From 0d4ef22478a470d86ce907beedcaa726d0d46c73 Mon Sep 17 00:00:00 2001
From: Paul Buetow <paul@buetow.org>
Date: Tue, 24 Feb 2026 12:09:31 +0200
Subject: statsengine: stop cloning snapshot accessors on reads

---
 internal/statsengine/snapshot.go      | 35 +++++++++++++++++++++--------------
 internal/statsengine/snapshot_test.go | 14 +++++++-------
 2 files changed, 28 insertions(+), 21 deletions(-)

(limited to 'internal/statsengine')

diff --git a/internal/statsengine/snapshot.go b/internal/statsengine/snapshot.go
index 6583514..6e38739 100644
--- a/internal/statsengine/snapshot.go
+++ b/internal/statsengine/snapshot.go
@@ -154,24 +154,28 @@ func (h HistogramSnapshot) Clone() HistogramSnapshot {
 	}
 }
 
-// LatencySeriesNs returns a defensive copy of latency sparkline samples.
+// LatencySeriesNs returns latency sparkline samples.
+// Callers must treat returned data as read-only.
 func (s Snapshot) LatencySeriesNs() []float64 {
-	return slices.Clone(s.latencySeriesNs)
+	return s.latencySeriesNs
 }
 
-// GapSeriesNs returns a defensive copy of inter-syscall gap sparkline samples.
+// GapSeriesNs returns inter-syscall gap sparkline samples.
+// Callers must treat returned data as read-only.
 func (s Snapshot) GapSeriesNs() []float64 {
-	return slices.Clone(s.gapSeriesNs)
+	return s.gapSeriesNs
 }
 
-// ThroughputSeriesB returns a defensive copy of throughput sparkline samples.
+// ThroughputSeriesB returns throughput sparkline samples.
+// Callers must treat returned data as read-only.
 func (s Snapshot) ThroughputSeriesB() []float64 {
-	return slices.Clone(s.throughputSeriesB)
+	return s.throughputSeriesB
 }
 
-// Syscalls returns a defensive copy of per-syscall snapshot rows.
+// Syscalls returns per-syscall snapshot rows.
+// Callers must treat returned data as read-only.
 func (s Snapshot) Syscalls() []SyscallSnapshot {
-	return slices.Clone(s.syscalls)
+	return s.syscalls
 }
 
 // SyscallsCount returns number of syscall rows without cloning backing slices.
@@ -179,9 +183,10 @@ func (s Snapshot) SyscallsCount() int {
 	return len(s.syscalls)
 }
 
-// Files returns a defensive copy of per-file snapshot rows.
+// Files returns per-file snapshot rows.
+// Callers must treat returned data as read-only.
 func (s Snapshot) Files() []FileSnapshot {
-	return slices.Clone(s.files)
+	return s.files
 }
 
 // FilesCount returns number of file rows without cloning backing slices.
@@ -189,9 +194,10 @@ func (s Snapshot) FilesCount() int {
 	return len(s.files)
 }
 
-// Processes returns a defensive copy of per-process snapshot rows.
+// Processes returns per-process snapshot rows.
+// Callers must treat returned data as read-only.
 func (s Snapshot) Processes() []ProcessSnapshot {
-	return slices.Clone(s.processes)
+	return s.processes
 }
 
 // ProcessesCount returns number of process rows without cloning backing slices.
@@ -199,7 +205,8 @@ func (s Snapshot) ProcessesCount() int {
 	return len(s.processes)
 }
 
-// Buckets returns a defensive copy of histogram buckets.
+// Buckets returns histogram buckets.
+// Callers must treat returned data as read-only.
 func (h HistogramSnapshot) Buckets() []HistogramBucketSnapshot {
-	return slices.Clone(h.buckets)
+	return h.buckets
 }
diff --git a/internal/statsengine/snapshot_test.go b/internal/statsengine/snapshot_test.go
index 065eded..e5c3caa 100644
--- a/internal/statsengine/snapshot_test.go
+++ b/internal/statsengine/snapshot_test.go
@@ -58,7 +58,7 @@ func TestNewSnapshotDefensivelyCopiesSlices(t *testing.T) {
 	}
 }
 
-func TestSnapshotAccessorsReturnCopies(t *testing.T) {
+func TestSnapshotAccessorsReturnReadOnlyViews(t *testing.T) {
 	s := NewSnapshot(
 		[]float64{1},
 		[]float64{2},
@@ -72,20 +72,20 @@ func TestSnapshotAccessorsReturnCopies(t *testing.T) {
 
 	lat := s.LatencySeriesNs()
 	lat[0] = 100
-	if got := s.LatencySeriesNs()[0]; got != 1 {
-		t.Fatalf("latency accessor leaked mutability: got %v", got)
+	if got := s.LatencySeriesNs()[0]; got != 100 {
+		t.Fatalf("expected accessor to return backing slice view, got %v", got)
 	}
 
 	syscalls := s.Syscalls()
 	syscalls[0].Name = "write"
-	if got := s.Syscalls()[0].Name; got != "read" {
-		t.Fatalf("syscalls accessor leaked mutability: got %q", got)
+	if got := s.Syscalls()[0].Name; got != "write" {
+		t.Fatalf("expected accessor to return backing slice view, got %q", got)
 	}
 
 	buckets := s.LatencyHistogram.Buckets()
 	buckets[0].Count = 99
-	if got := s.LatencyHistogram.Buckets()[0].Count; got != 1 {
-		t.Fatalf("bucket accessor leaked mutability: got %d", got)
+	if got := s.LatencyHistogram.Buckets()[0].Count; got != 99 {
+		t.Fatalf("expected accessor to return backing slice view, got %d", got)
 	}
 }
 
-- 
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