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.

src/backend/utils/init/globals.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/utils/init/globals.c,v 1.23 1998/05/29 17:00:18 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/utils/init/globals.c,v 1.24 1998/07/09 03:28:51 scrappy Exp $
  *
  * NOTES
  *	  Globals used all over the place should be declared here and not
@@ -44,6 +44,8 @@ bool		QueryCancel = false;
 
 int			MyProcPid;
 
+long		MyCancelKey;
+
 char	   *DataDir;
 
  /*