bpo-30768: Recompute timeout on interrupted lock (GH-4103)

Fix the pthread+semaphore implementation of
PyThread_acquire_lock_timed() when called with timeout > 0 and
intr_flag=0: recompute the timeout if sem_timedwait() is interrupted
by a signal (EINTR).

See also the PEP 475.

The pthread implementation of PyThread_acquire_lock() now fails with
a fatal error if the timeout is larger than PY_TIMEOUT_MAX, as done
in the Windows implementation.

The check prevents any risk of overflow in PyThread_acquire_lock().

Add also PY_DWORD_MAX constant.

Include/pyport.h

@@ -787,6 +787,9 @@ extern _invalid_parameter_handler _Py_silent_invalid_parameter_handler;
 #include <android/api-level.h>
 #endif
 
+/* Maximum value of the Windows DWORD type */
+#define PY_DWORD_MAX 4294967295U
+
 /* This macro used to tell whether Python was built with multithreading
  * enabled.  Now multithreading is always enabled, but keep the macro
  * for compatibility.

Include/pythread.h

@@ -42,16 +42,23 @@ PyAPI_FUNC(int) PyThread_acquire_lock(PyThread_type_lock, int);
    and floating-point numbers allowed.
 */
 #define PY_TIMEOUT_T long long
-#define PY_TIMEOUT_MAX PY_LLONG_MAX
 
-/* In the NT API, the timeout is a DWORD and is expressed in milliseconds */
-#if defined (NT_THREADS)
-#if 0xFFFFFFFFLL * 1000 < PY_TIMEOUT_MAX
-#undef PY_TIMEOUT_MAX
-#define PY_TIMEOUT_MAX (0xFFFFFFFFLL * 1000)
-#endif
+#if defined(_POSIX_THREADS)
+   /* PyThread_acquire_lock_timed() uses _PyTime_FromNanoseconds(us * 1000),
+      convert microseconds to nanoseconds. */
+#  define PY_TIMEOUT_MAX (PY_LLONG_MAX / 1000)
+#elif defined (NT_THREADS)
+   /* In the NT API, the timeout is a DWORD and is expressed in milliseconds */
+#  if 0xFFFFFFFFLL * 1000 < PY_LLONG_MAX
+#    define PY_TIMEOUT_MAX (0xFFFFFFFFLL * 1000)
+#  else
+#    define PY_TIMEOUT_MAX PY_LLONG_MAX
+#  endif
+#else
+#  define PY_TIMEOUT_MAX PY_LLONG_MAX
 #endif
 
+
 /* If microseconds == 0, the call is non-blocking: it returns immediately
    even when the lock can't be acquired.
    If microseconds > 0, the call waits up to the specified duration.

Misc/NEWS.d/next/Library/2017-10-24-12-00-16.bpo-30768.Om8Yj_.rst

+Fix the pthread+semaphore implementation of PyThread_acquire_lock_timed() when
+called with timeout > 0 and intr_flag=0: recompute the timeout if
+sem_timedwait() is interrupted by a signal (EINTR). See also the :pep:`475`.

Modules/_threadmodule.c

@@ -1363,9 +1363,11 @@ PyInit__thread(void)
     if (m == NULL)
         return NULL;
 
-    timeout_max = PY_TIMEOUT_MAX / 1000000;
-    time_max = floor(_PyTime_AsSecondsDouble(_PyTime_MAX));
+    timeout_max = (double)PY_TIMEOUT_MAX * 1e-6;
+    time_max = _PyTime_AsSecondsDouble(_PyTime_MAX);
     timeout_max = Py_MIN(timeout_max, time_max);
+    /* Round towards minus infinity */
+    timeout_max = floor(timeout_max);
 
     v = PyFloat_FromDouble(timeout_max);
     if (!v)

Modules/_winapi.c

@@ -61,8 +61,6 @@
 
 #define T_HANDLE T_POINTER
 
-#define DWORD_MAX 4294967295U
-
 /* Grab CancelIoEx dynamically from kernel32 */
 static int has_CancelIoEx = -1;
 static BOOL (CALLBACK *Py_CancelIoEx)(HANDLE, LPOVERLAPPED);
@@ -184,11 +182,11 @@ class DWORD_return_converter(CReturnConverter):
 
     def render(self, function, data):
         self.declare(data)
-        self.err_occurred_if("_return_value == DWORD_MAX", data)
+        self.err_occurred_if("_return_value == PY_DWORD_MAX", data)
         data.return_conversion.append(
             'return_value = Py_BuildValue("k", _return_value);\n')
 [python start generated code]*/
-/*[python end generated code: output=da39a3ee5e6b4b0d input=94819e72d2c6d558]*/
+/*[python end generated code: output=da39a3ee5e6b4b0d input=4527052fe06e5823]*/
 
 #include "clinic/_winapi.c.h"
 
@@ -1009,7 +1007,7 @@ _winapi_GetExitCodeProcess_impl(PyObject *module, HANDLE process)
 
     if (! result) {
         PyErr_SetFromWindowsErr(GetLastError());
-        exit_code = DWORD_MAX;
+        exit_code = PY_DWORD_MAX;
     }
 
     return exit_code;
@@ -1466,7 +1464,7 @@ _winapi_WriteFile_impl(PyObject *module, HANDLE handle, PyObject *buffer,
     }
 
     Py_BEGIN_ALLOW_THREADS
-    len = (DWORD)Py_MIN(buf->len, DWORD_MAX);
+    len = (DWORD)Py_MIN(buf->len, PY_DWORD_MAX);
     ret = WriteFile(handle, buf->buf, len, &written,
                     overlapped ? &overlapped->overlapped : NULL);
     Py_END_ALLOW_THREADS

