Subtract(delta)
+	cover.newSignal.Subtract(delta)
+	cover.dropSignal.Merge(delta)
+}