[EduardoAndradeMachado]

Summary

An export renders each batch's frames in parallel, so a clip's first frames reach MediaPool::resolve at the same moment. resolve read the cache, opened the file on a miss and stored the source afterwards, so every worker missed and every worker opened the file: one MediaSource, one GopCache and one hardware decoder each, each walking the GOP from the sync sample to deliver a single frame. Only the last source stored survived; the others were dropped with their decoders. perf.stats showed it as one hardware session and one seek per renderer for every clip an export entered.

The change

• One open per item. MediaPool holds a lock per item while that item is being opened (MediaPool::opening). The first worker opens; the others wait, re-check the cache under the lock and share the one source. An item already in the pool takes the read-locked lookup as before, and different items still open in parallel. The same gate covers proxy_source.
• The open runs on a pool of its own (codecs::gop::isolated_open, one thread per core, beside the decode pool of Export stops making frames for good with two or three iPhone HEVC clips (Linux, 0.4.0); full-range iPhone video probed as limited range #480). An opener that decodes as it opens (the EXR still, par_chunks_mut) starts rayon work; a frame worker waiting inside that work on its own pool could pick up another frame job of the batch, which would block on the gate that worker holds. Opens do not run on the decode pool, so a slow file never holds up the caches' decoding. An open started from an open runs in place.
• No pool, no gate. Where the open pool cannot be made, and on the web build (no threads), the open runs on the asking thread without the gate, as before. A deferred-read mark (media::pending) the thread already had stays set across the open, so a frame with a layer still being fetched is still known to be incomplete.
• A failing proxy is tried once. A proxy that fails to open is not tried again for that item for one second (PROXY_RETRY), so the requests waiting on the gate fall back to the original instead of each paying for the failure in turn (a share that times out).

How it showed, before → after

GPU export (gpuRendering and hardwareEncoding auto, NVENC), 1080p30 H.264 + AAC output, a timeline of 1080p H.264 camera clips at 30 and 59.94 fps with 0.2 s dip-to-black transitions; perf.stats decode after the export; Xeon E5-2680 v4 14C/28T, RTX 5060, Windows 11, release CLI; two alternated rounds on an otherwise idle machine, this PR against main at the same base.

Range	Build	HW sessions	seeks	samples decoded	decodeMs
5 s (1 media item, 150 frames)	main	4	5	621	2904–2942
5 s	this PR	1	2	348–349	682–694
5 s	main, RAYON_NUM_THREADS=1 (cannot race)	1	2	348	597
20 s (3 media items, 600 frames)	main	10	11	1436–1438	5501–5531
20 s	this PR	3	4–6	926–1173	1973–2342

Hardware sessions are HybridDecoder::new calls (one per decoder created). After the change the export opens one source per media item and, in the 20 s range, seeks once per clip entered, matching a single-threaded run, which cannot race. The samples decoded above the frames requested that remain are the GOP walks at clip in points and the 59.94 fps sources decoded for 30 fps output (not duplicate work; a follow-up).

Time (CLI process start to exit / the export's own seconds / process CPU / GPU average from nvidia-smi dmon):

Range	Build	wall	export	process CPU	GPU
5 s	main	4.6 s, 4.0 s	2.8 s, 2.7 s	10.4 s, 9.9 s	11 %, 24 %
5 s	this PR	3.9 s, 3.5 s	2.2 s, 2.2 s	9.4 s, 10.2 s	13 %, 26 %
20 s	main	9.4 s, 9.0 s	8.1 s, 7.6 s	35.5 s, 38.7 s	33 %, 30 %
20 s	this PR	8.7 s, 8.5 s	7.3 s, 7.1 s	35.4 s, 36.0 s	30 %, 27 %

The export is 20 % shorter over 5 s (one clip entered, where the duplicate GOP walks were most of the decode) and 8 % over 20 s. The duplicate decoders ran on the hardware decoder alongside the renderers, so the change removes more decode work (decodeMs −58 to −77 %) than wall time; process CPU is unchanged within noise. The round with 6 seeks had a late frame request behind the one decoder's position, which costs a seek whatever the number of sources.

