De-reserve most statement-introducing keywords in plpgsql.

Add a bit of context sensitivity to plpgsql_yylex() so that it can
recognize when the word it is looking at is the first word of a new
statement, and if so whether it is the target of an assignment statement.
When we are at start of statement and it's not an assignment, we can
prefer recognizing unreserved keywords over recognizing variable names,
thereby allowing most statements' initial keywords to be demoted from
reserved to unreserved status.  This is rather useful already (there are
15 such words that get demoted here), and what's more to the point is
that future patches proposing to add new plpgsql statements can avoid
objections about having to add new reserved words.

The keywords BEGIN, DECLARE, FOR, FOREACH, LOOP, WHILE need to remain
reserved because they can be preceded by block labels, and the logic
added here doesn't understand about block labels.  In principle we
could probably fix that, but it would take more than one token of
lookback and the benefit doesn't seem worth extra complexity.

Also note I didn't de-reserve EXECUTE, because it is used in more places
than just statement start.  It's possible it could be de-reserved with
more work, but that would be an independent fix.

In passing, also de-reserve COLLATE and DEFAULT, which shouldn't have
been reserved in the first place since they only need to be recognized
within DECLARE sections.

src/pl/plpgsql/src/pl_gram.y

@@ -2315,32 +2315,46 @@ unreserved_keyword	:
 				| K_ALIAS
 				| K_ARRAY
 				| K_BACKWARD
+				| K_CLOSE
+				| K_COLLATE
 				| K_COLUMN
 				| K_COLUMN_NAME
 				| K_CONSTANT
 				| K_CONSTRAINT
 				| K_CONSTRAINT_NAME
+				| K_CONTINUE
 				| K_CURRENT
 				| K_CURSOR
 				| K_DATATYPE
 				| K_DEBUG
+				| K_DEFAULT
 				| K_DETAIL
+				| K_DIAGNOSTICS
 				| K_DUMP
+				| K_ELSIF
 				| K_ERRCODE
 				| K_ERROR
+				| K_EXCEPTION
+				| K_EXIT
+				| K_FETCH
 				| K_FIRST
 				| K_FORWARD
+				| K_GET
 				| K_HINT
 				| K_INFO
+				| K_INSERT
 				| K_IS
 				| K_LAST
 				| K_LOG
 				| K_MESSAGE
 				| K_MESSAGE_TEXT
+				| K_MOVE
 				| K_NEXT
 				| K_NO
 				| K_NOTICE
+				| K_OPEN
 				| K_OPTION
+				| K_PERFORM
 				| K_PG_CONTEXT
 				| K_PG_DATATYPE_NAME
 				| K_PG_EXCEPTION_CONTEXT
@@ -2349,8 +2363,10 @@ unreserved_keyword	:
 				| K_PRINT_STRICT_PARAMS
 				| K_PRIOR
 				| K_QUERY
+				| K_RAISE
 				| K_RELATIVE
 				| K_RESULT_OID
+				| K_RETURN
 				| K_RETURNED_SQLSTATE
 				| K_REVERSE
 				| K_ROW_COUNT

src/pl/plpgsql/src/pl_scanner.c

@@ -31,21 +31,28 @@ IdentifierLookup plpgsql_IdentifierLookup = IDENTIFIER_LOOKUP_NORMAL;
  *
  * We keep reserved and unreserved keywords in separate arrays.  The
  * reserved keywords are passed to the core scanner, so they will be
- * recognized before (and instead of) any variable name.  Unreserved
- * words are checked for separately, after determining that the identifier
+ * recognized before (and instead of) any variable name.  Unreserved words
+ * are checked for separately, usually after determining that the identifier
  * isn't a known variable name.  If plpgsql_IdentifierLookup is DECLARE then
  * no variable names will be recognized, so the unreserved words always work.
  * (Note in particular that this helps us avoid reserving keywords that are
  * only needed in DECLARE sections.)
  *
  * In certain contexts it is desirable to prefer recognizing an unreserved
- * keyword over recognizing a variable name.  Those cases are handled in
- * pl_gram.y using tok_is_keyword().
+ * keyword over recognizing a variable name.  In particular, at the start
+ * of a statement we should prefer unreserved keywords unless the statement
+ * looks like an assignment (i.e., first token is followed by ':=' or '[').
+ * This rule allows most statement-introducing keywords to be kept unreserved.
+ * (We still have to reserve initial keywords that might follow a block
+ * label, unfortunately, since the method used to determine if we are at
+ * start of statement doesn't recognize such cases.  We'd also have to
+ * reserve any keyword that could legitimately be followed by ':=' or '['.)
+ * Some additional cases are handled in pl_gram.y using tok_is_keyword().
  *
- * For the most part, the reserved keywords are those that start a PL/pgSQL
- * statement (and so would conflict with an assignment to a variable of the
- * same name).  We also don't sweat it much about reserving keywords that
- * are reserved in the core grammar.  Try to avoid reserving other words.
+ * We try to avoid reserving more keywords than we have to; but there's
+ * little point in not reserving a word if it's reserved in the core grammar.
+ * Currently, the following words are reserved here but not in the core:
+ * BEGIN BY DECLARE EXECUTE FOREACH IF LOOP STRICT WHILE
  */
 
