root / env / lib / python2.7 / site-packages / django / utils / synch.py @ 1a305335
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| 1 | 1a305335 | officers | """
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| 2 | Synchronization primitives:
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| 3 |
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| 4 | - reader-writer lock (preference to writers)
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| 5 |
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| 6 | (Contributed to Django by eugene@lazutkin.com)
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| 7 | """
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| 8 | |||
| 9 | from __future__ import with_statement |
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| 10 | |||
| 11 | import contextlib |
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| 12 | try:
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| 13 | import threading |
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| 14 | except ImportError: |
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| 15 | import dummy_threading as threading |
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| 16 | |||
| 17 | |||
| 18 | class RWLock(object): |
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| 19 | """
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| 20 | Classic implementation of reader-writer lock with preference to writers.
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| 21 |
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| 22 | Readers can access a resource simultaneously.
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| 23 | Writers get an exclusive access.
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| 24 |
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| 25 | API is self-descriptive:
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| 26 | reader_enters()
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| 27 | reader_leaves()
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| 28 | writer_enters()
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| 29 | writer_leaves()
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| 30 | """
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| 31 | def __init__(self): |
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| 32 | self.mutex = threading.RLock()
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| 33 | self.can_read = threading.Semaphore(0) |
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| 34 | self.can_write = threading.Semaphore(0) |
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| 35 | self.active_readers = 0 |
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| 36 | self.active_writers = 0 |
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| 37 | self.waiting_readers = 0 |
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| 38 | self.waiting_writers = 0 |
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| 39 | |||
| 40 | def reader_enters(self): |
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| 41 | with self.mutex: |
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| 42 | if self.active_writers == 0 and self.waiting_writers == 0: |
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| 43 | self.active_readers += 1 |
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| 44 | self.can_read.release()
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| 45 | else:
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| 46 | self.waiting_readers += 1 |
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| 47 | self.can_read.acquire()
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| 48 | |||
| 49 | def reader_leaves(self): |
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| 50 | with self.mutex: |
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| 51 | self.active_readers -= 1 |
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| 52 | if self.active_readers == 0 and self.waiting_writers != 0: |
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| 53 | self.active_writers += 1 |
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| 54 | self.waiting_writers -= 1 |
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| 55 | self.can_write.release()
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| 56 | |||
| 57 | @contextlib.contextmanager
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| 58 | def reader(self): |
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| 59 | self.reader_enters()
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| 60 | try:
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| 61 | yield
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| 62 | finally:
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| 63 | self.reader_leaves()
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| 64 | |||
| 65 | def writer_enters(self): |
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| 66 | with self.mutex: |
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| 67 | if self.active_writers == 0 and self.waiting_writers == 0 and self.active_readers == 0: |
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| 68 | self.active_writers += 1 |
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| 69 | self.can_write.release()
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| 70 | else:
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| 71 | self.waiting_writers += 1 |
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| 72 | self.can_write.acquire()
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| 73 | |||
| 74 | def writer_leaves(self): |
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| 75 | with self.mutex: |
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| 76 | self.active_writers -= 1 |
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| 77 | if self.waiting_writers != 0: |
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| 78 | self.active_writers += 1 |
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| 79 | self.waiting_writers -= 1 |
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| 80 | self.can_write.release()
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| 81 | elif self.waiting_readers != 0: |
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| 82 | t = self.waiting_readers
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| 83 | self.waiting_readers = 0 |
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| 84 | self.active_readers += t
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| 85 | while t > 0: |
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| 86 | self.can_read.release()
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| 87 | t -= 1
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| 88 | |||
| 89 | @contextlib.contextmanager
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| 90 | def writer(self): |
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| 91 | self.writer_enters()
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| 92 | try:
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| 93 | yield
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| 94 | finally:
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| 95 | self.writer_leaves() |