From 8982cda8afc40f3268bfc2614faa202fe6dea6d3 Mon Sep 17 00:00:00 2001
From: Paul Buetow <paul@buetow.org>
Date: Thu, 30 Apr 2026 00:49:46 +0300
Subject: fix: seed random source in thumbnail generator

---
 internal/thumb/thumb.go      |  7 +++--
 internal/thumb/thumb_test.go | 65 ++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 68 insertions(+), 4 deletions(-)

(limited to 'internal')

diff --git a/internal/thumb/thumb.go b/internal/thumb/thumb.go
index 96c496b..53a139b 100644
--- a/internal/thumb/thumb.go
+++ b/internal/thumb/thumb.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"math/rand"
 	"os/exec"
+	"time"
 )
 
 // Generator creates a thumbnail for a given media file.
@@ -16,12 +17,14 @@ type Generator interface {
 // FFmpegGenerator uses ffmpeg to extract a random frame.
 type FFmpegGenerator struct {
 	execer func(ctx context.Context, name string, arg ...string) *exec.Cmd
+	rnd    *rand.Rand
 }
 
-// NewFFmpegGenerator creates a new FFmpegGenerator.
+// NewFFmpegGenerator creates a new FFmpegGenerator with a seeded random source.
 func NewFFmpegGenerator() *FFmpegGenerator {
 	return &FFmpegGenerator{
 		execer: exec.CommandContext,
+		rnd:    rand.New(rand.NewSource(time.Now().UnixNano())),
 	}
 }
 
@@ -30,7 +33,7 @@ func NewFFmpegGenerator() *FFmpegGenerator {
 func (g *FFmpegGenerator) Generate(ctx context.Context, inputPath, outputPath string, duration float64) error {
 	offset := 0.0
 	if duration > 0 {
-		offset = rand.Float64() * duration
+		offset = g.rnd.Float64() * duration
 		if offset < 1.0 {
 			offset = 1.0
 		}
diff --git a/internal/thumb/thumb_test.go b/internal/thumb/thumb_test.go
index 2cd6124..d6272a9 100644
--- a/internal/thumb/thumb_test.go
+++ b/internal/thumb/thumb_test.go
@@ -3,9 +3,12 @@ package thumb
 import (
 	"context"
 	"errors"
+	"math/rand"
 	"os/exec"
+	"strconv"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestFFmpegGenerator_Generate(t *testing.T) {
@@ -35,7 +38,7 @@ func TestFFmpegGenerator_Generate(t *testing.T) {
 		return exec.Command("true")
 	}
 
-	g := &FFmpegGenerator{execer: fakeExecer}
+	g := &FFmpegGenerator{execer: fakeExecer, rnd: rand.New(rand.NewSource(1))}
 	if err := g.Generate(ctx, "input.mp4", "output.jpg", 120.0); err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -58,12 +61,58 @@ func TestFFmpegGenerator_Generate_Error(t *testing.T) {
 	fakeExecer := func(_ context.Context, name string, arg ...string) *exec.Cmd {
 		return exec.Command("false")
 	}
-	g := &FFmpegGenerator{execer: fakeExecer}
+	g := &FFmpegGenerator{execer: fakeExecer, rnd: rand.New(rand.NewSource(1))}
 	if err := g.Generate(ctx, "input.mp4", "output.jpg", 10.0); err == nil {
 		t.Fatal("expected error from failing ffmpeg command")
 	}
 }
 
+func TestFFmpegGenerator_Generate_RandomOffset(t *testing.T) {
+	ctx := context.Background()
+	var offsets []float64
+	fakeExecer := func(_ context.Context, name string, arg ...string) *exec.Cmd {
+		for i := 0; i < len(arg); i++ {
+			if arg[i] == "-ss" && i+1 < len(arg) {
+				off, err := strconv.ParseFloat(arg[i+1], 64)
+				if err != nil {
+					t.Fatalf("failed to parse offset: %v", err)
+				}
+				offsets = append(offsets, off)
+			}
+		}
+		return exec.Command("true")
+	}
+
+	// Use two generators with different seeds.
+	g1 := &FFmpegGenerator{execer: fakeExecer, rnd: rand.New(rand.NewSource(time.Now().UnixNano()))}
+	g2 := &FFmpegGenerator{execer: fakeExecer, rnd: rand.New(rand.NewSource(time.Now().UnixNano() + 12345))}
+
+	for i := 0; i < 5; i++ {
+		if err := g1.Generate(ctx, "input.mp4", "out.jpg", 100.0); err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if err := g2.Generate(ctx, "input.mp4", "out.jpg", 100.0); err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	}
+
+	if len(offsets) != 10 {
+		t.Fatalf("expected 10 offsets, got %d", len(offsets))
+	}
+
+	// Check that not all offsets are identical (should be extremely unlikely with different seeds).
+	allSame := true
+	for i := 1; i < len(offsets); i++ {
+		if offsets[i] != offsets[0] {
+			allSame = false
+			break
+		}
+	}
+	if allSame {
+		t.Fatal("expected different random offsets, but all were identical")
+	}
+}
+
 func TestMockGenerator(t *testing.T) {
 	ctx := context.Background()
 	m := &MockGenerator{}
@@ -78,3 +127,15 @@ func TestMockGenerator(t *testing.T) {
 		t.Fatal("expected error from mock generator")
 	}
 }
+
+func TestNewFFmpegGenerator_Seeded(t *testing.T) {
+	g := NewFFmpegGenerator()
+	if g.rnd == nil {
+		t.Fatal("expected rnd to be initialized")
+	}
+	// Generate a value to ensure the source is functional.
+	v := g.rnd.Float64()
+	if v < 0 || v >= 1 {
+		t.Fatalf("expected float in [0,1), got %v", v)
+	}
+}
-- 
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