Race condition prevents UART "write" call from returning

[Jen Dority]

Hi Curran,

Thanks for the bug report. I'd actually run into this issue while 
investigating your last bug report. I have a fix out for review now.

Jen


On 9/13/14 4:49 PM, Curran D. Muhlberger wrote:
> One minor correction to my previous email: Contrary to my claim, the
> `waitForSignal` method doesn't quite handle the case when a signal already
> exists (though it looks like it was meant to).  It neglects to assign to
> `sig` the result of its `findTarget` call (leading to a null-pointer
> dereference if the signal does exist).  Adding this assignment lets my
> patch work as expected, at least in my limited testing.
>   - Curran
>
> On Sat, Sep 13, 2014 at 1:11 PM, Curran D. Muhlberger <
> curran at muhlbergerweb.com> wrote:
>
>> Hello again DIO developers,
>>
>> There appears to be a bug in how completed writes are signaled that can
>> prevent the UART `write` method from returning if a race condition is
>> satisfied.  Worse, the race condition seems to be satisfied most of the
>> time.
>>
>> The relevant function is `Java_com_oracle_dio_uart_impl_UARTImpl_write0`
>> in "jni_uart.cpp".  Basically, the code copies the data into a
>> newly-allocated buffer, signals a separate "write thread" that the buffer
>> is ready, then waits for a signal indicating that the write has completed.
>>   However, after waking the "write thread", it is possible for that thread
>> to finish its work and send the "write complete" signal before the JNI
>> thread has had a chance to start listening.  The JNI thread will the wait
>> forever for a signal that has already been sent, causing the program to
>> hang.
>>
>> To reproduce this problem reliably, simply add a `sleep` call at line 242
>> of "jni_uart.cpp" (just before it calls `waitForSignal`), then try to
>> invoke the `write` method on a UART device.  The method will never return.
>>
>> I'm not sure how the DIO signaling infrastructure is designed to work, so
>> I defer to the core developers on how best to fix this problem.  However, I
>> did find a patch that seems to do the job.  In the `generateSignal`
>> function in "dio_common.cpp", if `findTarget` returns NULL, then
>> `generateSignal` can create a new signal and add it to the list, so that a
>> call to `javacall_os_cond_signal` is made regardless of whether a signal
>> already existed or not.  It turns out that the `waitForSignal` method
>> already handles this case - if a signal for the target already exists, it
>> simply waits on it instead of creating a new one.  With this small patch, I
>> can make multiple `write` calls to a UART device without problems.
>>
>> Thank you for taking a look at this issue.
>>
>>   - Curran
>>