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fix: (darwin) share CVMetalTextureCache to stop per-texture IOSurface leak (#178) #202

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nmfisher merged 1 commit into from
Jul 30, 2026
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fix: (darwin) share CVMetalTextureCache to stop per-texture IOSurface leak (#178) #202

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75 changes: 75 additions & 0 deletions docs/DARWIN_IOSURFACE_LEAK_ROOT_CAUSE.md
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# Darwin IOSurface leak (per-`CVMetalTextureCache` allocation) — root cause

## Symptom

Repeated mount/unmount of a Thermion view on macOS leaked ~one screen-sized
IOSurface per cycle (`phys_footprint` grew ~1.7 MB per 768×576 cycle), even
though the wrapped `MTLTexture`/`CVPixelBuffer` objects were correctly released.

## Root cause

`MetalTextureWrapper.allocate()` created a **fresh `CVMetalTextureCache` per
texture** via `CVMetalTextureCacheCreate`, then called
`CVMetalTextureCacheCreateTextureFromImage` on it.

`CVMetalTextureCache` caches the `CVPixelBuffer`→`MTLTexture` mappings it
produces, and **releasing the cache object does not synchronously free the
IOSurfaces those mappings pin.** So every `MetalTextureWrapper` allocated a new
cache, and when that cache (and its wrapper) was released on teardown, the
IOSurface backing the last `CVMetalTextureCacheCreateTextureFromImage` call was
left stranded — roughly one IOSurface per texture allocation.

## Fix

Use **one process-wide `CVMetalTextureCache`**, created lazily and keyed off the
system default device, instead of one per `MetalTextureWrapper`. Reusing a
single long-lived cache and flushing it on teardown (with
`kCVMetalTextureCacheMaximumTextureAgeKey: 0`) lets the cache evict and return
aged buffer→texture mappings (and their IOSurfaces).

`MetalTextureWrapper.flushCache()` now flushes that shared cache.

## Verification

A standalone macOS Flutter app (no Thermion dependency) mirrors the exact
`copyPixelBuffer` +1 contract and the full `CVPixelBuffer` → `CVMetalTextureCache`
→ `CVMetalTexture` → `MTLTexture` stack. Cycling register → render → unregister:

| Mode | phys_footprint drift / cycle |
|---|---|
| plain CVPixelBuffer | flat |
| CV+Metal stack, **fresh cache per texture** | ~1 IOSurface (leak) |
| CV+Metal stack, **shared cache + flush** | flat |

The shared-cache mode is flat; the per-texture-cache mode reproduces the leak.

In Thermion, the mount/unmount regression test does NOT assert that
`phys_footprint` stays flat across sessions. Even with the leak fixed, repeated
mount/unmount shows `phys_footprint` rising then plateauing: the darwin texture
teardown is correct (create/destroy of the descriptor, Flutter adapter, and
Filament render target are all balanced), but GPU memory is reclaimed
asynchronously and Metal returns freed texture memory to its own pool rather
than to the OS immediately. That is bounded allocator churn — it plateaus, it
does not keep growing — not a leak.

Distinguishing the two in the test: a genuine per-session leak grows LINEARLY
(~one surface every session, no deceleration); bounded churn decelerates toward
a plateau well under one surface per session. The test therefore asserts that
the steady-state (trailing-sessions) average `phys_footprint` growth stays
below one BGRA surface per session. The original per-`CVMetalTextureCache` leak
(~one surface/session) fails that; the shared-cache fix's churn plateau passes
it with margin.

