CGIs and file exclusion

[Michele Simionato]

"Diez B. Roggisch" <deetsNOSPAM at web.de> wrote in message news:<cmecq2$ksm$00$1 at news.t-online.com>...
> I'd suggest using ZODB instead of directly pickling - then every cgi can
> open the database with its own connection. ZODB will manage concurrency
> issues.

Ok, but then I guess you need an external long-living process keeping the
DB open for you. If the cgi script opens and close the database by itself, 
the only thing the ZODB buys for you is raising

IOError: [Errno 11] Resource temporarily unavailable

(not unexpectedly). Here is what I tried:

$ cat concurrency.py
import ZODB
from ZODB.FileStorage import FileStorage

def openzodb(dbname):
    db = ZODB.DB(FileStorage(dbname + ".fs"))
    conn = db.open()
    return db, conn, conn.root()

if __name__ == "__main__":
    print "Opening the db ..."
    db, conn, root = openzodb("x")
    print "Storing something ..."
    root["somekey"] = "somedata"
    get_transaction().commit()
    print "Closing the db ..."
    conn.close(); db.close()

$ echo Makefile
default:
	python concurrency.py& 
	python concurrency.py

$ make
python concurrency.py&
python concurrency.py
Opening the db ...
Opening the db ...
Traceback (most recent call last):
  File "concurrency.py", line 12, in ?
    db, conn, root = openzodb("x")
  File "concurrency.py", line 6, in openzodb
    db = ZODB.DB(FileStorage(dbname + ".fs"))
  File "/opt/zope/lib/python/ZODB/FileStorage.py", line 232, in __init__
Storing something ...
    self._lock_file = LockFile(file_name + '.lock')
  File "/opt/zope/lib/python/ZODB/lock_file.py", line 62, in __init__
    lock_file(self._fp)
  File "/opt/zope/lib/python/ZODB/lock_file.py", line 42, in lock_file
    fcntl.flock(file.fileno(), _flags)
IOError: [Errno 11] Resource temporarily unavailable
Closing the db ...

BTW, it is clear from the traceback than internally ZODB uses fcntl
and probably a custom solution based on it would be simpler than 
installing the ZODB (unless the OP has some reason to want it).
But I guess you had in mind something different, care to explain?

  Michele Simionato