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/include/miscadmin.h

@@ -11,7 +11,7 @@
  *
  * Copyright (c) 1994, Regents of the University of California
  *
- * $Id: miscadmin.h,v 1.26 1998/06/09 17:13:06 momjian Exp $
+ * $Id: miscadmin.h,v 1.27 1998/07/09 03:28:55 scrappy Exp $
  *
  * NOTES
  *	  some of the information in this file will be moved to
@@ -42,6 +42,8 @@ extern char *DataDir;
 
 extern int	MyProcPid;
 
+extern long	MyCancelKey;
+
 extern char OutputFileName[];
 
 /*