Added "locmem" and "file" cache backends. "locmem" is a thread-safe local-memory cache, and "file" is a file-based cache.

This refs #515; much thanks to Eugene Lazutkin!


git-svn-id: http://code.djangoproject.com/svn/django/trunk@686 bcc190cf-cafb-0310-a4f2-bffc1f526a37

1 files changed, 85 insertions, 0 deletions

diff --git a/django/utils/synch.py b/django/utils/synch.py
new file mode 100644
index 0000000000..1e6b546a78
--- /dev/null
+++ b/django/utils/synch.py
@@ -0,0 +1,85 @@
+"""
+Synchronization primitives:
+    
+    - reader-writer lock (preference to writers)
+
+(Contributed to Django by eugene@lazutkin.com)
+"""
+
+import threading
+
+class RWLock:
+    """
+    Classic implementation of reader-writer lock with preference to writers.
+    
+    Readers can access a resource simultaneously.
+    Writers get an exclusive access.
+    
+    API is self-descriptive:
+        reader_enters()
+        reader_leaves()
+        writer_enters()
+        writer_leaves()
+    """
+    
+    def __init__(self):
+        self.mutex     = threading.RLock()
+        self.can_read  = threading.Semaphore(0)
+        self.can_write = threading.Semaphore(0)
+        self.active_readers  = 0
+        self.active_writers  = 0
+        self.waiting_readers = 0
+        self.waiting_writers = 0
+        
+    def reader_enters(self):
+        self.mutex.acquire()
+        try:
+            if self.active_writers == 0 and self.waiting_writers == 0:
+                self.active_readers += 1
+                self.can_read.release()
+            else:
+                self.waiting_readers += 1
+        finally:
+            self.mutex.release()
+        self.can_read.acquire()
+        
+    def reader_leaves(self):
+        self.mutex.acquire()
+        try:
+            self.active_readers -= 1
+            if self.active_readers == 0 and self.waiting_writers != 0:
+                self.active_writers  += 1
+                self.waiting_writers -= 1
+                self.can_write.release()
+        finally:
+            self.mutex.release()
+    
+    def writer_enters(self):
+        self.mutex.acquire()
+        try:
+            if self.active_writers == 0 and self.waiting_writers == 0 and self.active_readers == 0:
+                self.active_writers += 1
+                self.can_write.release()
+            else:
+                self.waiting_writers += 1
+        finally:
+            self.mutex.release()
+        self.can_write.acquire()
+        
+    def writer_leaves(self):
+        self.mutex.acquire()
+        try:
+            self.active_writers -= 1
+            if self.waiting_writers != 0:
+                self.active_writers  += 1
+                self.waiting_writers -= 1
+                self.can_write.release()
+            elif self.waiting_readers != 0:
+                t = self.waiting_readers
+                self.waiting_readers = 0
+                self.active_readers += t
+                while t > 0:
+                    self.can_read.release()
+                    t -= 1
+        finally:
+            self.mutex.release()