Outputs are byte-identical before and after (cmp on every 5 s and 20 s export; PSNR inf on every frame).

Tests

All in crates/engine/src/media_pool.rs. The requests run as frame jobs on a rayon pool (as an export renders a batch) against a Services whose reader takes 50 ms and records the opens, how many overlap and the thread each ran on. No hardware, no fonts. Planted bugs, one at a time:

Test	Planted bug	Result
parallel_first_requests_open_an_item_once	no gate (main) / no re-check under the gate	8 opens instead of 1
parallel_first_requests_open_a_proxy_once	proxy path without the gate	8 opens instead of 1
parallel_first_requests_try_a_missing_proxy_once	no PROXY_RETRY check	9 opens instead of 2
different_items_open_in_parallel	one gate for every item	the two opens never overlap
opens_run_on_the_open_pool	open run inline on the asking thread / on the decode pool	opened on frame-* / decode-*
an_opener_running_rayon_work_never_deadlocks_the_frame_workers	open run inline on the asking thread	the batch is stuck; fails after 30 s
opening_keeps_the_threads_deferred_read_mark	the thread's earlier mark not restored	the earlier mark is lost

rayon is added as a dev-dependency of filmcraft-engine for these tests.

Known and left as is

• The proxy and the original of one item share one gate, so an export's first open of an item waits behind a preview's proxy open of it.
• If the wait thread cannot be spawned (out of resources), isolated_open runs the open on the asking thread, as isolated already does for decoding.
• An open that fails (or panics on the open pool) caches the unreadable slate, as before; the waiters now share that result instead of racing to store their own.

Checks

cargo fmt --check, cargo clippy --workspace --all-targets -D warnings, cargo nextest run --workspace (3588 tests) + doctests, cargo xtask layers, assets, wasm: green, apart from failures that main has on Windows (relink_tests.rs unused import index_folder, virtual_extraction_keeps_existing_media, nvenc_hevc_hdr_export::the_factory_rejects_mismatched_pictures_and_clamps_hostile_floats) and play_ahead, which fails only under the parallel run's load and passes alone.

Docs: docs/performance.md (new results section), docs/architecture.md (sources paragraph).

Summary by CodeRabbit

• Performance
  • Parallel exports now share a single open of the same media source, reducing duplicate reads and decoding while allowing different sources to open concurrently.
  • If a proxy cannot be opened, exports fall back to the full-resolution media. Repeated attempts to open the same failed proxy are temporarily avoided.
  • Exported output remains unchanged.
• Documentation
  • Updated architecture and performance documentation with information and benchmark results for media-source opening during exports.

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📝 Walkthrough

Media source opening now uses a pool separate from decoding on non-Wasm targets. MediaPool coordinates concurrent source and proxy opens per item, reuses cached sources, and suppresses retries for recently failed proxies. Tests and documentation cover the coordination and report benchmark results.

Changes: Media Source Opening

Layer / File(s)	Summary
Separate opening and decode pools crates/codecs/src/gop.rs	Non-Wasm builds use separate pools for decoding and opening. The isolation helpers handle Rayon callers, non-Rayon callers, and unavailable pools. Calls from the open pool run directly, and Wasm execution remains in the cancellation context.
Coordinate source and proxy opens crates/engine/src/media_pool.rs	MediaPool uses per-item gates and keyed cache checks so concurrent requests can share opened sources. Proxy failures are recorded for one second before another attempt. Proxy errors fall back to full-resolution media. Opens carry deferred-read state.
Validate and describe coordinated opens crates/engine/Cargo.toml, crates/engine/src/media_pool.rs, docs/architecture.md, docs/performance.md	Tests cover shared and parallel opens, proxy fallback, opener thread placement, deadlock avoidance, and deferred-read marks. Documentation describes the coordination and reports benchmark measurements.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant FrameWorker
  participant MediaPool
  participant IsolatedOpener
  FrameWorker->>MediaPool: resolve item
  MediaPool->>MediaPool: check cache and acquire item gate
  MediaPool->>IsolatedOpener: open source through isolated_open
  IsolatedOpener-->>MediaPool: return source and deferred-read state
  MediaPool-->>FrameWorker: return shared source

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Possibly related PRs

• storytold/filmcraft#3016: Both changes coordinate concurrent source opens per item in MediaPool and reuse the cached source.

