Re: Masked cancellation mode draft

[Szabolcs Nagy <nsz@port70.net>]

* Rich Felker <dalias@libc.org> [2015-02-21 22:24:53 -0500]:
> When the cancellation state is set to MASKED, the first cancellation
> point (other than close, which is special) called with cancellation
> pending, or which has a cancellation request arrive while it's
> blocking, returns with an error of ECANCELED, and sets the
> cancellation state to DISABLE.
> 
> Even code which was not specifically written to be cancellation-aware
> is compatible with this behavior. As long as it is responding to
> errors, it will see the error, but will have the full repertoire of
> standard functions available to use while cleaning up and returning
> after the error. If the error is ignored, cancellation will be
> delayed, but the behavior is no worse than what could already happen
> from ignoring errors.
> 

so it works like a special signal that only acts at blocking calls

since the thread is not forcefully killed, only notified about the
cancellation, the cleanup mechanism is under the control of the
programmer

this seems like a relevant approach to c11 and c++11 which currently
lack any way to safely cancel blocking threads

the only difficulty i see is that posix has a lot of cancellation
points (some of which are optional) so code that wants to be
'masked cancellation safe' should properly do the error handling
at a lot of places (eg some stdio functions like printf maybe
cancellation points and usually not checked for errors directly
only in aggregate through ferror)

if i understood correctly code that does not want to immediately
act upon masked cancellation (only at specific calls) should reset
the cancellation state with

  pthread_setcancelstate(PTHREAD_CANCEL_MASKED, 0)

and then cancellation is deferred until the next cancellation point.

another issue is that pthread_testcancel() has no return value so it
cannot be used for non-blocking testing of masked cancel.