Reduce an unnecessary O(N^3) loop in lexer.

The lexer's handling of operators contained an O(N^3) hazard when
dealing with long strings of + or - characters; it seems hard to
prevent this case from being O(N^2), but the additional N multiplier
was not needed.

Backpatch all the way since this has been there since 7.x, and it
presents at least a mild hazard in that trying to do Bind, PREPARE or
EXPLAIN on a hostile query could take excessive time (without
honouring cancels or timeouts) even if the query was never executed.

src/backend/parser/scan.l

@@ -885,20 +885,33 @@ other			.
 					 * to forbid operator names like '?-' that could not be
 					 * sequences of SQL operators.
 					 */
-					while (nchars > 1 &&
-						   (yytext[nchars - 1] == '+' ||
-							yytext[nchars - 1] == '-'))
+					if (nchars > 1 &&
+						(yytext[nchars - 1] == '+' ||
+						 yytext[nchars - 1] == '-'))
 					{
 						int			ic;
 
 						for (ic = nchars - 2; ic >= 0; ic--)
 						{
-							if (strchr("~!@#^&|`?%", yytext[ic]))
+							char c = yytext[ic];
+							if (c == '~' || c == '!' || c == '@' ||
+								c == '#' || c == '^' || c == '&' ||
+								c == '|' || c == '`' || c == '?' ||
+								c == '%')
 								break;
 						}
-						if (ic >= 0)
-							break; /* found a char that makes it OK */
-						nchars--; /* else remove the +/-, and check again */
+						if (ic < 0)
+						{
+							/*
+							 * didn't find a qualifying character, so remove
+							 * all trailing [+-]
+							 */
+							do {
+								nchars--;
+							} while (nchars > 1 &&
+								 (yytext[nchars - 1] == '+' ||
+								  yytext[nchars - 1] == '-'));
+						}
 					}
 
 					SET_YYLLOC();

src/fe_utils/psqlscan.l

@@ -817,20 +817,33 @@ other			.
 					 * to forbid operator names like '?-' that could not be
 					 * sequences of SQL operators.
 					 */
-					while (nchars > 1 &&
-						   (yytext[nchars - 1] == '+' ||
-							yytext[nchars - 1] == '-'))
+					if (nchars > 1 &&
+						(yytext[nchars - 1] == '+' ||
+						 yytext[nchars - 1] == '-'))
 					{
 						int			ic;
 
 						for (ic = nchars - 2; ic >= 0; ic--)
 						{
-							if (strchr("~!@#^&|`?%", yytext[ic]))
+							char c = yytext[ic];
+							if (c == '~' || c == '!' || c == '@' ||
+								c == '#' || c == '^' || c == '&' ||
+								c == '|' || c == '`' || c == '?' ||
+								c == '%')
 								break;
 						}
-						if (ic >= 0)
-							break; /* found a char that makes it OK */
-						nchars--; /* else remove the +/-, and check again */
+						if (ic < 0)
+						{
+							/*
+							 * didn't find a qualifying character, so remove
+							 * all trailing [+-]
+							 */
+							do {
+								nchars--;
+							} while (nchars > 1 &&
+								 (yytext[nchars - 1] == '+' ||
+								  yytext[nchars - 1] == '-'));
+						}
 					}
 
 					if (nchars < yyleng)

src/interfaces/ecpg/preproc/pgc.l

@@ -690,20 +690,33 @@ cppline			{space}*#([^i][A-Za-z]*|{if}|{ifdef}|{ifndef}|{import})((\/\*[^*/]*\*+
 						 * to forbid operator names like '?-' that could not be
 						 * sequences of SQL operators.
 						 */
-						while (nchars > 1 &&
-							   (yytext[nchars-1] == '+' ||
-								yytext[nchars-1] == '-'))
+						if (nchars > 1 &&
+							(yytext[nchars - 1] == '+' ||
+							 yytext[nchars - 1] == '-'))
 						{
 							int		ic;
 
-							for (ic = nchars-2; ic >= 0; ic--)
+							for (ic = nchars - 2; ic >= 0; ic--)
 							{
-								if (strchr("~!@#^&|`?%", yytext[ic]))
+								char c = yytext[ic];
+								if (c == '~' || c == '!' || c == '@' ||
+									c == '#' || c == '^' || c == '&' ||
+									c == '|' || c == '`' || c == '?' ||
+									c == '%')
 									break;
 							}
-							if (ic >= 0)
-								break; /* found a char that makes it OK */
-							nchars--; /* else remove the +/-, and check again */
+							if (ic < 0)
+							{
+								/*
+								 * didn't find a qualifying character, so remove
+								 * all trailing [+-]
+								 */
+								do {
+									nchars--;
+								} while (nchars > 1 &&
+									 (yytext[nchars - 1] == '+' ||
+									  yytext[nchars - 1] == '-'));
+							}
 						}
 
 						if (nchars < yyleng)