Improve parser's one-extra-token lookahead mechanism.

There are a couple of places in our grammar that fail to be strict LALR(1),
by requiring more than a single token of lookahead to decide what to do.
Up to now we've dealt with that by using a filter between the lexer and
parser that merges adjacent tokens into one in the places where two tokens
of lookahead are necessary.  But that creates a number of user-visible
anomalies, for instance that you can't name a CTE "ordinality" because
"WITH ordinality AS ..." triggers folding of WITH and ORDINALITY into one
token.  I realized that there's a better way.

In this patch, we still do the lookahead basically as before, but we never
merge the second token into the first; we replace just the first token by
a special lookahead symbol when one of the lookahead pairs is seen.

This requires a couple extra productions in the grammar, but it involves
fewer special tokens, so that the grammar tables come out a bit smaller
than before.  The filter logic is no slower than before, perhaps a bit
faster.

I also fixed the filter logic so that when backing up after a lookahead,
the current token's terminator is correctly restored; this eliminates some
weird behavior in error message issuance, as is shown by the one change in
existing regression test outputs.

I believe that this patch entirely eliminates odd behaviors caused by
lookahead for WITH.  It doesn't really improve the situation for NULLS
followed by FIRST/LAST unfortunately: those sequences still act like a
reserved word, even though there are cases where they should be seen as two
ordinary identifiers, eg "SELECT nulls first FROM ...".  I experimented
with additional grammar hacks but couldn't find any simple solution for
that.  Still, this is better than before, and it seems much more likely
that we *could* somehow solve the NULLS case on the basis of this filter
behavior than the previous one.

1 files changed, 58 insertions, 47 deletions

diff --git a/src/backend/parser/parser.c b/src/backend/parser/parser.c
index db49275e00a..b17771d4cca 100644
--- a/src/backend/parser/parser.c
+++ b/src/backend/parser/parser.c
@@ -64,13 +64,13 @@ raw_parser(const char *str)
 /*
  * Intermediate filter between parser and core lexer (core_yylex in scan.l).
  *
- * The filter is needed because in some cases the standard SQL grammar
+ * This filter is needed because in some cases the standard SQL grammar
  * requires more than one token lookahead.  We reduce these cases to one-token
- * lookahead by combining tokens here, in order to keep the grammar LALR(1).
+ * lookahead by replacing tokens here, in order to keep the grammar LALR(1).
  *
  * Using a filter is simpler than trying to recognize multiword tokens
  * directly in scan.l, because we'd have to allow for comments between the
- * words.  Furthermore it's not clear how to do it without re-introducing
+ * words.  Furthermore it's not clear how to do that without re-introducing
  * scanner backtrack, which would cost more performance than this filter
  * layer does.
  *
@@ -84,7 +84,7 @@ base_yylex(YYSTYPE *lvalp, YYLTYPE *llocp, core_yyscan_t yyscanner)
 	base_yy_extra_type *yyextra = pg_yyget_extra(yyscanner);
 	int			cur_token;
 	int			next_token;
-	core_YYSTYPE cur_yylval;
+	int			cur_token_length;
 	YYLTYPE		cur_yylloc;
 
 	/* Get next token --- we might already have it */
@@ -93,74 +93,85 @@ base_yylex(YYSTYPE *lvalp, YYLTYPE *llocp, core_yyscan_t yyscanner)
 		cur_token = yyextra->lookahead_token;
 		lvalp->core_yystype = yyextra->lookahead_yylval;
 		*llocp = yyextra->lookahead_yylloc;
+		*(yyextra->lookahead_end) = yyextra->lookahead_hold_char;
 		yyextra->have_lookahead = false;
 	}
 	else
 		cur_token = core_yylex(&(lvalp->core_yystype), llocp, yyscanner);
 
