add symlinklockfile module

2 files changed, 73 insertions, 1 deletions

diff --git a/lockfile/symlinklockfile.py b/lockfile/symlinklockfile.py
new file mode 100644
index 0000000..0447e22
--- /dev/null
+++ b/lockfile/symlinklockfile.py
@@ -0,0 +1,66 @@
+from __future__ import absolute_import
+
+import time
+import os
+
+from . import (LockBase, LockFailed, NotLocked, NotMyLock, LockTimeout,
+               AlreadyLocked)
+
+class SymlinkLockFile(LockBase):
+    """Lock access to a file using symlink(2)."""
+
+    def __init__(self, path, threaded=True):
+        # super(SymlinkLockFile).__init(...)
+        LockBase.__init__(self, path, threaded)
+        # split it back!
+        self.unique_name = os.path.split(self.unique_name)[1]
+
+    def acquire(self, timeout=None):
+        # Hopefully unnecessary for symlink.
+        #try:
+        #    open(self.unique_name, "wb").close()
+        #except IOError:
+        #    raise LockFailed("failed to create %s" % self.unique_name)
+
+        end_time = time.time()
+        if timeout is not None and timeout > 0:
+            end_time += timeout
+
+        while True:
+            # Try and create a hard link to it.
+            try:
+                os.symlink(self.unique_name, self.lock_file)
+            except OSError:
+                # Link creation failed.  Maybe we've double-locked?
+                if self.i_am_locking():
+                    # Linked to out unique name. Proceed.
+                    return
+                else:
+                    # Otherwise the lock creation failed.
+                    if timeout is not None and time.time() > end_time:
+                        if timeout > 0:
+                            raise LockTimeout
+                        else:
+                            raise AlreadyLocked
+                    time.sleep(timeout/10 if timeout is not None else 0.1)
+            else:
+                # Link creation succeeded.  We're good to go.
+                return
+
+    def release(self):
+        if not self.is_locked():
+            raise NotLocked
+        elif not self.i_am_locking():
+            raise NotMyLock
+        os.unlink(self.lock_file)
+
+    def is_locked(self):
+        return os.path.islink(self.lock_file)
+
+    def i_am_locking(self):
+        return os.path.islink(self.lock_file) and \
+         os.readlink(self.lock_file) == self.unique_name
+
+    def break_lock(self):
+        if os.path.islink(self.lock_file):  # exists && link
+            os.unlink(self.lock_file)
diff --git a/test/test_lockfile.py b/test/test_lockfile.py
index 3b70cdd..e1f4f72 100644
--- a/test/test_lockfile.py
+++ b/test/test_lockfile.py
@@ -1,12 +1,18 @@
 import sys
 
-import lockfile.linklockfile, lockfile.mkdirlockfile, lockfile.pidlockfile
+import lockfile.linklockfile
+import lockfile.mkdirlockfile
+import lockfile.pidlockfile
+import lockfile.symlinklockfile
 
 from compliancetest import ComplianceTest
     
 class TestLinkLockFile(ComplianceTest):
     class_to_test = lockfile.linklockfile.LinkLockFile
 
+class TestSymlinkLockFile(ComplianceTest):
+    class_to_test = lockfile.symlinklockfile.SymlinkLockFile
+
 class TestMkdirLockFile(ComplianceTest):
     class_to_test = lockfile.mkdirlockfile.MkdirLockFile