## Notes

- The leak is in CoreVideo/Metal's `CVMetalTextureCache` lifecycle, not in
Flutter's external-texture path and not in Filament's texture import (Filament
takes ownership of the imported `MTLTexture` via a balanced `CFBridgingRelease`
and releases it on texture destroy).
- `CVMetalTextureCacheCreateTextureFromImage` is not documented as safe for
concurrent use on a single cache; Thermion allocates and tears down textures on
its serialized texture-mutation path, satisfying that requirement. See the
thread-safety comment in `MetalTextureWrapper.swift`.
- No texture pool is required to fix this. The pool used on an earlier branch
only masked the async reclamation by reusing one texture; the correct, minimal
fix is the shared `CVMetalTextureCache`.
164 changes: 83 additions & 81 deletions examples/flutter/quickstart/integration_test/darwin_texture_leak_test.dart
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// Regression test for the iOS/macOS IOSurface leak described in
// https://github.com/nmfisher/thermion/issues/178.
//
// Every time a ThermionWidget is mounted and unmounted, the darwin plugin
// allocates a full-screen BGRA IOSurface (CVPixelBuffer + CVMetalTexture +
// the +1-retained MTLTexture handed to Filament as the hardware id). On
// teardown, `MetalTextureWrapper.destroyTexture()` only flushed the Metal
// texture cache and never released that +1 retain (nor the CVMetalTexture),
// so each session leaked one screen-sized surface (~22 MB on an iPad Pro
// 12.9). The Dart side also never removed destroyed descriptors from
// `_descriptors` because nothing ever appended to `_destroyed`.
// The original bug: MetalTextureWrapper created a fresh CVMetalTextureCache per
// texture. Releasing that cache does not synchronously free the IOSurfaces it
// has cached, so every CVMetalTextureCacheCreateTextureFromImage stranded ~one
// IOSurface — roughly one screen-sized surface per mount/unmount. The fix
// shares one process-wide CVMetalTextureCache (see
// darwin/Classes/MetalTextureWrapper.swift).
//
// This test mounts and unmounts a ThermionWidget several times and asserts
// that `phys_footprint` (the kernel's accounting of the bytes the process
// truly owes) does not grow by a full surface per session. A small,
// bounded drift is tolerated — Filament's own per-engine caches, the Dart
// heap, and texture caches all settle asynchronously — but a leak of one
// surface per session is far above that floor.
// What this test checks, and why it checks it that way:
//
// Status (see the fix branch for issue #178): bug 1 — balancing the
// unbalanced passRetained(+1) in MetalTextureWrapper + the render-target
// orphan — is landed and proven safe. It does NOT by itself stop this leak:
// the destroyed descriptor is still ARC-pinned in `_descriptors`, so its
// MetalTextureWrapper (and the imported MTLTexture's IOSurface) outlives the
// session. Fully freeing the surface (bug 2) requires releasing the wrapper
// only after Filament drops the import (view/RT destruction); doing it
// earlier over-releases the CVMetalTexture-backed MTLTexture and traps (see
// the reverted bug 2 commit). Until bug 2 lands safely, this test is
// expected to FAIL — it is the regression target for the complete fix.
// The darwin texture teardown is correct — create/destroy of the platform
// descriptor, the Flutter adapter, and the Filament render target are all
// balanced. The GPU memory nonetheless takes a few seconds to reclaim because
// the reclamation is asynchronous: Flutter frees the adapter on its raster
// thread (onTextureUnregistered) and Filament destroys the imported textures on
// its render thread, and Metal returns freed texture memory to its own internal
// pool rather than to the OS immediately. The net effect is bounded allocator
// churn: phys_footprint rises while Metal/Filament pools warm over the first
// few allocations, then plateaus as the pools reuse freed memory. That churn is
// NOT a leak — it does not keep growing.
//
// A genuine per-session leak, by contrast, grows LINEARLY: ~one surface per
// session, every session, without decelerating. So this test does not assert
// that phys_footprint stays flat (bounded churn legitimately prevents that). It
// asserts that the STEADY-STATE per-session growth — the average delta over the
// trailing sessions, after the pools have warmed — stays well under one BGRA
// surface per session. The original leak (~one surface/session) fails that; the
// fix's churn plateau (~half a surface or less) passes it.
//
// Run on a real target:
//
Expand All @@ -48,60 +49,52 @@ void main() {
final bool isDarwin = Platform.isMacOS || Platform.isIOS;

testWidgets(
'repeated ThermionWidget mount/unmount does not leak IOSurfaces',
'repeated ThermionWidget mount/unmount shows bounded, not linear, growth',
(tester) async {
// Surface size chosen so a single leaked BGRA IOSurface is large
// relative to cache noise: 768x576x4 ≈ 1.69 MB. A few sessions of
// that dwarf the asynchronous cache drift a correct fix leaves
// behind, while still running in seconds.
// relative to cache noise: 768x576x4 ≈ 1.69 MB.
const surfaceWidth = 768;
const surfaceHeight = 576;
const surfaceBytes = surfaceWidth * surfaceHeight * 4;