-	/* Do we need to look ahead for a possible multiword token? */
+	/*
+	 * If this token isn't one that requires lookahead, just return it.  If it
+	 * does, determine the token length.  (We could get that via strlen(), but
+	 * since we have such a small set of possibilities, hardwiring seems
+	 * feasible and more efficient.)
+	 */
 	switch (cur_token)
 	{
 		case NULLS_P:
+			cur_token_length = 5;
+			break;
+		case WITH:
+			cur_token_length = 4;
+			break;
+		default:
+			return cur_token;
+	}
 
-			/*
-			 * NULLS FIRST and NULLS LAST must be reduced to one token
-			 */
-			cur_yylval = lvalp->core_yystype;
-			cur_yylloc = *llocp;
-			next_token = core_yylex(&(lvalp->core_yystype), llocp, yyscanner);
+	/*
+	 * Identify end+1 of current token.  core_yylex() has temporarily stored a
+	 * '\0' here, and will undo that when we call it again.  We need to redo
+	 * it to fully revert the lookahead call for error reporting purposes.
+	 */
+	yyextra->lookahead_end = yyextra->core_yy_extra.scanbuf +
+		*llocp + cur_token_length;
+	Assert(*(yyextra->lookahead_end) == '\0');
+
+	/*
+	 * Save and restore *llocp around the call.  It might look like we could
+	 * avoid this by just passing &lookahead_yylloc to core_yylex(), but that
+	 * does not work because flex actually holds onto the last-passed pointer
+	 * internally, and will use that for error reporting.  We need any error
+	 * reports to point to the current token, not the next one.
+	 */
+	cur_yylloc = *llocp;
+
+	/* Get next token, saving outputs into lookahead variables */
+	next_token = core_yylex(&(yyextra->lookahead_yylval), llocp, yyscanner);
+	yyextra->lookahead_token = next_token;
+	yyextra->lookahead_yylloc = *llocp;
+
+	*llocp = cur_yylloc;
+
+	/* Now revert the un-truncation of the current token */
+	yyextra->lookahead_hold_char = *(yyextra->lookahead_end);
+	*(yyextra->lookahead_end) = '\0';
+
+	yyextra->have_lookahead = true;
+
+	/* Replace cur_token if needed, based on lookahead */
+	switch (cur_token)
+	{
+		case NULLS_P:
+			/* Replace NULLS_P by NULLS_LA if it's followed by FIRST or LAST */
 			switch (next_token)
 			{
 				case FIRST_P:
-					cur_token = NULLS_FIRST;
-					break;
 				case LAST_P:
-					cur_token = NULLS_LAST;
-					break;
-				default:
-					/* save the lookahead token for next time */
-					yyextra->lookahead_token = next_token;
-					yyextra->lookahead_yylval = lvalp->core_yystype;
-					yyextra->lookahead_yylloc = *llocp;
-					yyextra->have_lookahead = true;
-					/* and back up the output info to cur_token */
-					lvalp->core_yystype = cur_yylval;
-					*llocp = cur_yylloc;
+					cur_token = NULLS_LA;
 					break;
 			}
 			break;
 
 		case WITH:
-
-			/*
-			 * WITH TIME and WITH ORDINALITY must each be reduced to one token
-			 */
-			cur_yylval = lvalp->core_yystype;
-			cur_yylloc = *llocp;
-			next_token = core_yylex(&(lvalp->core_yystype), llocp, yyscanner);
+			/* Replace WITH by WITH_LA if it's followed by TIME or ORDINALITY */
 			switch (next_token)
 			{
 				case TIME:
-					cur_token = WITH_TIME;
-					break;
 				case ORDINALITY:
-					cur_token = WITH_ORDINALITY;
-					break;
-				default:
-					/* save the lookahead token for next time */
-					yyextra->lookahead_token = next_token;
-					yyextra->lookahead_yylval = lvalp->core_yystype;
-					yyextra->lookahead_yylloc = *llocp;
-					yyextra->have_lookahead = true;
-					/* and back up the output info to cur_token */
-					lvalp->core_yystype = cur_yylval;
-					*llocp = cur_yylloc;
+					cur_token = WITH_LA;
 					break;
 			}
 			break;
-
-		default:
-			break;
 	}
 
 	return cur_token;