Async FilesystemStore #3931

joostjager · 2025-07-15T13:12:41Z

Async filesystem store with eventually consistent writes. It is just using tokio's spawn_blocking, because that is what tokio::fs would otherwise do as well. Using tokio::fs would make it complicated to reuse the sync code.

ldk-node try out: lightningdevkit/ldk-node@main...joostjager:ldk-node:async-fsstore

ldk-reviews-bot · 2025-07-15T13:12:44Z

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tnull · 2025-07-15T13:49:38Z

lightning-persister/src/fs_store_async.rs

+		let this = Arc::clone(&self.inner);
+
+		Box::pin(async move {
+			tokio::task::spawn_blocking(move || {


Mhh, so I'm not sure if spawning blocking tasks for every IO call is the way to go (see for example https://docs.rs/tokio/latest/tokio/fs/index.html#tuning-your-file-io: "To get good performance with file IO on Tokio, it is recommended to batch your operations into as few spawn_blocking calls as possible."). Maybe there are other designs that we should at least consider before moving forward with this approach. For example, we could create a dedicated pool of longer-lived worker task(s) that process a queue?

If we use spawn_blocking, can we give the user control over which runtime this exactly will be spawned on? Also, rather than just doing wrapping, should we be using tokio::fs?

Mhh, so I'm not sure if spawning blocking tasks for every IO call is the way to go (see for example https://docs.rs/tokio/latest/tokio/fs/index.html#tuning-your-file-io: "To get good performance with file IO on Tokio, it is recommended to batch your operations into as few spawn_blocking calls as possible.").

If we should batch operations, I think the current approach is better than using tokio::fs? Because it already batches the various operations inside kvstoresync::write.

Further batching probably needs to happen at a higher level in LDK, and might be a bigger change. Not sure if that is worth it just for FIlesystemStore, especially when that store is not the preferred store for real world usage?

For example, we could create a dedicated pool of longer-lived worker task(s) that process a queue?

Isn't Tokio doing that already when a task is spawned?

If we use spawn_blocking, can we give the user control over which runtime this exactly will be spawned on? Also, rather than just doing wrapping, should we be using tokio::fs?

With tokio::fs, the current runtime is used. I'd think that that is then also sufficient if we spawn ourselves, without a need to specifiy which runtime exactly?

More generally, I think the main purpose of this PR is to show how an async kvstore could be implemented, and to have something for testing potentially. Additionally if there are users that really want to use this type of store in production, they could. But I don't think it is something to spend too much time on. A remote database is probably the more important target to design for.

With tokio::fs, the current runtime is used. I'd think that that is then also sufficient if we spawn ourselves, without a need to specifiy which runtime exactly?

Hmm, I'm not entirely sure, especially for users that have multiple runtime contexts floating around, it might be important to make sure the store uses a particular one (cc @domZippilli ?). I'll also have to think through this for LDK Node when we make the switch to async KVStore there, but happy to leave as-is for now.

tnull · 2025-07-15T13:50:24Z

lightning/src/util/persist.rs

 }

 /// Provides additional interface methods that are required for [`KVStore`]-to-[`KVStore`]
 /// data migration.
-pub trait MigratableKVStore: KVStore {
+pub trait MigratableKVStore: KVStoreSync {


How will we solve this for an KVStore?

I think this comment belongs in #3905?

We might not need to solve it now, as long as we still require a sync implementation alongside an async one? If we support async-only kvstores, then we can create an async version of this trait?

lightning-persister/src/fs_store.rs

joostjager · 2025-07-15T15:38:14Z

Removed garbage collector, because we need to keep the last written version.

codecov · 2025-07-23T19:56:31Z

Codecov Report

❌ Patch coverage is 91.32231% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.78%. Comparing base (c2d9b97) to head (96091ec).
⚠️ Report is 60 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning-persister/src/fs_store.rs	91.32%	10 Missing and 11 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3931    +/-   ##
========================================
  Coverage   88.77%   88.78%            
========================================
  Files         175      176     +1     
  Lines      127760   128709   +949     
  Branches   127760   128709   +949     
========================================
+ Hits       113425   114276   +851     
- Misses      11780    11836    +56     
- Partials     2555     2597    +42

Flag	Coverage Δ
fuzzing	`22.38% <47.08%> (+0.28%)`	⬆️
tests	`88.61% <91.32%> (+<0.01%)`	⬆️

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lightning-persister/src/fs_store_async.rs

joostjager · 2025-07-25T13:48:12Z

Updated code to not use an async wrapper, but conditionally expose the async KVStore trait on FilesystemStore.

