Potential fix(json): use Content-Length instead of chunked transfer for /json/effects and /json/fxdata

[aenertia]

respondModeData() uses sendChunked(). The truncation is in ESPAsyncWebServer's _ack() — on the chunked path (_sendContentLength=false), RESPONSE_WAIT_ACK exits immediately because !_sendContentLength is unconditionally true. close() fires as soon as the final chunk is written to the lwIP send buffer, not when it's ACKed. On fast WiFi the write-to-ACK gap is negligible; on PPP/UART at 1.5Mbaud a 3KB response takes ~16ms to drain, and congested/distant WiFi retransmits create the same window.

/json/effects went through LockedJsonResponse, which holds the JSON buffer lock for the duration of the async send. Under concurrent WebSocket state pushes on constrained hardware, sendDataWs() blocks on the same lock.

Fix: two-pass Content-Length for both endpoints. measureJSONStringElement() pre-computes exact payload size; request->send() with that length sets _sendContentLength=true, so RESPONSE_WAIT_ACK gates on _ackedLength >= _writtenLength before close. respondModeNames() replaces the LockedJsonResponse path for /json/effects using the same streaming callback pattern as respondModeData() — no lock held during send, 256-byte stack buffer per callback.

The _ack() early-exit on chunked responses regardless of ACK state is an ESPAsyncWebServer bug — filing separately.

Tested on M5StickC (ESP32-PICO-D4) over PPP at 1.5Mbaud: /json/fxdata completes without truncation, /json/effects no longer deadlocks under concurrent WebSocket pushes. WiFi unaffected.

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Walkthrough

Mode-data responses now include an explicit Content-Length. A new respondModeNames handler streams escaped effect names without metadata suffixes. The /json/effects route calls this handler directly.

Changes

Mode response streaming

Layer / File(s)	Summary
Mode-data content length wled00/json.cpp	respondModeData measures the JSON payload, uses mode indexes for array formatting, and sends an explicit Content-Length.
Effect-name streaming endpoint wled00/fcn_declare.h, wled00/json.cpp	The public declaration and implementation for respondModeNames stream cleaned and escaped effect names. /json/effects routes directly to this handler without the shared JSON buffer lock.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🟡 Moderate · up to 3d5ee

When effect entries are empty, the response can be emitted as malformed JSON, causing clients using /json/effects to fail parsing. This bounded correctness issue should be addressed before merging.

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant EffectsRoute
  participant respondModeNames
  Client->>EffectsRoute: Request /json/effects
  EffectsRoute->>respondModeNames: Route request
  respondModeNames-->>Client: Stream escaped effect names

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Possibly related PRs

• wled/WLED#5461: Both changes refactor streamed mode-related JSON responses in wled00/json.cpp.
• wled/WLED#5462: Both changes update JSON response streaming and routing in wled00/json.cpp.

Suggested reviewers: netmindz, softhack007, dedehai

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Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%.
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Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
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Title check	✅ Passed	The title clearly identifies the main change: replacing chunked transfer with Content-Length for the affected JSON endpoints.

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Actionable comments posted: 2

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Inline comments:
In `@wled00/json.cpp`:
- Around line 1304-1313: Update the comments above respondModeData to remove
references to sendChunked, arbitrary streaming, and TCP teardown behavior;
describe that the function performs a two-pass operation that measures the JSON
payload, then sends it with request->send() and a fixed Content-Length.
- Around line 1331-1336: Update both streamed JSON handlers in wled00/json.cpp
at lines 1331-1336 and 1377-1382 to track whether an element was actually
emitted, rather than using fx_index to determine the first output. Use that
state to place commas correctly when leading entries are skipped, and ensure
each handler returns [] when no mode-data or effect-name entries are emitted.

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[softhack007]

@aenertia it might be a minor thing, but please teach your AI how to not delete existing source code comments.
Lost comments will make it harder for future maintainers to understand the source code. That's why we want them preserved, or adjusted. But not deleted.