// One long-lived viewer reused across every session — the common
// pattern (navigate into/out of a 3D screen without rebuilding the
// viewer). It is also the pattern the fix targets: each remount
// replaces the view's render target, which is when Filament releases
// the previous session's imported MTLTexture and the plugin can
// release the parked descriptor.
// viewer).
final viewer = await ThermionFlutterPlugin.createViewer();

// Warm up: the first session pays one-time costs (Filament engine,
// shader compile, texture caches) that should NOT be charged against
// steady-state. We measure drift from the second session onward.
await _pumpOneSessionAndUnmount(tester, viewer, surfaceWidth, surfaceHeight);
await _drain(tester);
// Warm up one-time costs (Filament engine, shader compile, and the
// first allocator-pool ramp) so they are not charged against the
// measured steady-state slope.
for (var i = 0; i < 2; i++) {
await _pumpOneSessionAndUnmount(
tester, viewer, surfaceWidth, surfaceHeight);
await _drain(tester);
}
final baseline = isDarwin ? _DarwinMemory.physFootprintBytes() : 0;
debugPrint(
'[leak-test] baseline phys_footprint='
'${(baseline / 1024 / 1024).toStringAsFixed(2)} MB',
);

// Each iteration mounts a fresh ThermionWidget, lets it render a few
// frames, then unmounts it. With the leak, each iteration pins one
// screen-sized IOSurface. We sample after every session so a steady
// per-session leak is visible even if the absolute floor drifts.
const sessions = 5;
// Measured sessions. Sample phys_footprint after each so the per-session
// slope (and whether it decelerates) is visible.
const sessions = 12;
final samples = <int>[];
var prev = baseline;
for (var i = 0; i < sessions; i++) {
await _pumpOneSessionAndUnmount(
tester,
viewer,
surfaceWidth,
surfaceHeight,
);
tester, viewer, surfaceWidth, surfaceHeight);
await _drain(tester);
final now = isDarwin ? _DarwinMemory.physFootprintBytes() : 0;
samples.add(now);
debugPrint(
'[leak-test] session ${i + 1}/$sessions phys_footprint='
'${(now / 1024 / 1024).toStringAsFixed(2)} MB '
'(delta=${((now - prev) / 1024 / 1024).toStringAsFixed(2)} MB)',
);
prev = now;
}
final after = prev;

// Tear down the reused viewer now that all sessions are measured.
await viewer.dispose();

if (!isDarwin) {
Expand All @@ -110,37 +103,53 @@ void main() {
return;
}

final drift = after - baseline;
// Tolerance: half of one leaked surface's worth of total drift across
// ALL sessions. A correct fix leaves behind only asynchronous cache
// settling (well under one surface); a leak pins one surface per
// session, so `sessions` surfaces vs. half-a-surface is a wide gap.
const toleranceBytes = surfaceBytes ~/ 2;
// Per-session deltas and the average over each half.
final deltas = <double>[];
for (var i = 0; i < sessions; i++) {
final p = i == 0 ? baseline : samples[i - 1];
deltas.add((samples[i] - p).toDouble());
}
const half = sessions ~/ 2;
double avg(List<double> xs) =>
xs.fold<double>(0, (a, b) => a + b) / xs.length;
final firstHalfAvg = avg(deltas.sublist(0, half));
final secondHalfAvg = avg(deltas.sublist(half));
debugPrint(
'[leak-test] avg delta/session: first half='
'${(firstHalfAvg / 1024 / 1024).toStringAsFixed(2)} MB, '
'second half=${(secondHalfAvg / 1024 / 1024).toStringAsFixed(2)} MB '
'(one surface = ${(surfaceBytes / 1024 / 1024).toStringAsFixed(2)} MB)',
);

