arrive even while idle.

 to be meaningful.

From: Tom Lane <tgl@sss.pgh.pa.us>

Making PQrequestCancel safe to call in a signal handler turned out to be
much easier than I feared.  So here are the diffs.

Some notes:
  * I modified the postmaster's packet "iodone" callback interface to allow
    the callback routine to return a continue-or-drop-connection return
    code; this was necessary to allow the connection to be closed after
    receiving a Cancel, rather than proceeding to launch a new backend...
    Being a neatnik, I also made the iodone proc have a typechecked
    parameter list.
  * I deleted all code I could find that had to do with OOB.
  * I made some edits to ensure that all signals mentioned in the code
    are referred to symbolically not by numbers ("SIGUSR2" not "2").
    I think Bruce may have already done at least some of the same edits;
    I hope that merging these patches is not too painful.

1. Rewritten libpq to allow asynchronous clients.

2. Implemented client side of cancel protocol in library,
   and patched psql.c to send a cancel request upon SIGINT.  The
   backend doesn't notice it yet :-(

3. Implemented 'Z' protocol message addition and renaming of
   copy in/out start messages.  These are implemented conditionally,
   ie, the client protocol version is checked; so the code should
   still work with 1.0 clients.

4. Revised protocol and libpq sgml documents (don't have an SGML
   compiler, though, so there may be some markup glitches here).


What remains to be done:

1. Implement addition of atttypmod field to RowDescriptor messages.
   The client-side code is there but ifdef'd out.  I have no idea
   what to change on the backend side.  The field should be sent
   only if protocol >= 2.0, of course.

2. Implement backend response to cancel requests received as OOB
   messages.  (This prolly need not be conditional on protocol
   version; just do it if you get SIGURG.)

3. Update libpq.3.  (I'm hoping this can be generated mechanically
   from libpq.sgml... if not, will do it by hand.)  Is there any
   other doco to fix?

4. Update non-libpq interfaces as necessary.  I patched libpgtcl
   so that it would compile, but haven't tested it.  Dunno what
   needs to be done with the other interfaces.

Have at it!

Tom Lane