 /*
@@ -63,37 +70,20 @@ static const ScanKeyword reserved_keywords[] = {
 	PG_KEYWORD("begin", K_BEGIN, RESERVED_KEYWORD)
 	PG_KEYWORD("by", K_BY, RESERVED_KEYWORD)
 	PG_KEYWORD("case", K_CASE, RESERVED_KEYWORD)
-	PG_KEYWORD("close", K_CLOSE, RESERVED_KEYWORD)
-	PG_KEYWORD("collate", K_COLLATE, RESERVED_KEYWORD)
-	PG_KEYWORD("continue", K_CONTINUE, RESERVED_KEYWORD)
 	PG_KEYWORD("declare", K_DECLARE, RESERVED_KEYWORD)
-	PG_KEYWORD("default", K_DEFAULT, RESERVED_KEYWORD)
-	PG_KEYWORD("diagnostics", K_DIAGNOSTICS, RESERVED_KEYWORD)
 	PG_KEYWORD("else", K_ELSE, RESERVED_KEYWORD)
-	PG_KEYWORD("elseif", K_ELSIF, RESERVED_KEYWORD)
-	PG_KEYWORD("elsif", K_ELSIF, RESERVED_KEYWORD)
 	PG_KEYWORD("end", K_END, RESERVED_KEYWORD)
-	PG_KEYWORD("exception", K_EXCEPTION, RESERVED_KEYWORD)
 	PG_KEYWORD("execute", K_EXECUTE, RESERVED_KEYWORD)
-	PG_KEYWORD("exit", K_EXIT, RESERVED_KEYWORD)
-	PG_KEYWORD("fetch", K_FETCH, RESERVED_KEYWORD)
 	PG_KEYWORD("for", K_FOR, RESERVED_KEYWORD)
 	PG_KEYWORD("foreach", K_FOREACH, RESERVED_KEYWORD)
 	PG_KEYWORD("from", K_FROM, RESERVED_KEYWORD)
-	PG_KEYWORD("get", K_GET, RESERVED_KEYWORD)
 	PG_KEYWORD("if", K_IF, RESERVED_KEYWORD)
 	PG_KEYWORD("in", K_IN, RESERVED_KEYWORD)
-	PG_KEYWORD("insert", K_INSERT, RESERVED_KEYWORD)
 	PG_KEYWORD("into", K_INTO, RESERVED_KEYWORD)
 	PG_KEYWORD("loop", K_LOOP, RESERVED_KEYWORD)
-	PG_KEYWORD("move", K_MOVE, RESERVED_KEYWORD)
 	PG_KEYWORD("not", K_NOT, RESERVED_KEYWORD)
 	PG_KEYWORD("null", K_NULL, RESERVED_KEYWORD)
-	PG_KEYWORD("open", K_OPEN, RESERVED_KEYWORD)
 	PG_KEYWORD("or", K_OR, RESERVED_KEYWORD)
-	PG_KEYWORD("perform", K_PERFORM, RESERVED_KEYWORD)
-	PG_KEYWORD("raise", K_RAISE, RESERVED_KEYWORD)
-	PG_KEYWORD("return", K_RETURN, RESERVED_KEYWORD)
 	PG_KEYWORD("strict", K_STRICT, RESERVED_KEYWORD)
 	PG_KEYWORD("then", K_THEN, RESERVED_KEYWORD)
 	PG_KEYWORD("to", K_TO, RESERVED_KEYWORD)
@@ -109,32 +99,47 @@ static const ScanKeyword unreserved_keywords[] = {
 	PG_KEYWORD("alias", K_ALIAS, UNRESERVED_KEYWORD)
 	PG_KEYWORD("array", K_ARRAY, UNRESERVED_KEYWORD)
 	PG_KEYWORD("backward", K_BACKWARD, UNRESERVED_KEYWORD)
+	PG_KEYWORD("close", K_CLOSE, UNRESERVED_KEYWORD)
+	PG_KEYWORD("collate", K_COLLATE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("column", K_COLUMN, UNRESERVED_KEYWORD)
 	PG_KEYWORD("column_name", K_COLUMN_NAME, UNRESERVED_KEYWORD)
 	PG_KEYWORD("constant", K_CONSTANT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("constraint", K_CONSTRAINT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("constraint_name", K_CONSTRAINT_NAME, UNRESERVED_KEYWORD)
+	PG_KEYWORD("continue", K_CONTINUE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("current", K_CURRENT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("cursor", K_CURSOR, UNRESERVED_KEYWORD)
