1/*
2 * Portable condition variable support for windows and pthreads.
3 * Everything is inline, this header can be included where needed.
4 *
5 * APIs generally return 0 on success and non-zero on error,
6 * and the caller needs to use its platform's error mechanism to
7 * discover the error (errno, or GetLastError())
8 *
9 * Note that some implementations cannot distinguish between a
10 * condition variable wait time-out and successful wait. Most often
11 * the difference is moot anyway since the wait condition must be
12 * re-checked.
13 * PyCOND_TIMEDWAIT, in addition to returning negative on error,
14 * thus returns 0 on regular success, 1 on timeout
15 * or 2 if it can't tell.
16 *
17 * There are at least two caveats with using these condition variables,
18 * due to the fact that they may be emulated with Semaphores on
19 * Windows:
20 * 1) While PyCOND_SIGNAL() will wake up at least one thread, we
21 *    cannot currently guarantee that it will be one of the threads
22 *    already waiting in a PyCOND_WAIT() call.  It _could_ cause
23 *    the wakeup of a subsequent thread to try a PyCOND_WAIT(),
24 *    including the thread doing the PyCOND_SIGNAL() itself.
25 *    The same applies to PyCOND_BROADCAST(), if N threads are waiting
26 *    then at least N threads will be woken up, but not necessarily
27 *    those already waiting.
28 *    For this reason, don't make the scheduling assumption that a
29 *    specific other thread will get the wakeup signal
30 * 2) The _mutex_ must be held when calling PyCOND_SIGNAL() and
31 *    PyCOND_BROADCAST().
32 *    While e.g. the posix standard strongly recommends that the mutex
33 *    associated with the condition variable is held when a
34 *    pthread_cond_signal() call is made, this is not a hard requirement,
35 *    although scheduling will not be "reliable" if it isn't.  Here
36 *    the mutex is used for internal synchronization of the emulated
37 *    Condition Variable.
38 */
39
40#ifndef _CONDVAR_H_
41#define _CONDVAR_H_
42
43#include "Python.h"
44
45#ifndef _POSIX_THREADS
46/* This means pthreads are not implemented in libc headers, hence the macro
47   not present in unistd.h. But they still can be implemented as an external
48   library (e.g. gnu pth in pthread emulation) */
49# ifdef HAVE_PTHREAD_H
50#  include <pthread.h> /* _POSIX_THREADS */
51# endif
52#endif
53
54#ifdef _POSIX_THREADS
55/*
56 * POSIX support
57 */
58#define Py_HAVE_CONDVAR
59
60#include <pthread.h>
61
62#define PyCOND_ADD_MICROSECONDS(tv, interval) \
63do { /* TODO: add overflow and truncation checks */ \
64    tv.tv_usec += (long) interval; \
65    tv.tv_sec += tv.tv_usec / 1000000; \
66    tv.tv_usec %= 1000000; \
67} while (0)
68
69/* We assume all modern POSIX systems have gettimeofday() */
70#ifdef GETTIMEOFDAY_NO_TZ
71#define PyCOND_GETTIMEOFDAY(ptv) gettimeofday(ptv)
72#else
73#define PyCOND_GETTIMEOFDAY(ptv) gettimeofday(ptv, (struct timezone *)NULL)
74#endif
75
76/* The following functions return 0 on success, nonzero on error */
77#define PyMUTEX_T pthread_mutex_t
78#define PyMUTEX_INIT(mut)       pthread_mutex_init((mut), NULL)
79#define PyMUTEX_FINI(mut)       pthread_mutex_destroy(mut)
80#define PyMUTEX_LOCK(mut)       pthread_mutex_lock(mut)
81#define PyMUTEX_UNLOCK(mut)     pthread_mutex_unlock(mut)
82
83#define PyCOND_T pthread_cond_t
84#define PyCOND_INIT(cond)       pthread_cond_init((cond), NULL)
85#define PyCOND_FINI(cond)       pthread_cond_destroy(cond)
86#define PyCOND_SIGNAL(cond)     pthread_cond_signal(cond)
87#define PyCOND_BROADCAST(cond)  pthread_cond_broadcast(cond)
88#define PyCOND_WAIT(cond, mut)  pthread_cond_wait((cond), (mut))
89
90/* return 0 for success, 1 on timeout, -1 on error */
91Py_LOCAL_INLINE(int)
92PyCOND_TIMEDWAIT(PyCOND_T *cond, PyMUTEX_T *mut, long long us)
93{
94    int r;
95    struct timespec ts;
96    struct timeval deadline;
97
98    PyCOND_GETTIMEOFDAY(&deadline);
99    PyCOND_ADD_MICROSECONDS(deadline, us);
100    ts.tv_sec = deadline.tv_sec;
101    ts.tv_nsec = deadline.tv_usec * 1000;
102
103    r = pthread_cond_timedwait((cond), (mut), &ts);
104    if (r == ETIMEDOUT)
105        return 1;
106    else if (r)
107        return -1;
108    else
109        return 0;
110}
111
112#elif defined(NT_THREADS)
113/*
114 * Windows (XP, 2003 server and later, as well as (hopefully) CE) support
115 *
116 * Emulated condition variables ones that work with XP and later, plus
117 * example native support on VISTA and onwards.