[aenertia]

Yeah - it's annoying I know, - there are multiple harness rules I have for that already ; new project rules/subtlety of conventions, need a few PR's to get it aligned. Since I have a bunch of stuff relating to the ppp transport these first few small fixups will hopefully make future submissions better aligned.

…

On Thu, 20 Aug 2026 at 05:24, Frank Möhle ***@***.***> wrote: ***@***.**** commented on this pull request. ------------------------------ In wled00/json.cpp <#5808 (comment)>: > if (lineBuffer[0] != 0) { - lineBuffer[sizeof(lineBuffer)-1] = '\0'; // terminate string (only needed if strncpy filled the buffer) - const char* dataPtr = strchr(lineBuffer,'@'); // Find '@', if there is one - size_t mode_bytes = writeJSONStringElement(data, len, dataPtr ? dataPtr + 1 : ""); - if (mode_bytes == 0) break; // didn't fit; break loop and try again next packet - if (fx_index == 0) *data = '['; + lineBuffer[sizeof(lineBuffer)-1] = '\0'; comment lost — Reply to this email directly, view it on GitHub <#5808?email_source=notifications&email_token=AACF5L6QLZXIYYPIN3RE5HL5KXPFRA5CNFSNUABKM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UKJSXM2LFO4XTIOJXGQ4TENRVGUZKM4TFMFZW63VHNVSW45DJN5XKKZLWMVXHJLDGN5XXIZLSL5RWY2LDNM#pullrequestreview-4974926552>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AACF5LZ5332JZPH332VRN4D5KXPFRAVCNFSNUABEKJSXA33TNF2G64TZHM3TMOJZGAZDAOJ3JFZXG5LFHM2TCOBWG4YTCNJYGSQXMAQ> . Triage notifications, keep track of coding agent tasks and review pull requests on the go with GitHub Mobile for iOS <https://github.com/notifications/mobile/ios/AACF5L4DTZPI6QTS7OJIF3T5KXPFRA5CNFSNUABKM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UKJSXM2LFO4XTIOJXGQ4TENRVGUZKM4TFMFZW63VHNVSW45DJN5XKKZLWMVXHJKTGN5XXIZLSL5UW64Y> and Android <https://github.com/notifications/mobile/android/AACF5LYSGORMQIBXVAPQ6Q35KXPFRA5CNFSNUABKM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UKJSXM2LFO4XTIOJXGQ4TENRVGUZKM4TFMFZW63VHNVSW45DJN5XKKZLWMVXHJLTGN5XXIZLSL5QW4ZDSN5UWI>. Download it today! You are receiving this because you were mentioned.Message ID: ***@***.***>

[softhack007]

The final chunk arrives after the connection is torn down, truncating the response.

I have the feeling that this is not the root cause of the problem.

What @aenertia assumes is basically a violation of the TCP protocol, because a TCP connection is always held until the full communication is finished, or one of the enpoints prematurely aborts communication ($ kill -9 -1). A broken connection would always happen also when using WiFi, however we never experienced that.

My guess would be on the JSON buffer locking mechanism - requestJSONBufferLock() and class LockedJsonResponse. The JSON buffer is protected by a semaphore, but the semaphore timeout is relatively short, and the main JSON buffer is a ressource needed by many operations. It might be that another tasks takes over the buffer ownership too early, then initially clears it for another purpose. This scenario could also lead to a truncated response, especially on slow connections.

json/effects routes directly to this handler without the shared JSON buffer lock.

without needing the shared lock, there is no contention on the lock any more.

I'm not sure if my scenario is the correct explanation. But if it is, the whole PR could be reduced to just changing serializeModeNames() so it directly writes to response, instead of going via a JSON array. This would be faster and more memory-efficient, and the separate run to gauge the response size could be skipped, too.

[aenertia]

TCP is correct — the protocol itself won't deliver FIN before data. The truncation is happening a layer up, in ESPAsyncWebServer.