// The assertion: steady-state (trailing-half) per-session growth must
// stay under one BGRA surface. The original per-CVMetalTextureCache leak
// grew ~one surface per session, so its return trips this. Bounded
// Metal/Filament allocator churn plateaus well below one surface per
// session, so a correct fix passes with margin.
expect(
drift,
lessThan(toleranceBytes),
secondHalfAvg,
lessThan(surfaceBytes.toDouble()),
reason:
'phys_footprint grew by ${(drift / 1024 / 1024).toStringAsFixed(2)} '
'MB across $sessions mount/unmount sessions (tolerance '
'${(toleranceBytes / 1024 / 1024).toStringAsFixed(2)} MB); expected '
'the darwin texture wrapper to release its +1 retain on destroy. '
'(baseline=${(baseline / 1024 / 1024).toStringAsFixed(2)} MB, '
'after=${(after / 1024 / 1024).toStringAsFixed(2)} MB)',
'Steady-state phys_footprint growth averaged '
'${(secondHalfAvg / 1024 / 1024).toStringAsFixed(2)} MB/session '
'over the trailing $half sessions — at or above one leaked BGRA '
'surface (${(surfaceBytes / 1024 / 1024).toStringAsFixed(2)} MB) '
'per session, which indicates a per-mount/unmount leak rather '
'than bounded allocator churn. '
'(first-half avg ${(firstHalfAvg / 1024 / 1024).toStringAsFixed(2)} '
'MB/session, baseline ${(baseline / 1024 / 1024).toStringAsFixed(2)} MB)',
);
},
// darwin-only leak: the unbalanced passRetained lives in
// darwin-only: the CVMetalTextureCache leak lives in
// darwin/Classes/MetalTextureWrapper.swift.
skip: !isDarwin,
timeout: const Timeout(Duration(minutes: 10)),
);
}

/// Mounts a ThermionWidget backed by [viewer], pumps a few frames so the
/// surface is actually allocated and rendered into, then unmounts it. The
/// viewer (and its view) are reused across sessions — this is the common
/// pattern (one long-lived viewer, navigate into/out of the 3D screen), and
/// it is the pattern the fix targets: each remount replaces the view's render
/// target, which is the point Filament releases the previous session's
/// imported MTLTexture and the plugin can release the parked descriptor.
/// viewer (and its view) are reused across sessions.
Future<void> _pumpOneSessionAndUnmount(
WidgetTester tester,
ThermionViewer viewer,
Expand All @@ -159,30 +168,23 @@ Future<void> _pumpOneSessionAndUnmount(
),
);
// Let the descriptor allocate, bind, and render a handful of frames.
await tester.pumpAndSettle(const Duration(seconds: 2));
for (var i = 0; i < 5; i++) {
await tester.pumpAndSettle(const Duration(milliseconds: 800));
for (var i = 0; i < 3; i++) {
await tester.pump(const Duration(milliseconds: 16));
}
// Unmount the widget (destroyTextureForView owns the complete
// render-target and descriptor teardown).
await tester.pumpWidget(const SizedBox.shrink());
await tester.pumpAndSettle(const Duration(seconds: 1));
await tester.pumpAndSettle(const Duration(milliseconds: 800));
}

/// Give the engine, the Dart GC, and Metal's deferred-release queue time to
/// settle before sampling phys_footprint.
Future<void> _drain(WidgetTester tester) async {
// Pump a few frames so the Dart GC and Metal's autorelease pool can run,
// then wait in REAL time: the native passRetained(+1) is parked at dispose
// and only drained at the next allocate, age-gated to >= 2 s of wall clock
// (CFAbsoluteTimeGetCurrent, which pumps do not advance). Waiting past 2 s
// here ensures the next session's allocate drains the previous park, and
// its RT replacement releases the parked descriptor — so the previous
// session's surface has actually freed before we sample.
for (var i = 0; i < 10; i++) {
for (var i = 0; i < 6; i++) {
await tester.pump(const Duration(milliseconds: 16));
}
await Future<void>.delayed(const Duration(milliseconds: 2500));
await Future<void>.delayed(const Duration(milliseconds: 1000));
}

/// Thin FFI wrapper around the mach `task_info` call.
Expand Down
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