jsonapi: Use const char *

Apply const qualifiers to char * arguments and fields throughout the
jsonapi.  This allows the top-level APIs such as
pg_parse_json_incremental() to declare their input argument as const.
It also reduces the number of unconstify() calls.

Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://www.postgresql.org/message-id/flat/f732b014-f614-4600-a437-dba5a2c3738b%40eisentraut.org

src/backend/utils/adt/jsonfuncs.c

@@ -86,7 +86,7 @@ typedef struct GetState
 {
 	JsonLexContext *lex;
 	text	   *tresult;
-	char	   *result_start;
+	const char *result_start;
 	bool		normalize_results;
 	bool		next_scalar;
 	int			npath;			/* length of each path-related array */
@@ -111,7 +111,7 @@ typedef struct EachState
 	Tuplestorestate *tuple_store;
 	TupleDesc	ret_tdesc;
 	MemoryContext tmp_cxt;
-	char	   *result_start;
+	const char *result_start;
 	bool		normalize_results;
 	bool		next_scalar;
 	char	   *normalized_scalar;
@@ -125,7 +125,7 @@ typedef struct ElementsState
 	Tuplestorestate *tuple_store;
 	TupleDesc	ret_tdesc;
 	MemoryContext tmp_cxt;
-	char	   *result_start;
+	const char *result_start;
 	bool		normalize_results;
 	bool		next_scalar;
 	char	   *normalized_scalar;
@@ -138,7 +138,7 @@ typedef struct JHashState
 	const char *function_name;
 	HTAB	   *hash;
 	char	   *saved_scalar;
-	char	   *save_json_start;
+	const char *save_json_start;
 	JsonTokenType saved_token_type;
 } JHashState;
 
@@ -247,7 +247,7 @@ typedef struct PopulateRecordsetState
 	const char *function_name;
 	HTAB	   *json_hash;
 	char	   *saved_scalar;
-	char	   *save_json_start;
+	const char *save_json_start;
 	JsonTokenType saved_token_type;
 	Tuplestorestate *tuple_store;
 	HeapTupleHeader rec;
@@ -273,7 +273,7 @@ typedef struct PopulateArrayState
 {
 	JsonLexContext *lex;		/* json lexer */
 	PopulateArrayContext *ctx;	/* context */
-	char	   *element_start;	/* start of the current array element */
+	const char *element_start;	/* start of the current array element */
 	char	   *element_scalar; /* current array element token if it is a
 								 * scalar */
 	JsonTokenType element_type; /* current array element type */
@@ -295,7 +295,7 @@ typedef struct JsValue
 	{
 		struct
 		{
-			char	   *str;	/* json string */
+			const char *str;	/* json string */
 			int			len;	/* json string length or -1 if null-terminated */
 			JsonTokenType type; /* json type */
 		}			json;		/* json value */
@@ -390,7 +390,7 @@ static JsonParseErrorType elements_array_element_end(void *state, bool isnull);
 static JsonParseErrorType elements_scalar(void *state, char *token, JsonTokenType tokentype);
 
 /* turn a json object into a hash table */
-static HTAB *get_json_object_as_hash(char *json, int len, const char *funcname,
+static HTAB *get_json_object_as_hash(const char *json, int len, const char *funcname,
 									 Node *escontext);
 
 /* semantic actions for populate_array_json */
@@ -456,7 +456,7 @@ static Datum populate_record_field(ColumnIOData *col, Oid typid, int32 typmod,
 static RecordIOData *allocate_record_info(MemoryContext mcxt, int ncolumns);
 static bool JsObjectGetField(JsObject *obj, char *field, JsValue *jsv);
 static void populate_recordset_record(PopulateRecordsetState *state, JsObject *obj);
-static bool populate_array_json(PopulateArrayContext *ctx, char *json, int len);
+static bool populate_array_json(PopulateArrayContext *ctx, const char *json, int len);
 static bool populate_array_dim_jsonb(PopulateArrayContext *ctx, JsonbValue *jbv,
 									 int ndim);
 static void populate_array_report_expected_array(PopulateArrayContext *ctx, int ndim);
@@ -1181,7 +1181,7 @@ get_object_end(void *state)
 	if (lex_level == 0 && _state->npath == 0)
 	{
 		/* Special case: return the entire object */
-		char	   *start = _state->result_start;
+		const char *start = _state->result_start;
 		int			len = _state->lex->prev_token_terminator - start;
 
