fix(windows): release FFmpeg memory and capture native crashes

pkg/download/crash_log_test.go

@@ -0,0 +1,26 @@
+package download
+
+import (
+	"os"
+	"path/filepath"
+	"runtime/debug"
+	"testing"
+)
+
+func TestLogPanicPreservesPreviousCrash(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "crash.log")
+	const previous = "previous crash traceback\n"
+	if err := os.WriteFile(path, []byte(previous), 0600); err != nil {
+		t.Fatal(err)
+	}
+	logPanic(dir)
+	t.Cleanup(func() { _ = debug.SetCrashOutput(nil, debug.CrashOptions{}) })
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != previous {
+		t.Fatalf("previous crash was overwritten: %q", data)
+	}
+}

pkg/download/downloader.go

@@ -2659,11 +2659,12 @@ func logPanic(logDir string) {
 	if err := util.CreateDirIfNotExist(logDir); err != nil {
 		return
 	}
-	f, err := os.Create(filepath.Join(logDir, "crash.log"))
+	f, err := os.OpenFile(filepath.Join(logDir, "crash.log"), os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0600)
 	if err != nil {
 		return
 	}
-	debug.SetCrashOutput(f, debug.CrashOptions{})
+	defer f.Close() // SetCrashOutput duplicates the descriptor.
+	_ = debug.SetCrashOutput(f, debug.CrashOptions{})
 }
 
 func (d *Downloader) assignFetcherManager(task *Task) error {

pkg/download/engine/ffmpeg/README.md

@@ -174,3 +174,12 @@ compilation memory, host buffers and JS allocations are additional to the
 linear-memory limit. Playback support depends on the selected codecs, not just
 the `.mp4` suffix. Platform execution and performance must be tested on target
 devices, especially mobile/32-bit devices.
+
+On 64-bit Windows, instance linear memory uses a reserved virtual address range with
+pages committed as it grows. Module completion, errors and cancellation release
+this range directly through `VirtualFree`, rather than waiting for Go's garbage
+collector to reclaim large buffers. Growing an instance preserves its address
+without copying the previous buffer. The 512 MiB limit is unchanged; reservation
+does not commit that entire amount. Shared compilation remains resident after
+the first merge, so process memory is not expected to return exactly to the
+pre-FFmpeg baseline. This does not diagnose unrelated idle memory growth.

pkg/download/engine/ffmpeg/memory.go

@@ -0,0 +1,32 @@
+package ffmpeg
+
+import (
+	"context"
+	"sync"
+
+	"github.com/tetratelabs/wazero/experimental"
+)
+
+// The runtime does not always invoke Free when a module's start function traps.
+// Keep host ownership until execution returns, including failed instantiation.
+func withOwnedMemory(ctx context.Context, allocator experimental.MemoryAllocator) (context.Context, func()) {
+	var memories []*ownedLinearMemory
+	ctx = experimental.WithMemoryAllocator(ctx, experimental.MemoryAllocatorFunc(
+		func(cap, max uint64) experimental.LinearMemory {
+			memory := &ownedLinearMemory{LinearMemory: allocator.Allocate(cap, max)}
+			memories = append(memories, memory)
+			return memory
+		}))
+	return ctx, func() {
+		for _, memory := range memories {
+			memory.Free()
+		}
+	}
+}
+
+type ownedLinearMemory struct {
+	experimental.LinearMemory
+	once sync.Once
+}
+
+func (m *ownedLinearMemory) Free() { m.once.Do(m.LinearMemory.Free) }

pkg/download/engine/ffmpeg/memory_other.go

@@ -0,0 +1,7 @@
+//go:build !windows || (!amd64 && !arm64)
+
+package ffmpeg
+
+import "context"
+
+func withLinearMemory(ctx context.Context) (context.Context, func()) { return ctx, func() {} }

