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Is there a guarantee that for multiple parallel responses header function and write function will not be interleaved?
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From: Arvind Srivaths via curl-library <curl-library_at_cool.haxx.se>
Date: Fri, 13 Nov 2020 13:59:01 +0000 (UTC)
Firstly, I would like to thank all the developers working on Curl for a reliable library that is easy to use for beginners and yet is customisable for very advanced work.
I am using curl to make REST calls and each request contains a request Id in the HTTP header.
I detect the request Id (copied back into the response by the server) in the header handler and use that in the data handler to map to the CommHandle (my class, which has a reference to the Curl handle) and copy the data into the handle specific buffer. With a single outstanding request, it's guaranteed that nothing will come between the call to the two handlers and I can rely on the fact that the request Id detected in the header handler can be used by the data handler without anything stomping on it.
I am not sure whether this will work if Curl uses multiple threads for multiple handles and if another response can overwrite the request Id before the data handler gets to it. Is there any guarantee that the library (as of today) will not interleave calls to header function and write function for two responses? While this may not be the case with Curl today, is there a chance that it may change in future?
I can think of several solutions to this:. Ideal solution is if Curl itself can provide a single handler to handle both header and data - then it's atomic and we can use locks to guarantee mutual exclusion.. I can lock the request Id in the header handler and unlock after getting it in the data handler. Depending on how Curl implements response handling it may result in deadlock. With two threads if it calls header handlers in order, the second can block waiting for the first (which will not release the lock till its data handler is called) and if curl waits for both to end before calling data handlers, you have a deadlock. If this Curl code is unlikely to be run in multiple threads, I can use this solution temporarily.
. I can assume in the header handler that the data starts at the end of the header and simulate a single handler. If Curl implementation changes (eg. for HTTP 2 header is compressed and decompressed header may not be contiguous with data) this may break.
I would like your opinion on what is the best solution to this issue.
Thanks,
Arvind
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Received on 2020-11-13
Date: Fri, 13 Nov 2020 13:59:01 +0000 (UTC)
Firstly, I would like to thank all the developers working on Curl for a reliable library that is easy to use for beginners and yet is customisable for very advanced work.
I am using curl to make REST calls and each request contains a request Id in the HTTP header.
I detect the request Id (copied back into the response by the server) in the header handler and use that in the data handler to map to the CommHandle (my class, which has a reference to the Curl handle) and copy the data into the handle specific buffer. With a single outstanding request, it's guaranteed that nothing will come between the call to the two handlers and I can rely on the fact that the request Id detected in the header handler can be used by the data handler without anything stomping on it.
I am not sure whether this will work if Curl uses multiple threads for multiple handles and if another response can overwrite the request Id before the data handler gets to it. Is there any guarantee that the library (as of today) will not interleave calls to header function and write function for two responses? While this may not be the case with Curl today, is there a chance that it may change in future?
I can think of several solutions to this:. Ideal solution is if Curl itself can provide a single handler to handle both header and data - then it's atomic and we can use locks to guarantee mutual exclusion.. I can lock the request Id in the header handler and unlock after getting it in the data handler. Depending on how Curl implements response handling it may result in deadlock. With two threads if it calls header handlers in order, the second can block waiting for the first (which will not release the lock till its data handler is called) and if curl waits for both to end before calling data handlers, you have a deadlock. If this Curl code is unlikely to be run in multiple threads, I can use this solution temporarily.
. I can assume in the header handler that the data starts at the end of the header and simulate a single handler. If Curl implementation changes (eg. for HTTP 2 header is compressed and decompressed header may not be contiguous with data) this may break.
I would like your opinion on what is the best solution to this issue.
Thanks,
Arvind
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Received on 2020-11-13