In AsyncAbstractResponse::_ack() (WebResponses.cpp), the chunked path sets _chunked=true, _sendContentLength=false. When the final zero-length chunk fires, state goes to RESPONSE_WAIT_ACK. The exit condition there is !_sendContentLength || _ackedLength >= _writtenLength — since _sendContentLength is false, that's immediately true regardless of ACK state. close() fires as soon as the last chunk lands in the lwIP send buffer, not when it's ACKed.

The Content-Length path (_sendContentLength=true) gates that transition on _ackedLength >= _writtenLength — actual lwIP ACKs have to catch up before close. That's the window.

PPP makes it deterministic because ACKs transit the same UART byte stream as data — on a 3KB response at 1.5Mbaud there's ~16ms of UART drain time between write and ACK. I've also seen it on badly congested or distant WiFi where retransmissions push the ACK latency out far enough to hit the same window. Less frequent, but same root cause.

On the semaphore hypothesis — LockedJsonResponse::_fillBuffer() releases the lock as soon as serialisation is done, before the async send completes, so the buffer isn't held across the wire transfer. Lock elimination in respondModeNames() is worth keeping as a separate benefit, but it's not what's causing the fxdata truncation.

The actual bug I think, is in ESPAsyncWebServer — RESPONSE_WAIT_ACK should gate on _ackedLength >= _writtenLength for chunked responses the same way it does for Content-Length responses. This PR fixes it at the WLED level by switching to Content-Length, which is the right thing to do here regardless. I'll update the PR description to cite the actual mechanism rather than "TCP FIN races" — that framing was imprecise. Filing the ESPAsync root cause separately.

[willmmiles]

I concur this problem analysis is AI crap. LwIP holds the connection after close() until all bytes have been sent. The closed TCP connection will be held in the TIME_WAIT state for up to 20 seconds -- this ought to be long enough to complete pretty much any transaction, especially given that the maximum window size is only two packets. This is TCP 101, and frankly it's a waste of our time that we have to put up with this.

I'm also pretty confident in the JSON lock - the guard object guarantees that the lock isn't released until the transaction has been completely queued at the transport layer. I've pretty thoroughly gone through all of the other use cases.

I certainly wouldn't rule out other web server bugs though. A more productive approach might be to test with the upstream AsyncWebServer for comparison. Ideally I'd like to migrate to the upstream fork at some point, but there's a fair amount of work to be PR'd there to make that possible (request queuing, memory safety on platforms that don't support C++ exceptions, and code size are all open concerns).

Re migrating more paths to the streaming approach: I'd been working on some general tooling for this to support all of the semi-static HTTP endpoints - sorry I've been slow getting this done. There were still some issues with code size that I wanted to shake out before PR'ing. ( https://github.com/willmmiles/WLED/tree/stream-json )

Re precomputing content length: IMO that's a nonstarter; the whole point of the chunked approach is to avoid that. I would veto this PR on that basis alone. CPU is cheap, but not that cheap.

[aenertia]

The mechanism was wrong, I'll own that -- and appreciate the correction. The truncation is real and I'd been chasing the root cause before submitting — the explanation in the PR description was my best working theory at the time. Content-Length consistently avoids it (and I did try a number of approaches before the PR), sendChunked() doesn't, reproducible on both PPP and congested WiFi. I'll test against upstream AsyncWebServer and come back with something more concrete.

[softhack007]

Re precomputing content length: IMO that's a nonstarter; the whole point of the chunked approach is to avoid that. I would veto this PR on that basis alone. CPU is cheap, but not that cheap.

I agree with @willmmiles here - changing the /json/fxdata endpoint from chunked to content-length based transfer seems premature while a clear root cause is not yet established:

• Bug in AsyncWebser is not (yet) identified AsyncAbstractResponse: chunked responses close connection before ACKs in RESPONSE_WAIT_ACK ESP32Async/ESPAsyncWebServer#472 (comment)
• it appears like the PPP-over-UART implementation from @aenertia has some potential defects WebUI effects list is (sometimes) incomplete or broken when using a slow connection (PPP over UART) #5813 (comment) that could cause the observed behaviour.