 	PG_KEYWORD("datatype", K_DATATYPE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("debug", K_DEBUG, UNRESERVED_KEYWORD)
+	PG_KEYWORD("default", K_DEFAULT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("detail", K_DETAIL, UNRESERVED_KEYWORD)
+	PG_KEYWORD("diagnostics", K_DIAGNOSTICS, UNRESERVED_KEYWORD)
 	PG_KEYWORD("dump", K_DUMP, UNRESERVED_KEYWORD)
+	PG_KEYWORD("elseif", K_ELSIF, UNRESERVED_KEYWORD)
+	PG_KEYWORD("elsif", K_ELSIF, UNRESERVED_KEYWORD)
 	PG_KEYWORD("errcode", K_ERRCODE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("error", K_ERROR, UNRESERVED_KEYWORD)
+	PG_KEYWORD("exception", K_EXCEPTION, UNRESERVED_KEYWORD)
+	PG_KEYWORD("exit", K_EXIT, UNRESERVED_KEYWORD)
+	PG_KEYWORD("fetch", K_FETCH, UNRESERVED_KEYWORD)
 	PG_KEYWORD("first", K_FIRST, UNRESERVED_KEYWORD)
 	PG_KEYWORD("forward", K_FORWARD, UNRESERVED_KEYWORD)
+	PG_KEYWORD("get", K_GET, UNRESERVED_KEYWORD)
 	PG_KEYWORD("hint", K_HINT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("info", K_INFO, UNRESERVED_KEYWORD)
+	PG_KEYWORD("insert", K_INSERT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("is", K_IS, UNRESERVED_KEYWORD)
 	PG_KEYWORD("last", K_LAST, UNRESERVED_KEYWORD)
 	PG_KEYWORD("log", K_LOG, UNRESERVED_KEYWORD)
 	PG_KEYWORD("message", K_MESSAGE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("message_text", K_MESSAGE_TEXT, UNRESERVED_KEYWORD)
+	PG_KEYWORD("move", K_MOVE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("next", K_NEXT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("no", K_NO, UNRESERVED_KEYWORD)
 	PG_KEYWORD("notice", K_NOTICE, UNRESERVED_KEYWORD)
+	PG_KEYWORD("open", K_OPEN, UNRESERVED_KEYWORD)
 	PG_KEYWORD("option", K_OPTION, UNRESERVED_KEYWORD)
+	PG_KEYWORD("perform", K_PERFORM, UNRESERVED_KEYWORD)
 	PG_KEYWORD("pg_context", K_PG_CONTEXT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("pg_datatype_name", K_PG_DATATYPE_NAME, UNRESERVED_KEYWORD)
 	PG_KEYWORD("pg_exception_context", K_PG_EXCEPTION_CONTEXT, UNRESERVED_KEYWORD)
@@ -143,8 +148,10 @@ static const ScanKeyword unreserved_keywords[] = {
 	PG_KEYWORD("print_strict_params", K_PRINT_STRICT_PARAMS, UNRESERVED_KEYWORD)
 	PG_KEYWORD("prior", K_PRIOR, UNRESERVED_KEYWORD)
 	PG_KEYWORD("query", K_QUERY, UNRESERVED_KEYWORD)
+	PG_KEYWORD("raise", K_RAISE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("relative", K_RELATIVE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("result_oid", K_RESULT_OID, UNRESERVED_KEYWORD)
+	PG_KEYWORD("return", K_RETURN, UNRESERVED_KEYWORD)
 	PG_KEYWORD("returned_sqlstate", K_RETURNED_SQLSTATE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("reverse", K_REVERSE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("row_count", K_ROW_COUNT, UNRESERVED_KEYWORD)
@@ -166,6 +173,19 @@ static const ScanKeyword unreserved_keywords[] = {
 
