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Immediate timeout on reused curl handle with low speed limit set #17139

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Immediate timeout on reused curl handle with low speed limit set#17139
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icing
@Coderlane

Description

@Coderlane
Coderlane
opened on Apr 22, 2025 · edited by Coderlane

I did this

I have a curl easy handle that is used with curl_easy_perform to periodically POST data. The easy handle is reused between requests to take advantage of the cached connection. Data is attached to the upload via CURLOPT_READFUNCTION. However, a CURLOPT_SEEKFUNCTION is not configured.

To avoid stalled connections, CURLOPT_LOW_SPEED_LIMIT is set:

curl_easy_setopt(easy, CURLOPT_LOW_SPEED_LIMIT, 1L);
curl_easy_setopt(easy, CURLOPT_LOW_SPEED_TIME, 60L);

Occasionally, I'll see the following behavior:

  1. (OK) A request succeeds and the connection is cached
  2. (OK) A request fails, as the cached connection is actually closed and the request can't be rewound
  3. (NOT OK) All subsequent requests successfully upload all data, but hit CURLOPT_LOW_SPEED_LIMIT before receiving the response

See annotated_log.txt for annotated logs from CURLOPT_DEBUGFUNCTION.

Suspicion:

I think the behavior in part 3 above occurs because the rewind bit is still set on the request from the previous (failed) request. When Curl_init_do is called at the start of the request in part 3, it calls Curl_req_start -> Curl_req_soft_reset -> Curl_client_start which will attempt to perform a rewind. This rewind is guaranteed to fail, because I'm using a read function and don't have a seek function.

Popping back to Curl_init_do, if Curl_req_start fails Curl_speedinit will not be called:

curl/lib/url.c

Lines 3853 to 3858 in 657aae7

result = Curl_req_start(&data->req, data);
if(!result) {
Curl_speedinit(data);
Curl_pgrsSetUploadCounter(data, 0);
Curl_pgrsSetDownloadCounter(data, 0);
}

None of the call sites of Curl_init_do check the result of the function:

curl/lib/url.c

Lines 3555 to 3558 in 657aae7

/* since we skip do_init() */
Curl_init_do(data, conn);
goto out;

curl/lib/url.c

Lines 3706 to 3714 in 657aae7

/* Setup and init stuff before DO starts, in preparing for the transfer. */
Curl_init_do(data, conn);
/*
* Setup whatever necessary for a resumed transfer
*/
result = setup_range(data);
if(result)
goto out;

This is ultimately what triggers the CURLOPT_LOW_SPEED_LIMIT failure on an otherwise successful request. The speed data on request 3 is still the speed data from request 2. Thus, it looks like request 3 has stalled.

I expected the following

I'd expect curl_easy_perform to fail if a required seek fails. IE: The result of Curl_init_do should be checked.

I'd expect the rewind bit to be reset after the request, but I'm not sure this is a reasonable expectation. For my use, each call to curl_easy_perform is a request and rewinding before sending any data has no meaning. I'm not sure if this holds for all use cases.

curl/libcurl version

libcurl 8.12.1 w/ boringssl

operating system

macOS 15.4.1 (24E263)

Darwin <REDACTED> 24.4.0 Darwin Kernel Version 24.4.0: Fri Apr 11 18:33:47 PDT 2025; root:xnu-11417.101.15~117/RELEASE_ARM64_T6000 arm64

Activity

icing
self-assigned this
on Apr 23, 2025
icing

icing commented on Apr 23, 2025

@icing
icing
on Apr 23, 2025
Contributor

@Coderlane thanks for your detailed report. I believe your analysis is correct and the failed rewind should lead to an early error, not a transfer failed unreliably later. Will work on a fix...

icing
added a commit that references this issue on Apr 23, 2025

init_do(): check result

177f7b3
icing
mentioned this on Apr 23, 2025
icing

icing commented on Apr 23, 2025

@icing
icing
on Apr 23, 2025
Contributor

I made the fix in #17150 (and found a bug in a test case that way). Could you test that in your setup? Thanks!

Coderlane

Coderlane commented on Apr 23, 2025

@Coderlane
Coderlane
on Apr 23, 2025
Author

Thank you! I'll give it a try and get back to you.

Coderlane

Coderlane commented on Apr 23, 2025

@Coderlane
Coderlane
on Apr 23, 2025 · edited by Coderlane
Author

177f7b3 seems correct to me. The request now fails with CURLE_SEND_FAIL_REWIND when I don't have a CURLOPT_SEEKFUNCTION registered. Thank you!

Here are the logs:

2025-04-23T19:25:14.470ZI LogCurlInfo == Info: upload completely sent off: 39 bytes
2025-04-23T19:25:18.167ZI LogCurlInfo == Info: Recv failure: No route to host
2025-04-23T19:25:18.167ZI LogCurlInfo == Info: BoringSSL SSL_read: No route to host, errno 65
2025-04-23T19:25:18.167ZI LogCurlInfo == Info: Connection died, retrying a fresh connect (retry count: 1)
2025-04-23T19:25:18.168ZI LogCurlInfo == Info: shutting down connection 0
2025-04-23T19:25:18.168ZI LogCurlInfo == Info: Send failure: Broken pipe
2025-04-23T19:25:18.169ZI LogCurlInfo == Info: seek callback returned error 2
2025-04-23T19:25:18.169ZI LogCurlInfo == Info: rewind of client reader 'cr-in' failed: 65
2025-04-23T19:25:18.169ZI LogCurlInfo == Info: Issue another request to this URL: '<REDACTED>'
2025-04-23T19:25:18.169ZI LogCurlInfo == Info: seek callback returned error 2
2025-04-23T19:25:18.169ZI LogCurlInfo == Info: rewind of client reader 'cr-in' failed: 65
2025-04-23T19:25:18.169ZI LogCurlInfo == Info: closing connection 3
bagder
closed this as completedin 5e95556on Apr 24, 2025
JimFuller-RedHat
added a commit that references this issue on Apr 24, 2025

multi: init_do(): check result

5e95556
nbaws
added a commit that references this issue on Apr 26, 2025

multi: init_do(): check result

6fe0ee7
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        Immediate timeout on reused curl handle with low speed limit set · Issue #17139 · curl/curl