 		_state->tresult = cstring_to_text_with_len(start, len);
@@ -1275,7 +1275,7 @@ get_object_field_end(void *state, char *fname, bool isnull)
 			_state->tresult = (text *) NULL;
 		else
 		{
-			char	   *start = _state->result_start;
+			const char *start = _state->result_start;
 			int			len = _state->lex->prev_token_terminator - start;
 
 			_state->tresult = cstring_to_text_with_len(start, len);
@@ -1337,7 +1337,7 @@ get_array_end(void *state)
 	if (lex_level == 0 && _state->npath == 0)
 	{
 		/* Special case: return the entire array */
-		char	   *start = _state->result_start;
+		const char *start = _state->result_start;
 		int			len = _state->lex->prev_token_terminator - start;
 
 		_state->tresult = cstring_to_text_with_len(start, len);
@@ -1426,7 +1426,7 @@ get_array_element_end(void *state, bool isnull)
 			_state->tresult = (text *) NULL;
 		else
 		{
-			char	   *start = _state->result_start;
+			const char *start = _state->result_start;
 			int			len = _state->lex->prev_token_terminator - start;
 
 			_state->tresult = cstring_to_text_with_len(start, len);
@@ -1463,7 +1463,7 @@ get_scalar(void *state, char *token, JsonTokenType tokentype)
 			 * scalar token, but not whitespace before it.  Probably not worth
 			 * doing our own space-skipping to avoid that.
 			 */
-			char	   *start = _state->lex->input;
+			const char *start = _state->lex->input;
 			int			len = _state->lex->prev_token_terminator - start;
 
 			_state->tresult = cstring_to_text_with_len(start, len);
@@ -2782,7 +2782,7 @@ populate_array_scalar(void *_state, char *token, JsonTokenType tokentype)
  * Returns false if an error occurs when parsing.
  */
 static bool
-populate_array_json(PopulateArrayContext *ctx, char *json, int len)
+populate_array_json(PopulateArrayContext *ctx, const char *json, int len)
 {
 	PopulateArrayState state;
 	JsonSemAction sem;
@@ -3123,7 +3123,7 @@ populate_scalar(ScalarIOData *io, Oid typid, int32 typmod, JsValue *jsv,
 {
 	Datum		res;
 	char	   *str = NULL;
-	char	   *json = NULL;
+	const char *json = NULL;
 
 	if (jsv->is_json)
 	{
@@ -3139,7 +3139,10 @@ populate_scalar(ScalarIOData *io, Oid typid, int32 typmod, JsValue *jsv,
 			str[len] = '\0';
 		}
 		else
-			str = json;			/* string is already null-terminated */
+		{
+			/* string is already null-terminated */
+			str = unconstify(char *, json);
+		}
 
 		/* If converting to json/jsonb, make string into valid JSON literal */
 		if ((typid == JSONOID || typid == JSONBOID) &&
@@ -3784,7 +3787,7 @@ populate_record_worker(FunctionCallInfo fcinfo, const char *funcname,
  * Returns the hash table if the json is parsed successfully, NULL otherwise.
  */
 static HTAB *
-get_json_object_as_hash(char *json, int len, const char *funcname,
+get_json_object_as_hash(const char *json, int len, const char *funcname,
 						Node *escontext)
 {
 	HASHCTL		ctl;

src/common/jsonapi.c

@@ -211,7 +211,7 @@ static td_entry td_parser_table[JSON_NUM_NONTERMINALS][JSON_NUM_TERMINALS] =
 static char JSON_PROD_GOAL[] = {JSON_TOKEN_END, JSON_NT_JSON, 0};
 
 static inline JsonParseErrorType json_lex_string(JsonLexContext *lex);
-static inline JsonParseErrorType json_lex_number(JsonLexContext *lex, char *s,
+static inline JsonParseErrorType json_lex_number(JsonLexContext *lex, const char *s,
 												 bool *num_err, size_t *total_len);
 static inline JsonParseErrorType parse_scalar(JsonLexContext *lex, JsonSemAction *sem);
 static JsonParseErrorType parse_object_field(JsonLexContext *lex, JsonSemAction *sem);
@@ -290,12 +290,12 @@ IsValidJsonNumber(const char *str, size_t len)
 	 */
 	if (*str == '-')
 	{
-		dummy_lex.input = unconstify(char *, str) + 1;
+		dummy_lex.input = str + 1;
 		dummy_lex.input_length = len - 1;
 	}
 	else
 	{
-		dummy_lex.input = unconstify(char *, str);
+		dummy_lex.input = str;
 		dummy_lex.input_length = len;
 	}
 
