From 5d908a4a37cfacd28f289c34e678b8006b8cbcea Mon Sep 17 00:00:00 2001
From: Alexander Yastrebov <alexander.yastrebov@zalando.de>
Date: Thu, 5 Dec 2024 16:26:46 +0100
Subject: [PATCH] predicates/traffic: deflake TestTrafficSegmentSplit

Use fixed random sequence to deflake TestTrafficSegmentSplit.

Fixes #2665

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>
---
 predicates/traffic/export_test.go  | 11 +++++++++++
 predicates/traffic/rand_test.go    | 22 ++++++++++++++++++++++
 predicates/traffic/segment.go      | 17 +++++++++++------
 predicates/traffic/segment_test.go |  4 +++-
 4 files changed, 47 insertions(+), 7 deletions(-)
 create mode 100644 predicates/traffic/rand_test.go

diff --git a/predicates/traffic/export_test.go b/predicates/traffic/export_test.go
index d1d32b2be2..b09d373e0e 100644
--- a/predicates/traffic/export_test.go
+++ b/predicates/traffic/export_test.go
@@ -1,3 +1,14 @@
 package traffic
 
+import "github.com/zalando/skipper/routing"
+
 var ExportRandomValue = randomValue
+
+func WithRandFloat64(ps routing.PredicateSpec, randFloat64 func() float64) routing.PredicateSpec {
+	if s, ok := ps.(*segmentSpec); ok {
+		s.randFloat64 = randFloat64
+	} else {
+		panic("invalid predicate spec, expected *segmentSpec")
+	}
+	return ps
+}
diff --git a/predicates/traffic/rand_test.go b/predicates/traffic/rand_test.go
new file mode 100644
index 0000000000..d7ede7b11a
--- /dev/null
+++ b/predicates/traffic/rand_test.go
@@ -0,0 +1,22 @@
+package traffic_test
+
+import (
+	"math/rand/v2"
+	"sync"
+)
+
+// newTestRandFloat64 returns a function that generates fixed sequence of random float64 values for testing.
+func newTestRandFloat64() func() float64 {
+	return rand.New(&lockedSource{s: rand.NewPCG(0x5EED_1, 0x5EED_2)}).Float64
+}
+
+type lockedSource struct {
+	mu sync.Mutex
+	s  rand.Source
+}
+
+func (s *lockedSource) Uint64() uint64 {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.s.Uint64()
+}
diff --git a/predicates/traffic/segment.go b/predicates/traffic/segment.go
index f9dc323860..799fd10c0a 100644
--- a/predicates/traffic/segment.go
+++ b/predicates/traffic/segment.go
@@ -9,8 +9,13 @@ import (
 )
 
 type (
-	segmentSpec      struct{}
-	segmentPredicate struct{ min, max float64 }
+	segmentSpec struct {
+		randFloat64 func() float64
+	}
+	segmentPredicate struct {
+		randFloat64 func() float64
+		min, max    float64
+	}
 )
 
 type contextKey struct{}
@@ -19,7 +24,7 @@ var randomValue contextKey
 
 // NewSegment creates a new traffic segment predicate specification
 func NewSegment() routing.WeightedPredicateSpec {
-	return &segmentSpec{}
+	return &segmentSpec{rand.Float64}
 }
 
 func (*segmentSpec) Name() string {
@@ -42,12 +47,12 @@ func (*segmentSpec) Name() string {
 //	r50: Path("/test") && TrafficSegment(0.0, 0.5) -> <shunt>;
 //	r30: Path("/test") && TrafficSegment(0.5, 0.8) -> <shunt>;
 //	r20: Path("/test") && TrafficSegment(0.8, 1.0) -> <shunt>;
-func (*segmentSpec) Create(args []any) (routing.Predicate, error) {
+func (s *segmentSpec) Create(args []any) (routing.Predicate, error) {
 	if len(args) != 2 {
 		return nil, predicates.ErrInvalidPredicateParameters
 	}
 
-	p, ok := &segmentPredicate{}, false
+	p, ok := &segmentPredicate{randFloat64: s.randFloat64}, false
 
 	if p.min, ok = args[0].(float64); !ok || p.min < 0 || p.min > 1 {
 		return nil, predicates.ErrInvalidPredicateParameters
@@ -72,7 +77,7 @@ func (*segmentSpec) Weight() int {
 }
 
 func (p *segmentPredicate) Match(req *http.Request) bool {
-	r := routing.FromContext(req.Context(), randomValue, rand.Float64)
+	r := routing.FromContext(req.Context(), randomValue, p.randFloat64)
 	// min == max defines a never-matching interval and always yields false
 	return p.min <= r && r < p.max
 }
diff --git a/predicates/traffic/segment_test.go b/predicates/traffic/segment_test.go
index 87e93397a8..78e005f205 100644
--- a/predicates/traffic/segment_test.go
+++ b/predicates/traffic/segment_test.go
@@ -148,7 +148,9 @@ func TestTrafficSegmentSplit(t *testing.T) {
 		RoutingOptions: routing.Options{
 			FilterRegistry: builtin.MakeRegistry(),
 			Predicates: []routing.PredicateSpec{
-				traffic.NewSegment(),
+				// Use fixed random sequence to deflake the test,
+				// see https://github.com/zalando/skipper/issues/2665
+				traffic.WithRandFloat64(traffic.NewSegment(), newTestRandFloat64()),
 			},
 		},
 		Routes: eskip.MustParse(`