Fix tab completion of "SET variable TO|=" to not offer bogus completions.

Don't think that the context "UPDATE tab SET var =" is a GUC-setting
command.

If we have "SET var =" but the "var" is not a known GUC variable,
don't offer any completions.  The most likely explanation is that
we've misparsed the context and it's not really a GUC-setting command.

Per gripe from Ken Tanzer.  Back-patch to 9.6.  The issue exists
further back, but before 9.6 the code looks very different and it
doesn't actually know whether the "var" name matches anything,
so I desisted from trying to fix it.

Discussion: https://postgr.es/m/CAD3a31XpXzrZA9TT3BqLSHghdTK+=cXjNCE+oL2Zn4+oWoc=qA@mail.gmail.com

src/bin/psql/tab-complete.c

@@ -3362,8 +3362,13 @@ psql_completion(const char *text, int start, int end)
 	else if (HeadMatches("ALTER", "DATABASE|FUNCTION|PROCEDURE|ROLE|ROUTINE|USER") &&
 			 TailMatches("SET", MatchAny))
 		COMPLETE_WITH("FROM CURRENT", "TO");
-	/* Suggest possible variable values */
-	else if (TailMatches("SET", MatchAny, "TO|="))
+
+	/*
+	 * Suggest possible variable values in SET variable TO|=, along with the
+	 * preceding ALTER syntaxes.
+	 */
+	else if (TailMatches("SET", MatchAny, "TO|=") &&
+			 !TailMatches("UPDATE", MatchAny, "SET", MatchAny, "TO|="))
 	{
 		/* special cased code for individual GUCs */
 		if (TailMatches("DateStyle", "TO|="))
@@ -3381,23 +3386,31 @@ psql_completion(const char *text, int start, int end)
 			/* generic, type based, GUC support */
 			char	   *guctype = get_guctype(prev2_wd);
 
-			if (guctype && strcmp(guctype, "enum") == 0)
+			/*
+			 * Note: if we don't recognize the GUC name, it's important to not
+			 * offer any completions, as most likely we've misinterpreted the
+			 * context and this isn't a GUC-setting command at all.
+			 */
+			if (guctype)
+			{
+				if (strcmp(guctype, "enum") == 0)
 				{
 					char		querybuf[1024];
 
-				snprintf(querybuf, sizeof(querybuf), Query_for_enum, prev2_wd);
+					snprintf(querybuf, sizeof(querybuf),
+							 Query_for_enum, prev2_wd);
 					COMPLETE_WITH_QUERY(querybuf);
 				}
-			else if (guctype && strcmp(guctype, "bool") == 0)
+				else if (strcmp(guctype, "bool") == 0)
 					COMPLETE_WITH("on", "off", "true", "false", "yes", "no",
 								  "1", "0", "DEFAULT");
 				else
 					COMPLETE_WITH("DEFAULT");
 
-			if (guctype)
 				free(guctype);
 			}
 		}
+	}
 
 /* START TRANSACTION */
 	else if (Matches("START"))