Error Domain=NSPOSIXErrorDomain Code=28 “No space left on device” UserInfo={_kCFStreamErrorCodeKey=28, _kCFStreamErrorDomainKey=1} - alamofire

Question

Lately numerous network requests with Alamofire made from our iOS device fail with the following error:
Error Domain=NSPOSIXErrorDomain Code=28 "No space left on device"
UserInfo={_NSURLErrorFailingURLSessionTaskErrorKey=LocalDataTask
.<3>,
_kCFStreamErrorDomainKey=1, _NSURLErrorRelatedURLSessionTaskErrorKey=( "LocalDataTask .<3>" ),
_kCFStreamErrorCodeKey=28}
Our app has a mechanism to send a network request if the user has moved +- 10 meters. This is checked every 5 seconds, so in theory every five seconds a call can be made. The network request fails occasionally with this message, returning no status code and the above error.
The message implies the error has to do with available disk/memory space on the device. However, after checking both there is no link to be found since there is plenty of space available. Also, the error occurs on multiple devices, all running iOS 14.4 or higher.
Is there information available regarding error code 28 and what could be the culprit on iOS devices? Even better; how can this error be prevented?

Answer 1

To answer the occurrence of the error itself:
NSPOSIXErrorDomain Code=28 "No space left on device"
With logs in the Xcode terminal:
2021-05-07 15:56:50.873428+0200 MYAPP[21757:7406020] [] nw_path_evaluator_create_flow_inner NECP_CLIENT_ACTION_ADD_FLOW 05CD829A-810D-412F-B86E-7524369359E8 [28: No space left on device]
2021-05-07 15:56:50.877243+0200 MYAPP[21757:7400322] Task <5504BCDF-7DFE-4045-BD4B-E75054636D5B>.<1> finished with error [28] Error Domain=NSPOSIXErrorDomain Code=28 "No space left on device" UserInfo={_NSURLErrorFailingURLSessionTaskErrorKey=LocalUploadTask <5504BCDF-7DFE-4045-BD4B-E75054636D5B>.<1>, _kCFStreamErrorDomainKey=1, _NSURLErrorRelatedURLSessionTaskErrorKey=(
"LocalUploadTask <5504BCDF-7DFE-4045-BD4B-E75054636D5B>.<1>"
), _kCFStreamErrorCodeKey=28}
It appears to get called when there are too many NSURLSessions created, reaching a limit of (in our tests) 600-700 sessions, which are not maintained or closed properly. The error started to get thrown since iOS 14, so it is interesting to see if there was a limit introduced.
Linked is a github issue raised stating the same issues on the ktor microservices framework by JetBrains, pointing in the same direction, mentioning the invalidation of sessions to prevent this issue:
https://github.com/ktorio/ktor/issues/1341
In our own project the origin of the problem turned out to be our implementation of the StarScream websocket library. This might not be relevant for the issues others are having, but explained anyways to create a complete picture of the problem. It is the cause and fix of our specific situation.
At first we assumed it had something to do with the URLSession created by Alamofire (networking library used) since POST requests started to get cancelled, and a kill of the app seemed the only solution to do requests again.
However, we also make use of websocket connections using the StarScream library, which attempts to connect to an socket, and if failed retry to connect every two seconds for a max time of two hours. This would mean for two hours, every two seconds, we connect to the socket -> receive a failure to connect -> disconnect the socket -> connect again. Using a singleton of the socket it was thought there was no possibility of creating multiple URLSessions, since the socket was only initiated once. However calling the connect to the socket again would create a new nw_connection object every single time, since the library did not handle the disconnect properly.
image of NWConcrete_nw_connection objects generated in socket connection
The way this was validated was using the instruments app to check for the creation of new nw_connection objects. Logged as a "memory leak" there, the creation of the nw_connection objects was getting logged and the solution was to make sure we disconnect the socket (invalidate the session) properly before connecting again.
I hope to answer a big part of the issue here, and I will mark my own question answered since this was the solution to the problem at hand. I think Apple should consider giving accurate reports on the number of objects created being limited, instead of giving an error "No space left on device".

Answer 2

Just wanted to chime in with more info, since we're experiencing the same issue.
Based on our analytics, this issue only started happening since iOS 14. We've verified it happening on 14.2, 14.4 and 14.5. Naturally the most straightforward cause for this error would be low memory or disk storage. We've excluded this option with additional logging, as you seem to have done as well.
A possibly related SO post has attributed the issue to a network inspecting framework that was enabled in their release build. It's worth checking if you use a similar tool.
Another report of this issue, this time on the Github of AFNetworking (predecessor to the Alamofire library you use), says they were able to fix it by limiting the creation of URLSession objects.
For us personally, neither of these did the trick. We created a support ticket with Apple, but this hasn't lead to a solution. They requested a small sample project that reproduces the issue, but the error only manifested after 7 days of continuous use in our app. If you have a faster way to reproduce this, it may be worth it to submit your own support ticket.
Hopefully this helps you find a solution, if you do please add this to your post to help others!