118 */
119#define Py_HAVE_CONDVAR
120
121
122/* include windows if it hasn't been done before */
123#define WIN32_LEAN_AND_MEAN
124#include <windows.h>
125
126/* options */
127/* non-emulated condition variables are provided for those that want
128 * to target Windows Vista.  Modify this macro to enable them.
129 */
130#ifndef _PY_EMULATED_WIN_CV
131#define _PY_EMULATED_WIN_CV 1  /* use emulated condition variables */
132#endif
133
134/* fall back to emulation if not targeting Vista */
135#if !defined NTDDI_VISTA || NTDDI_VERSION < NTDDI_VISTA
136#undef _PY_EMULATED_WIN_CV
137#define _PY_EMULATED_WIN_CV 1
138#endif
139
140
141#if _PY_EMULATED_WIN_CV
142
143/* The mutex is a CriticalSection object and
144   The condition variables is emulated with the help of a semaphore.
145   Semaphores are available on Windows XP (2003 server) and later.
146   We use a Semaphore rather than an auto-reset event, because although
147   an auto-resent event might appear to solve the lost-wakeup bug (race
148   condition between releasing the outer lock and waiting) because it
149   maintains state even though a wait hasn't happened, there is still
150   a lost wakeup problem if more than one thread are interrupted in the
151   critical place.  A semaphore solves that, because its state is counted,
152   not Boolean.
153   Because it is ok to signal a condition variable with no one
154   waiting, we need to keep track of the number of
155   waiting threads.  Otherwise, the semaphore's state could rise
156   without bound.  This also helps reduce the number of "spurious wakeups"
157   that would otherwise happen.
158
159   This implementation still has the problem that the threads woken
160   with a "signal" aren't necessarily those that are already
161   waiting.  It corresponds to listing 2 in:
162   http://birrell.org/andrew/papers/ImplementingCVs.pdf
163
164   Generic emulations of the pthread_cond_* API using
165   earlier Win32 functions can be found on the Web.
166   The following read can be give background information to these issues,
167   but the implementations are all broken in some way.
168   http://www.cse.wustl.edu/~schmidt/win32-cv-1.html
169*/
170
171typedef CRITICAL_SECTION PyMUTEX_T;
172
173Py_LOCAL_INLINE(int)
174PyMUTEX_INIT(PyMUTEX_T *cs)
175{
176    InitializeCriticalSection(cs);
177    return 0;
178}
179
180Py_LOCAL_INLINE(int)
181PyMUTEX_FINI(PyMUTEX_T *cs)
182{
183    DeleteCriticalSection(cs);
184    return 0;
185}
186
187Py_LOCAL_INLINE(int)
188PyMUTEX_LOCK(PyMUTEX_T *cs)
189{
190    EnterCriticalSection(cs);
191    return 0;
192}
193
194Py_LOCAL_INLINE(int)
195PyMUTEX_UNLOCK(PyMUTEX_T *cs)
196{
197    LeaveCriticalSection(cs);
198    return 0;
199}
200
201/* The ConditionVariable object.  From XP onwards it is easily emulated with
202 * a Semaphore
203 */
204
205typedef struct _PyCOND_T
206{
207    HANDLE sem;
208    int waiting; /* to allow PyCOND_SIGNAL to be a no-op */
209} PyCOND_T;
210
211Py_LOCAL_INLINE(int)
212PyCOND_INIT(PyCOND_T *cv)
213{
214    /* A semaphore with a "large" max value,  The positive value
215     * is only needed to catch those "lost wakeup" events and
216     * race conditions when a timed wait elapses.
217     */
218    cv->sem = CreateSemaphore(NULL, 0, 100000, NULL);
219    if (cv->sem==NULL)
220        return -1;
221    cv->waiting = 0;
222    return 0;
223}
224
225Py_LOCAL_INLINE(int)
226PyCOND_FINI(PyCOND_T *cv)
227{
228    return CloseHandle(cv->sem) ? 0 : -1;
229}
230
231/* this implementation can detect a timeout.  Returns 1 on timeout,
232 * 0 otherwise (and -1 on error)
233 */
234Py_LOCAL_INLINE(int)
235_PyCOND_WAIT_MS(PyCOND_T *cv, PyMUTEX_T *cs, DWORD ms)
236{
237    DWORD wait;
238    cv->waiting++;
239    PyMUTEX_UNLOCK(cs);
240    /* "lost wakeup bug" would occur if the caller were interrupted here,
241     * but we are safe because we are using a semaphore which has an internal
242     * count.
243     */
244    wait = WaitForSingleObjectEx(cv->sem, ms, FALSE);
245    PyMUTEX_LOCK(cs);
246    if (wait != WAIT_OBJECT_0)
247        --cv->waiting;
248        /* Here we have a benign race condition with PyCOND_SIGNAL.