Merge Risk: ⚪ Minimal · up to 3049e

No actionable risk remains in the selected documentation update; it is mergeable after normal checks.

Security architecture risk: 🟡 Moderate · up to e0ec5

The change reduces duplicate work, but a thread-creation failure can leave a render permanently blocked instead of recovering. The identified risk concerns application availability; broader access or increased privileges were not established.

Retained concerns

• Medium · reliability · inferred: If the open pool exists but spawning its waiting thread fails, the fallback runs opening on the frame worker while its item gate remains locked. A Rayon-yielding opener can execute another request for that item and block on its own gate, stranding the render even after thread capacity recovers. The no-pool gate bypass does not cover this distinct failure state.

Security review details

Security Blast Radius

• inferred — The identified failure can strand requests sharing the affected item's gate and prevent their render batch from completing. The inspected path establishes an in-process availability impact, not cross-tenant access, credential exposure, or privilege escalation.

Security Findings and Attack Paths

• inferred — A conditional availability path exists when opening media invokes Rayon work, concurrent requests target the same item, and creation of the waiting thread fails. The source supports the resulting lock cycle, but neither a practical attacker-controlled thread-exhaustion route nor an independent remote trigger was established.

Trust Boundaries and Controls

• observed — Native filesystem access and web virtual-file access remain owned by their Services implementations. Services is already Send and Sync. The checked changes preserve path resolution and do not add an alternate privileged opening route.

Resilience and Maintainability Implications

• observed — The normal waiting-thread path prevents the frame worker from executing other Rayon jobs while holding an item gate. Added tests exercise ordinary pool placement and a yielding opener, but their inspected setup does not force waiting-thread creation to fail.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 76.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 2 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title names the two main areas touched by the change (engine and codecs) and the main behavior: opening an item's media source once when parallel first requests arrive. The raw summary confirms a …
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title names the two main areas touched by the change (engine and codecs) and the main behavior: opening an item's media source once when parallel first requests arrive. The raw summary confirms a per-item open gate in media_pool.rs, the isolated_open helper in codecs gop.rs, and matching docs. The title is specific and matches the PR objective of preventing duplicate first opens.

Full details: Docstring Coverage

Explanation

Docstring coverage is 76.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 2 files. (1 skipped: 1 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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Inline comments:
Review comments at @docs/performance.md:
- Line 412: Update the published `decodeMs` reduction range in the performance
description to start at −58 % instead of −60 %, preserving the −77 % upper bound
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[Exalted5373]

I tested this on Linux with NVDEC decoding and NVENC encoding, on a 24-core laptop CPU with an RTX 50-series laptop GPU, release CLI. The export was 8 s (192 frames) of 1080p23.976 H.264 + AAC to H.264. This PR was merged locally on top of #3068, which sends untouched frames to the encoder as decoded and so makes the export decode-bound. The comparison is #3068 alone against #3068 + this PR, three alternating rounds:

	#3068	#3068 + #3046
NVENC export	1.37–1.45 s	0.93–0.94 s
process CPU	6.0–6.2 s	2.1–2.2 s
hardware decoder sessions	16	1
seeks	16	1
frames decoded (192 asked)	322–326	193
decodeMs	1916–2091	784–797

• Byte-identical: both the software export and the NVENC export have the same MD5 with and without this PR.
• Software export: with the built-in encoder, wall time stayed within noise because that export is bound by the encoder.

[echelon]

Thanks for this. We're holding it for a maintainer review: it changes how the engine opens media under concurrent first requests and adds a dependency, and that kind of concurrency change needs closer testing than we can give it in a batch.

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[EduardoAndradeMachado]

Thanks! A note on the dependency: rayon is already a workspace dependency (used by codecs, export, gpu, h264 and others); engine only adds the workspace entry, with no new crate in Cargo.lock. Exalted5373's Linux NVDEC run above also confirms the effect independently (16 → 1 decoder sessions, byte-identical output).