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Thread-blocking (was: socket.gethostbyname() thread-safe?)

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Michael Ströder <michael at stroeder.com> wrote:>The whole HTTP serving stuff seems to work just fine. But the>underlying LDAP libs (OpenLDAP 1.2.x in my case) are known not to be>thread-safe. Therefore I'm using locks to protect calls into>python-ldap methods. My problem is that sometimes the whole thing>blocks. Mainly if the ldap.open() call (creating a new connection)>does not return or the client aborts the connection.Sounds like a C code problem. gdb is your friend.Hint: libpthread redefines some libc functions for thread safety. Youmust link ldapmodule with -lpthread. You must also compile with-D_REENTRANT, *even if ldapmodule itself does not use pthreadfunctions directly*.>Two questions:>- Does the python-ldap module also has to set the global interpreter>lock as mentioned in the API docs (api/threads.html)?When the threads are created in Python and call out to a C extensionmodule, the global interpreter lock is already held by that thread.Blocking C functions in the extension module should be surroundedby Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS to *release* theglobal interpreter lock and allow other threads to run. This isdone correctly in ldapmodule.>- What is a good practice of exception handling to avoid blocking?>Should I catch all exceptions to avoid that the thread won't call>properly thread.exit(). Is this already done by>SocketServer.ThreadingMixIn?Calling thread.exit() explicitely is unnecessary.But see below.>  def handler(self,*args):>    req = args[0]>    try:>      w2lhandler.HandleHTTPRequest(req.inp,req.out,req.err,req.env)>    except:>      # Avoid blocking>      pass>    req.Finish()>    thread.exit()Looks fishy. I assume it "blocks" when req.Finish() isn't called(unterminated fcgi call)? That's what try/finally is for:def handler(self,*args):  req = args[0]  try:    w2lhandler.HandleHTTPRequest(req.inp,req.out,req.err,req.env)  finally:    req.Finish()HTH,Johannes

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