Fix JSON error reporting for many cases of erroneous string values.

The majority of error exit cases in json_lex_string() failed to
set lex->token_terminator, causing problems for the error context
reporting code: it would see token_terminator less than token_start
and do something more or less nuts.  In v14 and up the end result
could be as bad as a crash in report_json_context().  Older
versions accidentally avoided that fate; but all versions produce
error context lines that are far less useful than intended,
because they'd stop at the end of the prior token instead of
continuing to where the actually-bad input is.

To fix, invent some macros that make it less notationally painful
to do the right thing.  Also add documentation about what the
function is actually required to do; and in >= v14, add an assertion
in report_json_context about token_terminator being sufficiently
far advanced.

Per report from Nikolay Shaplov.  Back-patch to all supported
versions.

Discussion: https://postgr.es/m/7332649.x5DLKWyVIX@thinkpad-pgpro

1 files changed, 47 insertions, 29 deletions

diff --git a/src/common/jsonapi.c b/src/common/jsonapi.c
index e4ff3f36029..2e86589cfd8 100644
--- a/src/common/jsonapi.c
+++ b/src/common/jsonapi.c
@@ -697,6 +697,14 @@ json_lex(JsonLexContext *lex)
 
 /*
  * The next token in the input stream is known to be a string; lex it.
+ *
+ * If lex->strval isn't NULL, fill it with the decoded string.
+ * Set lex->token_terminator to the end of the decoded input, and in
+ * success cases, transfer its previous value to lex->prev_token_terminator.
+ * Return JSON_SUCCESS or an error code.
+ *
+ * Note: be careful that all error exits advance lex->token_terminator
+ * to the point after the character we detected the error on.
  */
 static inline JsonParseErrorType
 json_lex_string(JsonLexContext *lex)
@@ -705,6 +713,19 @@ json_lex_string(JsonLexContext *lex)
 	char	   *const end = lex->input + lex->input_length;
 	int			hi_surrogate = -1;
 
+	/* Convenience macros for error exits */
+#define FAIL_AT_CHAR_START(code) \
+	do { \
+		lex->token_terminator = s; \
+		return code; \
+	} while (0)
+#define FAIL_AT_CHAR_END(code) \
+	do { \
+		lex->token_terminator = \
+			s + pg_encoding_mblen_bounded(lex->input_encoding, s); \
+		return code; \
+	} while (0)
+
 	if (lex->strval != NULL)
 		resetStringInfo(lex->strval);
 
@@ -715,10 +736,7 @@ json_lex_string(JsonLexContext *lex)
 		s++;
 		/* Premature end of the string. */
 		if (s >= end)
-		{
-			lex->token_terminator = s;
-			return JSON_INVALID_TOKEN;
-		}
+			FAIL_AT_CHAR_START(JSON_INVALID_TOKEN);
 		else if (*s == '"')
 			break;
 		else if (*s == '\\')
@@ -726,10 +744,7 @@ json_lex_string(JsonLexContext *lex)
 			/* OK, we have an escape character. */
 			s++;
 			if (s >= end)
-			{
-				lex->token_terminator = s;
-				return JSON_INVALID_TOKEN;
-			}
+				FAIL_AT_CHAR_START(JSON_INVALID_TOKEN);
 			else if (*s == 'u')
 			{
 				int			i;
@@ -739,10 +754,7 @@ json_lex_string(JsonLexContext *lex)
 				{
 					s++;
 					if (s >= end)
-					{
-						lex->token_terminator = s;
-						return JSON_INVALID_TOKEN;
-					}
+						FAIL_AT_CHAR_START(JSON_INVALID_TOKEN);
 					else if (*s >= '0' && *s <= '9')
 						ch = (ch * 16) + (*s - '0');
 					else if (*s >= 'a' && *s <= 'f')
@@ -750,10 +762,7 @@ json_lex_string(JsonLexContext *lex)
 					else if (*s >= 'A' && *s <= 'F')
 						ch = (ch * 16) + (*s - 'A') + 10;
 					else
-					{
-						lex->token_terminator = s + pg_encoding_mblen_bounded(lex->input_encoding, s);
-						return JSON_UNICODE_ESCAPE_FORMAT;
-					}
+						FAIL_AT_CHAR_END(JSON_UNICODE_ESCAPE_FORMAT);
 				}
 				if (lex->strval != NULL)
 				{
@@ -763,20 +772,20 @@ json_lex_string(JsonLexContext *lex)
 					if (is_utf16_surrogate_first(ch))
 					{
 						if (hi_surrogate != -1)
-							return JSON_UNICODE_HIGH_SURROGATE;
+							FAIL_AT_CHAR_END(JSON_UNICODE_HIGH_SURROGATE);
 						hi_surrogate = ch;
 						continue;
 					}
 					else if (is_utf16_surrogate_second(ch))
 					{
 						if (hi_surrogate == -1)
-							return JSON_UNICODE_LOW_SURROGATE;
+							FAIL_AT_CHAR_END(JSON_UNICODE_LOW_SURROGATE);
 						ch = surrogate_pair_to_codepoint(hi_surrogate, ch);
 						hi_surrogate = -1;
 					}
 