 static const int num_unreserved_keywords = lengthof(unreserved_keywords);
 
+/*
+ * This macro must recognize all tokens that can immediately precede a
+ * PL/pgSQL executable statement (that is, proc_sect or proc_stmt in the
+ * grammar).  Fortunately, there are not very many, so hard-coding in this
+ * fashion seems sufficient.
+ */
+#define AT_STMT_START(prev_token) \
+	((prev_token) == ';' || \
+	 (prev_token) == K_BEGIN || \
+	 (prev_token) == K_THEN || \
+	 (prev_token) == K_ELSE || \
+	 (prev_token) == K_LOOP)
+
 
 /* Auxiliary data about a token (other than the token type) */
 typedef struct
@@ -192,6 +212,9 @@ static const char *scanorig;
 /* Current token's length (corresponds to plpgsql_yylval and plpgsql_yylloc) */
 static int	plpgsql_yyleng;
 
+/* Current token's code (corresponds to plpgsql_yylval and plpgsql_yylloc) */
+static int	plpgsql_yytoken;
+
 /* Token pushback stack */
 #define MAX_PUSHBACKS 4
 
@@ -315,6 +338,39 @@ plpgsql_yylex(void)
 		{
 			/* not A.B, so just process A */
 			push_back_token(tok2, &aux2);
+
+			/*
+			 * If we are at start of statement, prefer unreserved keywords
+			 * over variable names, unless the next token is assignment or
+			 * '[', in which case prefer variable names.  (Note we need not
+			 * consider '.' as the next token; that case was handled above,
+			 * and we always prefer variable names in that case.)  If we are
+			 * not at start of statement, always prefer variable names over
+			 * unreserved keywords.
+			 */
+			if (AT_STMT_START(plpgsql_yytoken) &&
+				!(tok2 == '=' || tok2 == COLON_EQUALS || tok2 == '['))
+			{
+				/* try for unreserved keyword, then for variable name */
+				if (core_yy.scanbuf[aux1.lloc] != '"' &&
+					(kw = ScanKeywordLookup(aux1.lval.str,
+											unreserved_keywords,
+											num_unreserved_keywords)))
+				{
+					aux1.lval.keyword = kw->name;
+					tok1 = kw->value;
+				}
+				else if (plpgsql_parse_word(aux1.lval.str,
+											core_yy.scanbuf + aux1.lloc,
+											&aux1.lval.wdatum,
+											&aux1.lval.word))
+					tok1 = T_DATUM;
+				else
+					tok1 = T_WORD;
+			}
+			else
+			{
+				/* try for variable name, then for unreserved keyword */
 				if (plpgsql_parse_word(aux1.lval.str,
 									   core_yy.scanbuf + aux1.lloc,
 									   &aux1.lval.wdatum,
@@ -332,14 +388,25 @@ plpgsql_yylex(void)
 					tok1 = T_WORD;
 			}
 		}
+	}
 	else
 	{
-		/* Not a potential plpgsql variable name, just return the data */
+		/*
+		 * Not a potential plpgsql variable name, just return the data.
+		 *
+		 * Note that we also come through here if the grammar pushed back a
+		 * T_DATUM, T_CWORD, T_WORD, or unreserved-keyword token returned by a
+		 * previous lookup cycle; thus, pushbacks do not incur extra lookup
+		 * work, since we'll never do the above code twice for the same token.
+		 * This property also makes it safe to rely on the old value of
+		 * plpgsql_yytoken in the is-this-start-of-statement test above.
+		 */
 	}
 
 	plpgsql_yylval = aux1.lval;
 	plpgsql_yylloc = aux1.lloc;
 	plpgsql_yyleng = aux1.leng;
+	plpgsql_yytoken = tok1;
 	return tok1;
 }
 
@@ -645,6 +712,7 @@ plpgsql_scanner_init(const char *str)
 
 	/* Other setup */
 	plpgsql_IdentifierLookup = IDENTIFIER_LOOKUP_NORMAL;
+	plpgsql_yytoken = 0;
 
 	num_pushbacks = 0;
 

src/test/regress/expected/plpgsql.out

@@ -4906,6 +4906,20 @@ select unreserved_test();
               42
 (1 row)
 
+create or replace function unreserved_test() returns int as $$
+declare
+  return int := 42;
+begin
+  return := return + 1;
+  return return;
+end
+$$ language plpgsql;
+select unreserved_test();
+ unreserved_test 
+-----------------
+              43
+(1 row)
+
 drop function unreserved_test();
 --
 -- Test FOREACH over arrays

src/test/regress/sql/plpgsql.sql

@@ -3940,6 +3940,17 @@ $$ language plpgsql;
 
 select unreserved_test();
 
+create or replace function unreserved_test() returns int as $$
+declare
+  return int := 42;
+begin
+  return := return + 1;
+  return return;
+end
+$$ language plpgsql;
+
+select unreserved_test();
+
 drop function unreserved_test();
 
 --