Improve error reporting in format()

Clarify invalid format conversion type error message and add hint.

Author: Jim Nasby

src/backend/utils/adt/varlena.c

@@ -4788,7 +4788,8 @@ text_reverse(PG_FUNCTION_ARGS)
 		if (++(ptr) >= (end_ptr)) \
 			ereport(ERROR, \
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE), \
-					 errmsg("unterminated format specifier"))); \
+					 errmsg("unterminated format() type specifier"), \
+					 errhint("For a single \"%%\" use \"%%%%\"."))); \
 	} while (0)
 
 /*
@@ -4920,8 +4921,9 @@ text_format(PG_FUNCTION_ARGS)
 		if (strchr("sIL", *cp) == NULL)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-					 errmsg("unrecognized conversion type specifier \"%c\"",
-							*cp)));
+					 errmsg("unrecognized format() type specifier \"%c\"",
+							*cp),
+					 errhint("For a single \"%%\" use \"%%%%\".")));
 
 		/* If indirect width was specified, get its value */
 		if (widthpos >= 0)
@@ -4932,7 +4934,7 @@ text_format(PG_FUNCTION_ARGS)
 			if (arg >= nargs)
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-						 errmsg("too few arguments for format")));
+						 errmsg("too few arguments for format()")));
 
 			/* Get the value and type of the selected argument */
 			if (!funcvariadic)
@@ -5040,8 +5042,9 @@ text_format(PG_FUNCTION_ARGS)
 				/* should not get here, because of previous check */
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-					  errmsg("unrecognized conversion type specifier \"%c\"",
-							 *cp)));
+					  errmsg("unrecognized format() type specifier \"%c\"",
+							 *cp),
+					  errhint("For a single \"%%\" use \"%%%%\".")));
 				break;
 		}
 	}

src/test/regress/expected/text.out

@@ -211,7 +211,8 @@ ERROR:  too few arguments for format
 select format('Hello %s');
 ERROR:  too few arguments for format
 select format('Hello %x', 20);
-ERROR:  unrecognized conversion type specifier "x"
+ERROR:  unrecognized format() type specifier "x"
+HINT:  For a single "%" use "%%".
 -- check literal and sql identifiers
 select format('INSERT INTO %I VALUES(%L,%L)', 'mytab', 10, 'Hello');
                  format                 
@@ -263,9 +264,11 @@ ERROR:  format specifies argument 0, but arguments are numbered from 1
 select format('%*0$s', 'Hello');
 ERROR:  format specifies argument 0, but arguments are numbered from 1
 select format('%1$', 1);
-ERROR:  unterminated format specifier
+ERROR:  unterminated format() type specifier
+HINT:  For a single "%" use "%%".
 select format('%1$1', 1);
-ERROR:  unterminated format specifier
+ERROR:  unterminated format() type specifier
+HINT:  For a single "%" use "%%".
 -- check mix of positional and ordered placeholders
 select format('Hello %s %1$s %s', 'World', 'Hello again');
             format