 					if (hi_surrogate != -1)
-						return JSON_UNICODE_LOW_SURROGATE;
+						FAIL_AT_CHAR_END(JSON_UNICODE_LOW_SURROGATE);
 
 					/*
 					 * Reject invalid cases.  We can't have a value above
@@ -786,7 +795,7 @@ json_lex_string(JsonLexContext *lex)
 					if (ch == 0)
 					{
 						/* We can't allow this, since our TEXT type doesn't */
-						return JSON_UNICODE_CODE_POINT_ZERO;
+						FAIL_AT_CHAR_END(JSON_UNICODE_CODE_POINT_ZERO);
 					}
 
 					/*
@@ -800,7 +809,7 @@ json_lex_string(JsonLexContext *lex)
 						char		cbuf[MAX_UNICODE_EQUIVALENT_STRING + 1];
 
 						if (!pg_unicode_to_server_noerror(ch, (unsigned char *) cbuf))
-							return JSON_UNICODE_UNTRANSLATABLE;
+							FAIL_AT_CHAR_END(JSON_UNICODE_UNTRANSLATABLE);
 						appendStringInfoString(lex->strval, cbuf);
 					}
 #else
@@ -820,14 +829,14 @@ json_lex_string(JsonLexContext *lex)
 						appendStringInfoChar(lex->strval, (char) ch);
 					}
 					else
-						return JSON_UNICODE_HIGH_ESCAPE;
+						FAIL_AT_CHAR_END(JSON_UNICODE_HIGH_ESCAPE);
 #endif							/* FRONTEND */
 				}
 			}
 			else if (lex->strval != NULL)
 			{
 				if (hi_surrogate != -1)
-					return JSON_UNICODE_LOW_SURROGATE;
+					FAIL_AT_CHAR_END(JSON_UNICODE_LOW_SURROGATE);
 
 				switch (*s)
 				{
@@ -852,10 +861,14 @@ json_lex_string(JsonLexContext *lex)
 						appendStringInfoChar(lex->strval, '\t');
 						break;
 					default:
-						/* Not a valid string escape, so signal error. */
+
+						/*
+						 * Not a valid string escape, so signal error.  We
+						 * adjust token_start so that just the escape sequence
+						 * is reported, not the whole string.
+						 */
 						lex->token_start = s;
-						lex->token_terminator = s + pg_encoding_mblen_bounded(lex->input_encoding, s);
-						return JSON_ESCAPING_INVALID;
+						FAIL_AT_CHAR_END(JSON_ESCAPING_INVALID);
 				}
 			}
 			else if (strchr("\"\\/bfnrt", *s) == NULL)
@@ -868,8 +881,7 @@ json_lex_string(JsonLexContext *lex)
 				 * shown it's not a performance win.
 				 */
 				lex->token_start = s;
-				lex->token_terminator = s + pg_encoding_mblen_bounded(lex->input_encoding, s);
-				return JSON_ESCAPING_INVALID;
+				FAIL_AT_CHAR_END(JSON_ESCAPING_INVALID);
 			}
 		}
 		else
@@ -877,7 +889,7 @@ json_lex_string(JsonLexContext *lex)
 			char	   *p = s;
 
 			if (hi_surrogate != -1)
-				return JSON_UNICODE_LOW_SURROGATE;
+				FAIL_AT_CHAR_END(JSON_UNICODE_LOW_SURROGATE);
 
 			/*
 			 * Skip to the first byte that requires special handling, so we
@@ -917,12 +929,18 @@ json_lex_string(JsonLexContext *lex)
 	}
 
 	if (hi_surrogate != -1)
+	{
+		lex->token_terminator = s + 1;
 		return JSON_UNICODE_LOW_SURROGATE;
+	}
 
 	/* Hooray, we found the end of the string! */
 	lex->prev_token_terminator = lex->token_terminator;
 	lex->token_terminator = s + 1;
 	return JSON_SUCCESS;
+
+#undef FAIL_AT_CHAR_START
+#undef FAIL_AT_CHAR_END
 }
 
 /*