libpthread: Implement sem_trywait() and sem_getvalue()

libpthread: Implement sem_trywait() and sem_getvalue()

Now that semaphores are implemented using futexes, this is easy to do.
It resolves a longstanding omission.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=715
TEST=run_semaphore_tests

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=11763

[Roland McGrath, 2013-07-12 21:15:58]

https://codereview.chromium.org/18796006/diff/1/src/untrusted/pthread/nc_sema...
File src/untrusted/pthread/nc_semaphore.c (right):

https://codereview.chromium.org/18796006/diff/1/src/untrusted/pthread/nc_sema...
src/untrusted/pthread/nc_semaphore.c:107: if (NULL == sem) {
Failing gracefully rather than crashing when passed a bogus sem_t is arguably an
anti-feature.  POSIX makes EINVAL for this case a "may fail" error (vs "shall
fail"), so no conforming application is entitled to rely on it.  Passing a null
pointer here is undefined behavior.  The existing functions diagnosed EINVAL
already and that was historical behavior and so I didn't argue for their
replacement implementations behaving differently.

But these are new functions and there is no possibility of preexisting
applications being broken by a recompile.  So I don't think we should diagnose
EINVAL for null pointers.

Note that glibc doesn't diagnose EINVAL for these cases; it will just crash.

https://codereview.chromium.org/18796006/diff/1/src/untrusted/pthread/nc_sema...
src/untrusted/pthread/nc_semaphore.c:150: if (NULL == sem || NULL == value) {
See above about checking |sem| being a null pointer.
There is no excuse whatsoever for checking |value|.
Passing a null pointer is undefined behavior and the best thing to do is crash
(which doesn't require any extra effort to ensure).

[Mark Seaborn, 2013-07-15 15:51:21]

https://codereview.chromium.org/18796006/diff/1/src/untrusted/pthread/nc_sema...
File src/untrusted/pthread/nc_semaphore.c (right):

https://codereview.chromium.org/18796006/diff/1/src/untrusted/pthread/nc_sema...
src/untrusted/pthread/nc_semaphore.c:107: if (NULL == sem) {
On 2013/07/12 21:15:58, Roland McGrath wrote:
> Failing gracefully rather than crashing when passed a bogus sem_t is
> arguably an anti-feature.

OK, I've removed the NULL checks.

The NULL check was to be consistent with the existing code, but I've removed
those other NULL checks in a separate change
(https://codereview.chromium.org/18711004/).

https://codereview.chromium.org/18796006/diff/1/src/untrusted/pthread/nc_sema...
src/untrusted/pthread/nc_semaphore.c:150: if (NULL == sem || NULL == value) {
On 2013/07/12 21:15:58, Roland McGrath wrote:
> See above about checking |sem| being a null pointer.
> There is no excuse whatsoever for checking |value|.
> Passing a null pointer is undefined behavior and the best thing to do is crash
> (which doesn't require any extra effort to ensure).

Done.

[Roland McGrath, 2013-07-15 16:43:22]

lgtm

[Mark Seaborn, 2013-07-15 16:51:05]

Committed patchset #2 manually as r11763 (presubmit successful).