pkg/download/engine/ffmpeg/memory_test.go

@@ -0,0 +1,88 @@
+package ffmpeg
+
+import (
+	"context"
+	"encoding/hex"
+	"testing"
+	"time"
+
+	"github.com/tetratelabs/wazero"
+	"github.com/tetratelabs/wazero/experimental"
+)
+
+// Verify the pinned runtime's allocator lifecycle on every development host:
+// host-owned buffers must not be released before execution ends, or retained
+// after failed instantiation, traps or cancellation.
+func TestLinearMemoryModuleLifecycle(t *testing.T) {
+	testLinearMemoryModuleLifecycle(t, func(_, _ uint64) experimental.LinearMemory {
+		return &testLinearMemory{}
+	})
+}
+
+type testLinearMemory struct{ buffer []byte }
+
+func (m *testLinearMemory) Reallocate(size uint64) []byte {
+	m.buffer = append(m.buffer, make([]byte, int(size)-len(m.buffer))...)
+	return m.buffer
+}
+func (m *testLinearMemory) Free() { m.buffer = nil }
+
+type trackedLinearMemory struct {
+	experimental.LinearMemory
+	freeCount int
+}
+
+func (m *trackedLinearMemory) Free() {
+	m.freeCount++
+	m.LinearMemory.Free()
+}
+
+func testLinearMemoryModuleLifecycle(t *testing.T, allocate experimental.MemoryAllocatorFunc) {
+	t.Helper()
+	for _, tt := range []struct {
+		name string
+		// A module with one memory (min=1/max=2 pages), optionally a start
+		// function that traps or loops until its context is cancelled.
+		wasm    string
+		wantErr bool
+	}{
+		{"close", "0061736d01000000050401010102", false},
+		{"trap", "0061736d01000000010401600000030201000504010101020801000a05010300000b", true},
+		{"cancel", "0061736d01000000010401600000030201000504010101020801000a0901070003400c000b0b", true},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+			defer cancel()
+			var memory *trackedLinearMemory
+			ctx, release := withOwnedMemory(ctx, experimental.MemoryAllocatorFunc(
+				func(_, max uint64) experimental.LinearMemory {
+					memory = &trackedLinearMemory{LinearMemory: allocate(0, max)}
+					return memory
+				}))
+			defer release()
+			rt := wazero.NewRuntimeWithConfig(ctx, wazero.NewRuntimeConfig().WithCloseOnContextDone(true))
+			defer rt.Close(context.Background())
+			binary, err := hex.DecodeString(tt.wasm)
+			if err != nil {
+				t.Fatal(err)
+			}
+			module, err := rt.Instantiate(ctx, binary)
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("instantiate: %v", err)
+			}
+			if !tt.wantErr {
+				if memory == nil || memory.freeCount != 0 {
+					t.Fatal("module did not allocate native memory")
+				}
+				if err := module.Close(ctx); err != nil {
+					t.Fatal(err)
+				}
+			}
+			release()
+			release()
+			if memory == nil || memory.freeCount != 1 {
+				t.Fatal("host did not release memory exactly once")
+			}
+		})
+	}
+}

pkg/download/engine/ffmpeg/memory_windows.go

@@ -0,0 +1,70 @@
+//go:build windows && (amd64 || arm64)
+
+package ffmpeg
+
+import (
+	"context"
+	"fmt"
+	"unsafe"
+
+	"github.com/tetratelabs/wazero/experimental"
+	"golang.org/x/sys/windows"
+)
+
+// Keep large, short-lived WASM buffers out of the Go heap. Closing a module
+// releases its committed pages immediately, without waiting for GC/scavenging.
+func withLinearMemory(ctx context.Context) (context.Context, func()) {
+	return withOwnedMemory(ctx, experimental.MemoryAllocatorFunc(
+		func(_, max uint64) experimental.LinearMemory {
+			return &windowsLinearMemory{max: max}
+		}))
+}
+
+// Reserve the address range once, committing zero-filled pages on demand. This
+// avoids both copying the previous buffer and temporarily retaining two large
+// Go allocations when memory.grow runs. The backing address never changes.
+type windowsLinearMemory struct {
+	address uintptr
+	max     uint64
+	size    uint64
+	freed   bool
+}
+
+func (m *windowsLinearMemory) Reallocate(size uint64) []byte {
+	if m.freed || size > m.max || m.max > uint64(^uint(0)>>1) {
+		return nil
+	}
+	if size == 0 {
+		return []byte{}
+	}
+	if m.address == 0 {
+		address, err := windows.VirtualAlloc(0, uintptr(m.max), windows.MEM_RESERVE, windows.PAGE_READWRITE)
+		if err != nil {
+			panic(fmt.Errorf("ffmpeg: reserve WASM memory: %w", err))
+		}
+		m.address = address
+	}
+	if size > m.size {
+		_, err := windows.VirtualAlloc(m.address, uintptr(size), windows.MEM_COMMIT, windows.PAGE_READWRITE)
+		if err != nil {
+			if m.size == 0 {
+				// Instantiation cannot proceed without the minimum memory. Wazero
+				// has not taken ownership yet, so release the reservation here.
+				m.Free()
+				panic(fmt.Errorf("ffmpeg: commit WASM memory: %w", err))
+			}
+			return nil // memory.grow reports failure to the guest.
+		}
+		m.size = size
+	}
+	return unsafe.Slice((*byte)(unsafe.Pointer(m.address)), int(size))
+}
+
+func (m *windowsLinearMemory) Free() {
+	if m.address != 0 {
+		_ = windows.VirtualFree(m.address, 0, windows.MEM_RELEASE)
+		m.address = 0
+	}
+	m.size = 0
+	m.freed = true
+}

