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Date:	Sat, 09 Apr 2022 23:26:51 +0300
From:	Alexey Izbyshev <izbyshev AT ispras DOT ru>
To:	Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
Subject:	Re: Deadlock of the process tree when running make
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On 2022-04-09 22:35, Alexey Izbyshev wrote:
> On 2022-04-09 20:54, Takashi Yano wrote:
>> Thanks for checking. This seems to be normal. Then, I cannot
>> understand why the ClosePseudoConsole() call is blocked...
>> 
>> The document by Microsoft mentions the blocking conditions of
>> ClosePseudoConsole():
>> https://docs.microsoft.com/en-us/windows/console/closepseudoconsole
>> however, the thread above is draining the channel.
> 
> I've decided to check what object ClosePseudoConsole() waits for. The
> wait happens inside unexported KERNELBASE!_ClosePseudoConsoleMembers
> function. Here is the relevant part:
> 
> 76589fb5 8b4e08          mov     ecx,dword ptr [esi+8]
> 76589fb8 e8c2fdffff      call    KERNELBASE!_HandleIsValid (76589d7f)
> 76589fbd 84c0            test    al,al
> 76589fbf 7456            je
> KERNELBASE!_ClosePseudoConsoleMembers+0x89 (7658a017)
> 76589fc1 8d45fc          lea     eax,[ebp-4]
> 76589fc4 895dfc          mov     dword ptr [ebp-4],ebx
> 76589fc7 50              push    eax
> 76589fc8 51              push    ecx
> 76589fc9 e8c23ef5ff      call    KERNELBASE!GetExitCodeProcess 
> (764dde90)
> 76589fce 85c0            test    eax,eax
> 76589fd0 7414            je
> KERNELBASE!_ClosePseudoConsoleMembers+0x58 (76589fe6)
> 76589fd2 817dfc03010000  cmp     dword ptr [ebp-4],103h
> 76589fd9 750b            jne
> KERNELBASE!_ClosePseudoConsoleMembers+0x58 (76589fe6)
> 76589fdb 53              push    ebx
> 76589fdc 6aff            push    0FFFFFFFFh
> 76589fde ff7608          push    dword ptr [esi+8]
> 76589fe1 e8ba74f6ff      call    KERNELBASE!WaitForSingleObjectEx 
> (764f14a0)
> 
> "esi" is the argument of ClosePseudoConsole(), so the first mov
> dereferences it with an offset and loads a process handle. Then, if
> this handle is valid, it calls GetExitCodeProcess(), and if it
> succeeds and returns STILL_ACTIVE, it waits for that process.
> 
> I've checked that hanging bash process has only 3 process handles: for
> itself, for dead javac, and for conhost.exe. So obviously it waits for
> the latter to terminate. (After I did all this, I realized there was
> much easier way to get this result via "Analyze wait chain" feature of
> Task Manager).
> 
> Unfortunately, I don't know anything about Windows consoles, but just
> in case I also checked what 5 threads of conhost.exe are waiting for:
> 
> 1. Tries to enter a critical section (Task Manager claims it waits for
> thread 4, so probably the latter owns it).
> 2. Waits on a handle for "pty1-from-master-nat" named pipe.
> 3. Waits for an anonymous event.
> 4. Waits on a handle for "\Device\ConDrv" (in DeviceIoControl()).
> 5. Blocked in GetMessageW().
> 
> It's also worth of note that this conhost.exe seems to be the only one
> related to the Cygwin process tree (as well as the only related
> non-Cygwin process). All other conhost.exe processes were created
> before I started my stress test.
> 
> My guess is that this conhost.exe was created for a native app started
> from a Cygwin process. Could it be some race condition/bug that
> prevented conhost.exe from terminating once the native process
> (probably javac?) died?
> 
A few more things that might be important:

* Clarification: thread 2 of conhost.exe waits in KernelBase!ReadFile().

* In the assembly part I omitted, before waiting on the conhost process, 
_ClosePseudoConsoleMembers() closes the handle obtained from "dword ptr 
[esi]", i.e. "hWritePipe" member of HPCON_INTERNAL struct.

Alexey

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