I didn't yet update the ldk-node branch using this PR, because it seems many other things broke in main again.

joostjager · 2025-08-22T11:14:14Z

Fuzzer found an issue, fixup commit "f: fix remove clean up"

tnull

Took a first look at the fuzzer parts. I wonder if we would get any notable performance benefit from running the FilesystemStore fuzzer on a ramdisk? Or would we even lose some coverage going this way as it's exactly the IO latency that increases the chances of running into race conditions etc?

tnull · 2025-08-22T11:47:42Z

fuzz/Cargo.toml

 bech32 = "0.11.0"
 bitcoin = { version = "0.32.2", features = ["secp-lowmemory"] }
+tokio = { version = "1.35.*", default-features = false, features = ["rt-multi-thread"] }


nit: This is more common (note not 100% equivalent, but probably preferable):

Suggested change

tokio = { version = "1.35.*", default-features = false, features = ["rt-multi-thread"] }

tokio = { version = "1.35", default-features = false, features = ["rt-multi-thread"] }

Or is there any reason we don't want any API-compatible version 1.36 and above?

Yes, it doesn't work with rust 1.63

Yes, it doesn't work with rust 1.63

Huh, but why can we get away with 1.35 below in the actual lightning-persister dependency then? Also, while the * works, you'd usually rather see ~1.35 used.

For some reason, the compiler decided that 1.35 could safely be bumped to 1.47. Also happened in CI.

error: package `tokio v1.47.1` cannot be built because it requires rustc 1.70 or newer, while the currently active rustc version is 1.63.0 ~/repo/rust-lightning/fuzz (async-fsstore ✗) cargo tree -i tokio tokio v1.47.1 ├── lightning-fuzz v0.0.1 (/Users/joost/repo/rust-lightning/fuzz) └── lightning-persister v0.2.0+git (/Users/joost/repo/rust-lightning/lightning-persister) └── lightning-fuzz v0.0.1 (/Users/joost/repo/rust-lightning/fuzz) ```

Made it ~1.35, reads nicer indeed.

tnull · 2025-08-22T11:54:06Z

fuzz/src/bin/fs_store_target.rs

+	use lightning_fuzz::utils::test_logger::StringBuffer;
+
+	use std::sync::{atomic, Arc};
+	// {


nit: Remove commented-out code.

Ah yes. I was still wondering what that code was for. Some default fuzz string sanity check?

tnull · 2025-08-22T11:59:09Z

fuzz/src/fs_store.rs

+	let secondary_namespace = "secondary";
+	let key = "key";
+
+	// Remove the key in case something was left over from a previous run.


Hmm, rather than doing this, do we want to add a random suffix to temp_path above, so that we're sure to start with a clean directory every time? Also, do we want to clean up the filesystem store directory at the end of the run, similar to what we do in lightning-persister tests?

Added random suffixes. It is also necessary because fuzzing runs in parallel. I used uuid for simplicity, but can also generate names differently if preferred.

Also added clean up. I couldn't just copy the Drop trait, because the FilesystemStore isn't in the same crate. So created a wrapper for it. Maybe there is a better way to do it.

tnull · 2025-08-22T12:03:01Z

fuzz/src/fs_store.rs

+				let fut = futures.remove(fut_idx);
+
+				fut.await.unwrap();
+			},


It shouldn't change anything, but do we want to throw in some coverage for KVStore::list for good measure?

Added. Only I don't think we can assert anything because things may be in flight. It does add some extra variation to the test to also list during async ops.

Also added read. Some story, nothing to assert, but we do cover read execution during writes.

lightning-persister/src/fs_store.rs

joostjager · 2025-08-22T14:46:41Z

Considered the RAM disk, but it is platform-specific. @TheBlueMatt suggested an alternative option which is to allow the injection of the actual disk write handler into FilesystemStore and supply an in-memory implementation for fuzzing. But perhaps we are stretching the scope of this PR too much then, so wanted to see if we can keep it to what it is currently?

joostjager · 2025-08-25T14:27:54Z

Fuzz passes, but some deviating log lines show up:

Sz:16 Tm:78,882us (i/b/h/e/p/c) New:0/0/0/0/0/9, Cur:0/0/0/1108/49/48567
Sz:5 Tm:17,602us (i/b/h/e/p/c) New:0/0/0/0/0/16, Cur:0/0/0/1108/49/48583
Sz:1 Tm:17,008us (i/b/h/e/p/c) New:0/0/0/0/0/5, Cur:0/0/0/1108/49/48588
Sz:22 Tm:75,985us (i/b/h/e/p/c) New:0/0/0/1/0/27, Cur:0/0/0/1109/49/48615
[2025-08-25T13:56:42+0000][W][28702] subproc_checkTimeLimit():532 pid=28711 took too much time (limit 1 s). Killing it with SIGKILL
[2025-08-25T13:56:42+0000][W][28703] subproc_checkTimeLimit():532 pid=28715 took too much time (limit 1 s). Killing it with SIGKILL
Sz:44 Tm:137,892us (i/b/h/e/p/c) New:0/0/0/1/0/32, Cur:0/0/0/1110/49/48647
Sz:7 Tm:71,188us (i/b/h/e/p/c) New:0/0/0/0/0/5, Cur:0/0/0/1110/49/48652
[2025-08-25T13:56:42+0000][W][28703] arch_checkWait():237 Persistent mode: pid=28715 exited with status: SIGNALED, signal: 9 (Killed)
Sz:3138 Tm:1,007,070us (i/b/h/e/p/c) New:0/0/0/263/3/13664, Cur:0/0/0/1373/52/62316
Sz:7 Tm:38,881us (i/b/h/e/p/c) New:0/0/0/3/0/55, Cur:0/0/0/1376/52/62371
[2025-08-25T13:56:42+0000][W][28702] arch_checkWait():237 Persistent mode: pid=28711 exited with status: SIGNALED, signal: 9 (Killed)
Sz:5662 Tm:1,013,168us (i/b/h/e/p/c) New:0/0/0/2/0/41, Cur:0/0/0/1378/52/62412
Persistent mode: Launched new persistent pid=30858
Persistent mode: Launched new persistent pid=30878

joostjager · 2025-08-25T16:41:39Z

Using /dev/shm as ramdisk if present fixed the timeouts.

joostjager · 2025-08-25T19:28:59Z

Tested with a RAM disk on macos using the tool https://github.com/conorarmstrong/macOS-ramdisk, to see if it isn't too fast now to catch problems. I think it is ok. On my machine RAM disk is about 10x faster than disk. Also when removing the is_stale_version check, it is caught by the fuzzer.

joostjager · 2025-08-26T11:24:37Z

fs_store fuzz stats now:

Summary iterations:100008 time:969 speed:103 crashes_count:0 timeout_count:339 new_units_added:1085 slowest_unit_ms:1114 guard_nb:686400 branch_coverage_percent:0 peak_rss_mb:67

969 secs, maybe reduce iterations somewhat?

joostjager · 2025-08-26T11:29:14Z

Squashed the commits. With the number of fixup commits that were there, I don't think it would be helpful anymore to have them.

TheBlueMatt

A few minor notes, but all of them can reasonably be addressed in a followup.

fuzz/Cargo.toml

TheBlueMatt · 2025-08-26T19:25:04Z

fuzz/src/fs_store.rs

+
+impl Drop for TempFilesystemStore {
+	fn drop(&mut self) {
+		_ = fs::remove_dir_all(&self.temp_path)


We need to make sure all the spawned tasks have finished before we do this. Otherwise cleanup wont work because the async task will re-create the directory as a part of its write.

Good point. First I thought to do something smart with drop, but at that point we don't have the list of handles, and also can't await in drop I think. So just added the final wait to the end of the test fn and avoid early returns.

lightning-persister/src/fs_store.rs

joostjager · 2025-08-26T20:51:24Z

I will address comments in this PR. I definitely want to merge, but there are no conflicts with other PRs, so an early merge isn't really justified.

TheBlueMatt · 2025-08-27T13:53:23Z

Oh, I guess this needs squashing, but feel free.

joostjager · 2025-08-27T13:57:48Z

Yes, kept the fixup because in this case I thought it would make re-review easier. Will let Elias take a look next week and then squash.

ldk-reviews-bot · 2025-08-30T00:01:10Z

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joostjager changed the title ~~Async fsstore~~ Async FilesystemStore Jul 15, 2025

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Async Persistence TODOs #3052

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Add async implementation of FilesystemStore

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	tokio = { version = "1.35.*", default-features = false, features = ["rt-multi-thread"] }
	tokio = { version = "1.35", default-features = false, features = ["rt-multi-thread"] }

Async FilesystemStore #3931

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