249         * When failure occurs or timeout, it is possible that
250         * PyCOND_SIGNAL also decrements this value
251         * and signals releases the mutex.  This is benign because it
252         * just means an extra spurious wakeup for a waiting thread.
253         * ('waiting' corresponds to the semaphore's "negative" count and
254         * we may end up with e.g. (waiting == -1 && sem.count == 1).  When
255         * a new thread comes along, it will pass right throuhgh, having
256         * adjusted it to (waiting == 0 && sem.count == 0).
257         */
258
259    if (wait == WAIT_FAILED)
260        return -1;
261    /* return 0 on success, 1 on timeout */
262    return wait != WAIT_OBJECT_0;
263}
264
265Py_LOCAL_INLINE(int)
266PyCOND_WAIT(PyCOND_T *cv, PyMUTEX_T *cs)
267{
268    int result = _PyCOND_WAIT_MS(cv, cs, INFINITE);
269    return result >= 0 ? 0 : result;
270}
271
272Py_LOCAL_INLINE(int)
273PyCOND_TIMEDWAIT(PyCOND_T *cv, PyMUTEX_T *cs, long long us)
274{
275    return _PyCOND_WAIT_MS(cv, cs, (DWORD)(us/1000));
276}
277
278Py_LOCAL_INLINE(int)
279PyCOND_SIGNAL(PyCOND_T *cv)
280{
281    /* this test allows PyCOND_SIGNAL to be a no-op unless required
282     * to wake someone up, thus preventing an unbounded increase of
283     * the semaphore's internal counter.
284     */
285    if (cv->waiting > 0) {
286        /* notifying thread decreases the cv->waiting count so that
287         * a delay between notify and actual wakeup of the target thread
288         * doesn't cause a number of extra ReleaseSemaphore calls.
289         */
290        cv->waiting--;
291        return ReleaseSemaphore(cv->sem, 1, NULL) ? 0 : -1;
292    }
293    return 0;
294}
295
296Py_LOCAL_INLINE(int)
297PyCOND_BROADCAST(PyCOND_T *cv)
298{
299    int waiting = cv->waiting;
300    if (waiting > 0) {
301        cv->waiting = 0;
302        return ReleaseSemaphore(cv->sem, waiting, NULL) ? 0 : -1;
303    }
304    return 0;
305}
306
307#else
308
309/* Use native Win7 primitives if build target is Win7 or higher */
310
311/* SRWLOCK is faster and better than CriticalSection */
312typedef SRWLOCK PyMUTEX_T;
313
314Py_LOCAL_INLINE(int)
315PyMUTEX_INIT(PyMUTEX_T *cs)
316{
317    InitializeSRWLock(cs);
318    return 0;
319}
320
321Py_LOCAL_INLINE(int)
322PyMUTEX_FINI(PyMUTEX_T *cs)
323{
324    return 0;
325}
326
327Py_LOCAL_INLINE(int)
328PyMUTEX_LOCK(PyMUTEX_T *cs)
329{
330    AcquireSRWLockExclusive(cs);
331    return 0;
332}
333
334Py_LOCAL_INLINE(int)
335PyMUTEX_UNLOCK(PyMUTEX_T *cs)
336{
337    ReleaseSRWLockExclusive(cs);
338    return 0;
339}
340
341
342typedef CONDITION_VARIABLE  PyCOND_T;
343
344Py_LOCAL_INLINE(int)
345PyCOND_INIT(PyCOND_T *cv)
346{
347    InitializeConditionVariable(cv);
348    return 0;
349}
350Py_LOCAL_INLINE(int)
351PyCOND_FINI(PyCOND_T *cv)
352{
353    return 0;
354}
355
356Py_LOCAL_INLINE(int)
357PyCOND_WAIT(PyCOND_T *cv, PyMUTEX_T *cs)
358{
359    return SleepConditionVariableSRW(cv, cs, INFINITE, 0) ? 0 : -1;
360}
361
362/* This implementation makes no distinction about timeouts.  Signal
363 * 2 to indicate that we don't know.
364 */
365Py_LOCAL_INLINE(int)
366PyCOND_TIMEDWAIT(PyCOND_T *cv, PyMUTEX_T *cs, long long us)
367{
368    return SleepConditionVariableSRW(cv, cs, (DWORD)(us/1000), 0) ? 2 : -1;
369}
370
371Py_LOCAL_INLINE(int)
372PyCOND_SIGNAL(PyCOND_T *cv)
373{
374     WakeConditionVariable(cv);
375     return 0;
376}
377
378Py_LOCAL_INLINE(int)
379PyCOND_BROADCAST(PyCOND_T *cv)
380{
381     WakeAllConditionVariable(cv);
382     return 0;
383}
384
385
386#endif /* _PY_EMULATED_WIN_CV */
387
388#endif /* _POSIX_THREADS, NT_THREADS */
389
390#endif /* _CONDVAR_H_ */
391