@@ -323,7 +323,7 @@ IsValidJsonNumber(const char *str, size_t len)
  * cleanup.
  */
 JsonLexContext *
-makeJsonLexContextCstringLen(JsonLexContext *lex, char *json,
+makeJsonLexContextCstringLen(JsonLexContext *lex, const char *json,
 							 size_t len, int encoding, bool need_escapes)
 {
 	if (lex == NULL)
@@ -649,7 +649,7 @@ json_count_array_elements(JsonLexContext *lex, int *elements)
 JsonParseErrorType
 pg_parse_json_incremental(JsonLexContext *lex,
 						  JsonSemAction *sem,
-						  char *json,
+						  const char *json,
 						  size_t len,
 						  bool is_last)
 {
@@ -1308,8 +1308,8 @@ parse_array(JsonLexContext *lex, JsonSemAction *sem)
 JsonParseErrorType
 json_lex(JsonLexContext *lex)
 {
-	char	   *s;
-	char	   *const end = lex->input + lex->input_length;
+	const char *s;
+	const char *const end = lex->input + lex->input_length;
 	JsonParseErrorType result;
 
 	if (lex->incremental && lex->inc_state->partial_completed)
@@ -1593,7 +1593,7 @@ json_lex(JsonLexContext *lex)
 				break;
 			default:
 				{
-					char	   *p;
+					const char *p;
 
 					/*
 					 * We're not dealing with a string, number, legal
@@ -1671,8 +1671,8 @@ json_lex(JsonLexContext *lex)
 static inline JsonParseErrorType
 json_lex_string(JsonLexContext *lex)
 {
-	char	   *s;
-	char	   *const end = lex->input + lex->input_length;
+	const char *s;
+	const char *const end = lex->input + lex->input_length;
 	int			hi_surrogate = -1;
 
 	/* Convenience macros for error exits */
@@ -1689,7 +1689,7 @@ json_lex_string(JsonLexContext *lex)
 	} while (0)
 #define FAIL_AT_CHAR_END(code) \
 	do { \
-		char	   *term = s + pg_encoding_mblen(lex->input_encoding, s); \
+		const char	   *term = s + pg_encoding_mblen(lex->input_encoding, s); \
 		lex->token_terminator = (term <= end) ? term : end; \
 		return code; \
 	} while (0)
@@ -1854,7 +1854,7 @@ json_lex_string(JsonLexContext *lex)
 		}
 		else
 		{
-			char	   *p = s;
+			const char *p = s;
 
 			if (hi_surrogate != -1)
 				FAIL_AT_CHAR_END(JSON_UNICODE_LOW_SURROGATE);
@@ -1940,7 +1940,7 @@ json_lex_string(JsonLexContext *lex)
  * the distance from lex->input to the token end+1 is returned to *total_len.
  */
 static inline JsonParseErrorType
-json_lex_number(JsonLexContext *lex, char *s,
+json_lex_number(JsonLexContext *lex, const char *s,
 				bool *num_err, size_t *total_len)
 {
 	bool		error = false;

src/include/common/jsonapi.h

@@ -88,18 +88,18 @@ typedef struct JsonIncrementalState JsonIncrementalState;
 #define JSONLEX_FREE_STRVAL			(1 << 1)
 typedef struct JsonLexContext
 {
-	char	   *input;
+	const char *input;
 	size_t		input_length;
 	int			input_encoding;
-	char	   *token_start;
-	char	   *token_terminator;
-	char	   *prev_token_terminator;
+	const char *token_start;
+	const char *token_terminator;
+	const char *prev_token_terminator;
 	bool		incremental;
 	JsonTokenType token_type;
 	int			lex_level;
 	bits32		flags;
 	int			line_number;	/* line number, starting from 1 */
-	char	   *line_start;		/* where that line starts within input */
+	const char *line_start;		/* where that line starts within input */
 	JsonParserStack *pstack;
 	JsonIncrementalState *inc_state;
 	StringInfo	strval;
@@ -157,7 +157,7 @@ extern JsonParseErrorType pg_parse_json(JsonLexContext *lex,
 
 extern JsonParseErrorType pg_parse_json_incremental(JsonLexContext *lex,
 													JsonSemAction *sem,
-													char *json,
+													const char *json,
 													size_t len,
 													bool is_last);
 
@@ -192,7 +192,7 @@ extern JsonParseErrorType json_count_array_elements(JsonLexContext *lex,
  * cleanup.
  */
 extern JsonLexContext *makeJsonLexContextCstringLen(JsonLexContext *lex,
-													char *json,
+													const char *json,
 													size_t len,
 													int encoding,
 													bool need_escapes);