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Date: Fri, 16 May 2025 15:08:16 -0400
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Subject: Re: FIFO hangs (Probably a bug of cygwin fifo)
To: cygwin AT cygwin DOT com
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From: Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Ken Brown <kbrown AT cornell DOT edu>
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On 5/16/2025 4:59 AM, Takashi Yano via Cygwin wrote:
> On Fri, 16 May 2025 08:46:40 +0900
> Takashi Yano wrote:
>> diff --git a/winsup/cygwin/local_includes/cygheap.h b/winsup/cygwin/local_includes/cygheap.h
>> index fed87ec2b..7d11fbb37 100644
>> --- a/winsup/cygwin/local_includes/cygheap.h
>> +++ b/winsup/cygwin/local_includes/cygheap.h
>> @@ -604,6 +604,8 @@ class cygheap_fdnew : public cygheap_fdmanip
>>     {
>>       if (cygheap->fdtab[fd])
>>         cygheap->fdtab[fd]->inc_refcnt ();
>> +    if (locked)
>> +      cygheap->fdtab.unlock ();
>>     }
>>     void operator = (fhandler_base *fh) {cygheap->fdtab[fd] = fh;}
>>   };
> 
> This should not be done, because the parent class cygheap_fdmanip
> does that.
Right.  But the other part of the patch (to syscalls.cc) looks right to 
me, and I agree that it fixes the hang.  Here's my understanding of why 
it works: The main thread tries to open the fifo for reading, but 
fhandler_fifo::open blocks until it detects that someone is opening the 
fifo for writing.  The other thread wants to do that, but it never gets 
to the point of calling fhandler_fifo::open because it is stuck waiting 
for the lock on cygheap->fdtab.  To fix this, we need to delay the 
construction of the cygheap_fdnew object fd until after 
fhandler_fifo::open has been called.

Do you agree with this explanation, or is there something else going on? 
  In either case, I think it would be good to include at least a brief 
explanation in your commit message, since this is a pretty subtle bug. 
And thanks for finding the fix!

Ken

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