pkg/download/engine/ffmpeg/memory_windows_test.go

@@ -0,0 +1,54 @@
+//go:build windows && (amd64 || arm64)
+
+package ffmpeg
+
+import (
+	"testing"
+	"unsafe"
+
+	"github.com/tetratelabs/wazero/experimental"
+	"golang.org/x/sys/windows"
+)
+
+func TestWindowsLinearMemoryGrowthAndRelease(t *testing.T) {
+	const page = 65536
+	m := &windowsLinearMemory{max: 4 * page}
+	defer m.Free()
+	first := m.Reallocate(page)
+	if len(first) != page {
+		t.Fatalf("initial allocation: %d", len(first))
+	}
+	first[0], first[page-1] = 17, 29
+	address := m.address
+	grown := m.Reallocate(3 * page)
+	if unsafe.SliceData(first) != unsafe.SliceData(grown) || grown[0] != 17 || grown[page-1] != 29 {
+		t.Fatal("growing memory moved or lost existing bytes")
+	}
+	for _, b := range grown[page:] {
+		if b != 0 {
+			t.Fatal("newly committed memory is not zero-filled")
+		}
+	}
+	if m.Reallocate(5*page) != nil || m.address != address || m.size != 3*page {
+		t.Fatal("failed growth changed the existing allocation")
+	}
+	m.Free()
+	m.Free()
+	if m.address != 0 || m.Reallocate(page) != nil {
+		t.Fatal("freed memory can still be used")
+	}
+	var info windows.MemoryBasicInformation
+	if err := windows.VirtualQuery(address, &info, unsafe.Sizeof(info)); err != nil {
+		t.Fatal(err)
+	}
+	const memFree = 0x10000 // MEMORY_BASIC_INFORMATION.State: MEM_FREE.
+	if info.State != memFree {
+		t.Fatalf("pages remain reserved after Free: state=%x", info.State)
+	}
+}
+
+func TestWindowsLinearMemoryModuleLifecycle(t *testing.T) {
+	testLinearMemoryModuleLifecycle(t, func(_, max uint64) experimental.LinearMemory {
+		return &windowsLinearMemory{max: max}
+	})
+}

pkg/download/engine/ffmpeg/runtime.go

@@ -151,7 +151,9 @@ func RunAcquired(ctx context.Context, video, audio Input, out io.Writer, format
 	files := &inputFS{files: map[string]Input{"video": video, "audio": audio}}
 	fsconfig := wazero.NewFSConfig().(sysfs.FSConfig).WithSysFSMount(files, "/inputs")
 	logs := &tailLog{}
-	rc, err := wasm.Run(ctx, shared.runtime, shared.module, wasm.Args{
+	runCtx, freeMemory := withLinearMemory(ctx)
+	defer freeMemory()
+	rc, err := wasm.Run(runCtx, shared.runtime, shared.module, wasm.Args{
 		Name: "ffmpeg", Args: args, Stdout: out, Stderr: logs,
 		Config: func(c wazero.ModuleConfig) wazero.ModuleConfig { return c.WithFSConfig(fsconfig) },
 	})