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Re: Adding CURL handles to running CURLM

From: Ray Satiro via curl-library <>
Date: Fri, 29 May 2015 01:41:01 -0400

On 5/25/2015 11:52 AM, wrote:
>> I don't know. Can you show us a recipe on how to reproduce this problem? Your description hasn't been detailed enough for me to understand.
> I tried reproducing this problem with pure CURL and our server. What I found is that returning exact number of read bytes from CURLOPT_READFUNCTION callback causes server to hang. Our read function receives 16384 byte buffer from CURL, fills it and returns number of bytes read. If we reach end of upload body - we return what was left to read - for example 2304. Returning 2305 (read bytes count incremented by one) for some reason fixes this issue.
> Tested workflow, each request creates new CURL handle and uses same CURLM handle:
> 1. [Application specific] Send upload handshake request with some initial data, receive 308 and ETag to use with each chunk
> 2. Upload 4 000 000 bytes chunk. Read function calls return 16384 (244 times) and then 2304, receive 308
> 3. Upload 4 000 000 bytes chunk. Read function calls return 16384 (227 times) and server hangs
> Reproduces if:
> - HTTPS (HTTP works)
> - Windows Server 2008 (2012 works)
> - Using same CURLM (using seperate CURML handles or seperate CURL hangles works)
> - Returning exact number of bytes that were read to buffer (works if returning same buffer size or incrementing value by one when read less than buffer size)
> mentions bug and server "hangs" in some workflows, can that be related to this?

I don't know what's going on here but the server hang described there is
if you return 0 from the read function prematurely, and as described
that's usually because you've sent less than you stated. For example
that could happen if you open the file in the default translation mode
and forget to specify binary mode. When you call fread it returns 0
early even though it hasn't read the complete file because EOF char has
already been reached.

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Received on 2015-05-29