Modules/clinic/_winapi.c.h

@@ -460,7 +460,7 @@ _winapi_GetExitCodeProcess(PyObject *module, PyObject *arg)
         goto exit;
     }
     _return_value = _winapi_GetExitCodeProcess_impl(module, process);
-    if ((_return_value == DWORD_MAX) && PyErr_Occurred()) {
+    if ((_return_value == PY_DWORD_MAX) && PyErr_Occurred()) {
         goto exit;
     }
     return_value = Py_BuildValue("k", _return_value);
@@ -487,7 +487,7 @@ _winapi_GetLastError(PyObject *module, PyObject *Py_UNUSED(ignored))
     DWORD _return_value;
 
     _return_value = _winapi_GetLastError_impl(module);
-    if ((_return_value == DWORD_MAX) && PyErr_Occurred()) {
+    if ((_return_value == PY_DWORD_MAX) && PyErr_Occurred()) {
         goto exit;
     }
     return_value = Py_BuildValue("k", _return_value);
@@ -889,4 +889,4 @@ _winapi_WriteFile(PyObject *module, PyObject **args, Py_ssize_t nargs, PyObject
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=afa6bd61eb0f18d2 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=fba2ad7bf1a87e4a input=a9049054013a1b77]*/

Modules/posixmodule.c

@@ -390,8 +390,6 @@ static int win32_can_symlink = 0;
 #endif
 #endif
 
-#define DWORD_MAX 4294967295U
-
 #ifdef MS_WINDOWS
 #define INITFUNC PyInit_nt
 #define MODNAME "nt"
@@ -3817,7 +3815,7 @@ os__getvolumepathname_impl(PyObject *module, PyObject *path)
     /* Volume path should be shorter than entire path */
     buflen = Py_MAX(buflen, MAX_PATH);
 
-    if (buflen > DWORD_MAX) {
+    if (buflen > PY_DWORD_MAX) {
         PyErr_SetString(PyExc_OverflowError, "path too long");
         return NULL;
     }

Python/thread_nt.h

@@ -283,12 +283,13 @@ PyThread_acquire_lock_timed(PyThread_type_lock aLock,
         milliseconds = microseconds / 1000;
         if (microseconds % 1000 > 0)
             ++milliseconds;
-        if ((DWORD) milliseconds != milliseconds)
-            Py_FatalError("Timeout too large for a DWORD, "
-                           "please check PY_TIMEOUT_MAX");
+        if (milliseconds > PY_DWORD_MAX) {
+            Py_FatalError("Timeout larger than PY_TIMEOUT_MAX");
+        }
     }
-    else
+    else {
         milliseconds = INFINITE;
+    }
 
     dprintf(("%lu: PyThread_acquire_lock_timed(%p, %lld) called\n",
              PyThread_get_thread_ident(), aLock, microseconds));

Python/thread_pthread.h

@@ -318,23 +318,66 @@ PyThread_acquire_lock_timed(PyThread_type_lock lock, PY_TIMEOUT_T microseconds,
     sem_t *thelock = (sem_t *)lock;
     int status, error = 0;
     struct timespec ts;
+    _PyTime_t deadline = 0;
 
     (void) error; /* silence unused-but-set-variable warning */
     dprintf(("PyThread_acquire_lock_timed(%p, %lld, %d) called\n",
              lock, microseconds, intr_flag));
 
-    if (microseconds > 0)
+    if (microseconds > PY_TIMEOUT_MAX) {
+        Py_FatalError("Timeout larger than PY_TIMEOUT_MAX");
+    }
+
+    if (microseconds > 0) {
         MICROSECONDS_TO_TIMESPEC(microseconds, ts);
-    do {
-        if (microseconds > 0)
+
+        if (!intr_flag) {
+            /* cannot overflow thanks to (microseconds > PY_TIMEOUT_MAX)
+               check done above */
+            _PyTime_t timeout = _PyTime_FromNanoseconds(microseconds * 1000);
+            deadline = _PyTime_GetMonotonicClock() + timeout;
+        }
+    }
+
+    while (1) {
+        if (microseconds > 0) {
             status = fix_status(sem_timedwait(thelock, &ts));
-        else if (microseconds == 0)
+        }
+        else if (microseconds == 0) {
             status = fix_status(sem_trywait(thelock));
-        else
+        }
+        else {
             status = fix_status(sem_wait(thelock));
+        }
+
         /* Retry if interrupted by a signal, unless the caller wants to be
            notified.  */
-    } while (!intr_flag && status == EINTR);
+        if (intr_flag || status != EINTR) {
+            break;
+        }
+
+        if (microseconds > 0) {
+            /* wait interrupted by a signal (EINTR): recompute the timeout */
+            _PyTime_t dt = deadline - _PyTime_GetMonotonicClock();
+            if (dt < 0) {
+                status = ETIMEDOUT;
+                break;
+            }
+            else if (dt > 0) {
+                _PyTime_t realtime_deadline = _PyTime_GetSystemClock() + dt;
+                if (_PyTime_AsTimespec(realtime_deadline, &ts) < 0) {
+                    /* Cannot occur thanks to (microseconds > PY_TIMEOUT_MAX)
+                       check done above */
+                    Py_UNREACHABLE();
+                }
+                /* no need to update microseconds value, the code only care
+                   if (microseconds > 0 or (microseconds == 0). */
+            }
+            else {
+                microseconds = 0;
+            }
+        }
+    }
 
     /* Don't check the status if we're stopping because of an interrupt.  */
     if (!(intr_flag && status == EINTR)) {