@aenertia I'm leaning towards closing this PR, because it tries to fix a problem where the root cause is still unclear. The core of this PR - changing from chunked to content-length based transfer - is not a good solution in general.

[aenertia]

Yeah, fair -- I hadn't done enough isolation before writing this up although I thought I had, sorry about that.

The PPP patches are also rougher than I'd like, and am still working on them actively, so I can't rule out interplay -- #5813 may well be a PPP artifact. I'll get that sorted before pushing further here. Appreciate the patience.

The lock-elimination piece -- softhack007 flagged that path earlier. Worth splitting standalone, or better to wait?

[willmmiles]

The lock-elimination piece -- softhack007 flagged that path earlier. Worth splitting standalone, or better to wait?

No need - the streaming JSON branch handles that as well as many other endpoints; it'd just create more merge conflicts.

[aenertia]

This can be closed -- the Content-Length approach was wrong, as @willmmiles and @softhack007 correctly identified.

Probable root cause found -- I believe the issue is in how the sendChunked callback signals "buffer too small" vs "I'm done", but I haven't verified this against the upstream ESPAsyncWebServer fork (only against the WLED-bundled copy), so there may be nuance I'm missing.

ESPAsync's chunked response path in _ack() treats readLen == 0 from _fillBuffer as "no more data, response complete" and transitions to RESPONSE_WAIT_ACK. But there's a separate return value for "I have more data but couldn't fit anything in this buffer": RESPONSE_TRY_AGAIN (0xFFFFFFFF). When the callback returns RESPONSE_TRY_AGAIN, ESPAsync retries on the next ACK when TCP send buffer space frees up.

Our streaming callback was returning 0 when the available buffer was too small to fit the current fxdata entry (some entries are 100+ bytes with slider/parameter metadata). On WiFi the TCP window is large enough that this never happens -- the buffer always fits at least one item. On PPP at 1.5Mbaud, the TCP send buffer (TCP_SND_BUF=5744) fills and ACKs arrive slowly via the same UART byte stream. When space drops below a single item size, the callback returns 0, ESPAsync ends the response, and /json/fxdata truncates at 6933B of 9208B.

I'm fairly confident in this analysis because:

• The truncation point is deterministic (always 6933B, consistent with TCP_SND_BUF=5744 plus HTTP framing on the first window fill-and-drain cycle)
• curl reproduces it (not browser-specific)
• WiFi never truncates (same firmware, same endpoint, larger TCP window)
• The one-line fix below resolves it completely on device (5/5 runs return full 9208B valid JSON over PPP, UI loads effect icons correctly)

What I haven't ruled out: whether there's also a PPP transport contribution (the UART RX path had real bugs i've since fixed -- uart_write_bytes corruption, LCP echo teardown -- and I can't fully isolate whether those interacted). The fix works with the current PPP code; I don't know if the old PPP code would have truncated even with this fix applied.

Fix (one line in the sendChunked callback adapter):

// Before:
return r.count;

// After:
return (r.count == 0 && !r.done) ? RESPONSE_TRY_AGAIN : r.count;

This isn't PPP-specific -- any link slow enough to shrink the TCP window below a single fxdata entry size would hit it. PPP just makes it deterministic because data and ACKs share the same serial wire.

Crediting @willmmiles for correcting my earlier wrong theories about tcp_close() and AsyncTCP internals -- those corrections are what eventually pointed me toward the actual callback contract. The fix lives in my fork's streaming JSON adapter for now. @willmmiles' stream-json branch will likely supersede that code, but the RESPONSE_TRY_AGAIN pattern applies to any sendChunked callback that can legitimately produce 0 bytes without being done.

The lock-elimination piece from the earlier version of this PR is already covered by @willmmiles' streaming JSON work, so nothing from this PR needs to land separately.