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Viewing / Editing File: sqlitelockfile.py

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from __future__ import absolute_import, division

import time
import os

try:
    unicode
except NameError:
    unicode = str

from . import LockBase, NotLocked, NotMyLock, LockTimeout, AlreadyLocked


class SQLiteLockFile(LockBase):
    "Demonstrate SQL-based locking."

    testdb = None

    def __init__(self, path, threaded=True, timeout=None):
        """
        >>> lock = SQLiteLockFile('somefile')
        >>> lock = SQLiteLockFile('somefile', threaded=False)
        """
        LockBase.__init__(self, path, threaded, timeout)
        self.lock_file = unicode(self.lock_file)
        self.unique_name = unicode(self.unique_name)

        if SQLiteLockFile.testdb is None:
            import tempfile
            _fd, testdb = tempfile.mkstemp()
            os.close(_fd)
            os.unlink(testdb)
            del _fd, tempfile
            SQLiteLockFile.testdb = testdb

        import sqlite3
        self.connection = sqlite3.connect(SQLiteLockFile.testdb)

        c = self.connection.cursor()
        try:
            c.execute("create table locks"
                      "("
                      "   lock_file varchar(32),"
                      "   unique_name varchar(32)"
                      ")")
        except sqlite3.OperationalError:
            pass
        else:
            self.connection.commit()
            import atexit
            atexit.register(os.unlink, SQLiteLockFile.testdb)

    def acquire(self, timeout=None):
        timeout = timeout if timeout is not None else self.timeout
        end_time = time.time()
        if timeout is not None and timeout > 0:
            end_time += timeout

        if timeout is None:
            wait = 0.1
        elif timeout <= 0:
            wait = 0
        else:
            wait = timeout / 10

        cursor = self.connection.cursor()

        while True:
            if not self.is_locked():
                # Not locked.  Try to lock it.
                cursor.execute("insert into locks"
                               "  (lock_file, unique_name)"
                               "  values"
                               "  (?, ?)",
                               (self.lock_file, self.unique_name))
                self.connection.commit()

                # Check to see if we are the only lock holder.
                cursor.execute("select * from locks"
                               "  where unique_name = ?",
                               (self.unique_name,))
                rows = cursor.fetchall()
                if len(rows) > 1:
                    # Nope.  Someone else got there.  Remove our lock.
                    cursor.execute("delete from locks"
                                   "  where unique_name = ?",
                                   (self.unique_name,))
                    self.connection.commit()
                else:
                    # Yup.  We're done, so go home.
                    return
            else:
                # Check to see if we are the only lock holder.
                cursor.execute("select * from locks"
                               "  where unique_name = ?",
                               (self.unique_name,))
                rows = cursor.fetchall()
                if len(rows) == 1:
                    # We're the locker, so go home.
                    return

            # Maybe we should wait a bit longer.
            if timeout is not None and time.time() > end_time:
                if timeout > 0:
                    # No more waiting.
                    raise LockTimeout("Timeout waiting to acquire"
                                      " lock for %s" %
                                      self.path)
                else:
                    # Someone else has the lock and we are impatient..
                    raise AlreadyLocked("%s is already locked" % self.path)

            # Well, okay.  We'll give it a bit longer.
            time.sleep(wait)

    def release(self):
        if not self.is_locked():
            raise NotLocked("%s is not locked" % self.path)
        if not self.i_am_locking():
            raise NotMyLock("%s is locked, but not by me (by %s)" %
                            (self.unique_name, self._who_is_locking()))
        cursor = self.connection.cursor()
        cursor.execute("delete from locks"
                       "  where unique_name = ?",
                       (self.unique_name,))
        self.connection.commit()

    def _who_is_locking(self):
        cursor = self.connection.cursor()
        cursor.execute("select unique_name from locks"
                       "  where lock_file = ?",
                       (self.lock_file,))
        return cursor.fetchone()[0]

    def is_locked(self):
        cursor = self.connection.cursor()
        cursor.execute("select * from locks"
                       "  where lock_file = ?",
                       (self.lock_file,))
        rows = cursor.fetchall()
        return not not rows

    def i_am_locking(self):
        cursor = self.connection.cursor()
        cursor.execute("select * from locks"
                       "  where lock_file = ?"
                       "    and unique_name = ?",
                       (self.lock_file, self.unique_name))
        return not not cursor.fetchall()

    def break_lock(self):
        cursor = self.connection.cursor()
        cursor.execute("delete from locks"
                       "  where lock_file = ?",
                       (self.lock_file,))
        self.connection.commit()
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