osdep: add POSIX semaphore emulation for OSX

OSX is POSIX conformant, but it's a sad joke: it provides the
<semaphore.h> prototype as the standard demands, but they're empty
wrappers, and all functions just return ENOSYS.

Emulate them similar to how osdep/io.h emulate filesystem functions on
Windows. By including the header, working sem_* functions become
available.

To make it async-signal safe, use a pipe for wakeup (write() is AS-safe,
but mutexes can't be). Actually I'm not sure anymore if we really need
AS-safety, but for now the emulation can do it.

On Linux, the system provides a far more efficient and robust
implementation. We definitely want to avoid using the emulation if
possible, so this code is active on OSX only. For convenience we always
build the source file though, even if the implementation is disabled and
no actual code is generated.

(Linux provides working semaphores, but is formally not POSIX
conformant. On OSX it's the opposite. Is POSIX a complete joke?)

1 files changed, 58 insertions, 0 deletions

diff --git a/osdep/semaphore.h b/osdep/semaphore.h
new file mode 100644
index 0000000000..832f059df4
--- /dev/null
+++ b/osdep/semaphore.h
@@ -0,0 +1,58 @@
+#ifndef MP_SEMAPHORE_H_
+#define MP_SEMAPHORE_H_
+
+#include <semaphore.h>
+
+// OSX provides non-working empty stubs, so we emulate them.
+// This should be AS-safe, but cancellation issues were ignored.
+// sem_getvalue() is not provided.
+// sem_post() won't always correctly return an error on overflow.
+// Process-shared semantics are not provided.
+
+#ifdef __APPLE__
+
+#define MP_SEMAPHORE_EMULATION
+
+#include <pthread.h>
+
+#define MP_SEM_VALUE_MAX 4096
+
+typedef struct {
+    int wakeup_pipe[2];
+    pthread_mutex_t lock;
+    // protected by lock
+    unsigned int count;
+} mp_sem_t;
+
+int mp_sem_init(mp_sem_t *sem, int pshared, unsigned int value);
+int mp_sem_wait(mp_sem_t *sem);
+int mp_sem_trywait(mp_sem_t *sem);
+int mp_sem_timedwait(mp_sem_t *sem, const struct timespec *abs_timeout);
+int mp_sem_post(mp_sem_t *sem);
+int mp_sem_destroy(mp_sem_t *sem);
+
+#undef sem_init
+#undef sem_wait
+#undef sem_trywait
+#undef sem_timedwait
+#undef sem_post
+#undef sem_getvalue
+#undef sem_destroy
+#undef sem_getvalue
+#undef sem_t
+#undef SEM_VALUE_MAX
+
+#define sem_init        mp_sem_init
+#define sem_wait        mp_sem_wait
+#define sem_trywait     mp_sem_trywait
+#define sem_timedwait   mp_sem_timedwait
+#define sem_post        mp_sem_post
+#define sem_destroy     mp_sem_destroy
+#define sem_t           mp_sem_t
+#define SEM_VALUE_MAX   MP_SEM_VALUE_MAX
+
+#define sem_getvalue    (void)
+
+#endif
+
+#endif