Invent a new, more thread-safe version of PQrequestCancel, called PQcancel.

Use this new function in psql.  Implement query cancellation in psql for
Windows.  Code by Magnus Hagander, documentation and minor editorialization
by Tom Lane.

src/interfaces/libpq/libpq-fe.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2004, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/interfaces/libpq/libpq-fe.h,v 1.112 2004/10/18 22:00:42 tgl Exp $
+ * $PostgreSQL: pgsql/src/interfaces/libpq/libpq-fe.h,v 1.113 2004/10/30 23:11:27 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -116,6 +116,12 @@ typedef struct pg_conn PGconn;
  */
 typedef struct pg_result PGresult;
 
+/* PGcancel encapsulates the information needed to cancel a running
+ * query on an existing connection.
+ * The contents of this struct are not supposed to be known to applications.
+ */
+typedef struct pg_cancel PGcancel;
+
 /* PGnotify represents the occurrence of a NOTIFY message.
  * Ideally this would be an opaque typedef, but it's so simple that it's
  * unlikely to change.
@@ -234,7 +240,16 @@ extern PostgresPollingStatusType PQresetPoll(PGconn *conn);
 /* Synchronous (blocking) */
 extern void PQreset(PGconn *conn);
 
+/* request a cancel structure */
+extern PGcancel *PQgetCancel(PGconn *conn);
+
+/* free a cancel structure */
+extern void PQfreeCancel(PGcancel *cancel);
+
 /* issue a cancel request */
+extern int	PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);
+
+/* backwards compatible version of PQcancel; not thread-safe */
 extern int	PQrequestCancel(PGconn *conn);
 
 /* Accessor functions for PGconn objects */