Fix shared slice mutation in fork schedule builder

pkg/beacon/state/scheduled_fork.go

@@ -14,11 +14,16 @@ type ScheduledFork struct {
 
 // ForkScheduleFromForkEpochs returns a fork schedule from a list of forks.
 func ForkScheduleFromForkEpochs(forks ForkEpochs) ([]*ScheduledFork, error) {
-	// Sort them by Epoch.
-	sort.Slice(forks, func(i, j int) bool {
-		return (forks)[i].Epoch < (forks)[j].Epoch
+	// Sort a copy by Epoch so we don't reorder the caller's backing array.
+	sorted := make(ForkEpochs, len(forks))
+	copy(sorted, forks)
+
+	sort.Slice(sorted, func(i, j int) bool {
+		return sorted[i].Epoch < sorted[j].Epoch
 	})
 
+	forks = sorted
+
 	scheduled := make([]*ScheduledFork, 0, len(forks))
 
 	for i, fork := range forks {

pkg/beacon/state/scheduled_fork_test.go

@@ -0,0 +1,73 @@
+package state
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/ethpandaops/go-eth2-client/spec"
+	"github.com/ethpandaops/go-eth2-client/spec/phase0"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestForkScheduleFromForkEpochs_DoesNotMutateInput(t *testing.T) {
+	forks := ForkEpochs{
+		{Epoch: 74240, Name: spec.DataVersionAltair, Version: "0x01000000"},
+		{Epoch: 0, Name: spec.DataVersionPhase0, Version: "0x00000000"},
+		{Epoch: 194048, Name: spec.DataVersionBellatrix, Version: "0x02000000"},
+	}
+
+	before := make([]spec.DataVersion, len(forks))
+	for i, f := range forks {
+		before[i] = f.Name
+	}
+
+	scheduled, err := ForkScheduleFromForkEpochs(forks)
+	assert.NoError(t, err)
+	assert.Len(t, scheduled, 3)
+
+	after := make([]spec.DataVersion, len(forks))
+	for i, f := range forks {
+		after[i] = f.Name
+	}
+
+	assert.Equal(t, before, after, "ForkScheduleFromForkEpochs must not reorder the caller's slice")
+
+	// The returned schedule should still be sorted ascending by epoch,
+	// independent of the input order.
+	assert.Equal(t, "0", scheduled[0].Epoch)
+	assert.Equal(t, "74240", scheduled[1].Epoch)
+	assert.Equal(t, "194048", scheduled[2].Epoch)
+}
+
+func TestForkScheduleFromForkEpochs_ConcurrentWithReader(t *testing.T) {
+	forks := ForkEpochs{}
+	for i := range 50 {
+		forks = append(forks, &ForkEpoch{Epoch: phase0.Epoch(i * 1000), Name: spec.DataVersionPhase0})
+	}
+
+	var wg sync.WaitGroup
+
+	// Writer: repeatedly builds a schedule from the shared slice, the same
+	// way a consumer rendering a fork schedule page would.
+	for range 4 {
+		wg.Go(func() {
+			for range 50 {
+				_, err := ForkScheduleFromForkEpochs(forks)
+				assert.NoError(t, err)
+			}
+		})
+	}
+
+	// Reader: the same access pattern ForkMetrics.calculateCurrent uses.
+	for range 4 {
+		wg.Go(func() {
+			for range 50 {
+				for _, fork := range forks {
+					_ = fork.Name.String()
+				}
+			}
+		})
+	}
+
+	wg.Wait()
+}