Fix jsonb Unicode escape processing, and in consequence disallow \u0000.

We've been trying to support \u0000 in JSON values since commit
78ed8e0, and have introduced increasingly worse hacks to try to
make it work, such as commit 0ad1a81.  However, it fundamentally
can't work in the way envisioned, because the stored representation looks
the same as for \\u0000 which is not the same thing at all.  It's also
entirely bogus to output \u0000 when de-escaped output is called for.

The right way to do this would be to store an actual 0x00 byte, and then
throw error only if asked to produce de-escaped textual output.  However,
getting to that point seems likely to take considerable work and may well
never be practical in the 9.4.x series.

To preserve our options for better behavior while getting rid of the nasty
side-effects of 0ad1a81, revert that commit in toto and instead
throw error if \u0000 is used in a context where it needs to be de-escaped.
(These are the same contexts where non-ASCII Unicode escapes throw error
if the database encoding isn't UTF8, so this behavior is by no means
without precedent.)

In passing, make both the \u0000 case and the non-ASCII Unicode case report
ERRCODE_UNTRANSLATABLE_CHARACTER / "unsupported Unicode escape sequence"
rather than claiming there's something wrong with the input syntax.

Back-patch to 9.4, where we have to do something because 0ad1a81
broke things for many cases having nothing to do with \u0000.  9.3 also has
bogus behavior, but only for that specific escape value, so given the lack
of field complaints it seems better to leave 9.3 alone.

Author: tglsfdc

doc/src/sgml/json.sgml

@@ -69,12 +69,14 @@
   regardless of the database encoding, and are checked only for syntactic
   correctness (that is, that four hex digits follow <literal>\u</>).
   However, the input function for <type>jsonb</> is stricter: it disallows
-  Unicode escapes for non-ASCII characters (those
-  above <literal>U+007F</>) unless the database encoding is UTF8.  It also
-  insists that any use of Unicode surrogate pairs to designate characters
-  outside the Unicode Basic Multilingual Plane be correct.  Valid Unicode
-  escapes, except for <literal>\u0000</>, are then converted to the
-  equivalent ASCII or UTF8 character for storage.
+  Unicode escapes for non-ASCII characters (those above <literal>U+007F</>)
+  unless the database encoding is UTF8.  The <type>jsonb</> type also
+  rejects <literal>\u0000</> (because that cannot be represented in
+  <productname>PostgreSQL</productname>'s <type>text</> type), and it insists
+  that any use of Unicode surrogate pairs to designate characters outside
+  the Unicode Basic Multilingual Plane be correct.  Valid Unicode escapes
+  are converted to the equivalent ASCII or UTF8 character for storage;
+  this includes folding surrogate pairs into a single character.
  </para>
 
  <note>
@@ -101,7 +103,7 @@
   constitutes valid <type>jsonb</type> data that do not apply to
   the <type>json</type> type, nor to JSON in the abstract, corresponding
   to limits on what can be represented by the underlying data type.
-  Specifically, <type>jsonb</> will reject numbers that are outside the
+  Notably, <type>jsonb</> will reject numbers that are outside the
   range of the <productname>PostgreSQL</productname> <type>numeric</> data
   type, while <type>json</> will not.  Such implementation-defined
   restrictions are permitted by <acronym>RFC</> 7159.  However, in
@@ -134,7 +136,8 @@
        <row>
         <entry><type>string</></entry>
         <entry><type>text</></entry>
-        <entry>See notes above concerning encoding restrictions</entry>
+        <entry><literal>\u0000</> is disallowed, as are non-ASCII Unicode
+         escapes if database encoding is not UTF8</entry>
        </row>
        <row>
         <entry><type>number</></entry>

doc/src/sgml/release-9.4.sgml

@@ -101,22 +101,6 @@
      </para>
     </listitem>
 
-    <listitem>
-     <para>
-      Unicode escapes in <link linkend="datatype-json"><type>JSON</type></link>
-      text values are no longer rendered with the backslash escaped
-      (Andrew Dunstan)
-     </para>
-
-     <para>
-      Previously, all backslashes in text values being formed into JSON
-      were escaped. Now a backslash followed by <literal>u</> and four
-      hexadecimal digits is not escaped, as this is a legal sequence in a
-      JSON string value, and escaping the backslash led to some perverse
-      results.
-     </para>
-    </listitem>
-
     <listitem>
      <para>
       When converting values of type <type>date</>, <type>timestamp</>

src/backend/utils/adt/json.c

@@ -806,14 +806,17 @@ json_lex_string(JsonLexContext *lex)
 					 * For UTF8, replace the escape sequence by the actual
 					 * utf8 character in lex->strval. Do this also for other
 					 * encodings if the escape designates an ASCII character,
-					 * otherwise raise an error. We don't ever unescape a
-					 * \u0000, since that would result in an impermissible nul
-					 * byte.
+					 * otherwise raise an error.
 					 */
 
 					if (ch == 0)
 					{
-						appendStringInfoString(lex->strval, "\\u0000");
+						/* We can't allow this, since our TEXT type doesn't */
+						ereport(ERROR,
+								(errcode(ERRCODE_UNTRANSLATABLE_CHARACTER),
+							   errmsg("unsupported Unicode escape sequence"),
+						   errdetail("\\u0000 cannot be converted to text."),
+								 report_json_context(lex)));
 					}
 					else if (GetDatabaseEncoding() == PG_UTF8)
 					{
@@ -833,8 +836,8 @@ json_lex_string(JsonLexContext *lex)
 					else
 					{
 						ereport(ERROR,
-								(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
-								 errmsg("invalid input syntax for type json"),
+								(errcode(ERRCODE_UNTRANSLATABLE_CHARACTER),
+							   errmsg("unsupported Unicode escape sequence"),
 								 errdetail("Unicode escape values cannot be used for code point values above 007F when the server encoding is not UTF8."),
 								 report_json_context(lex)));
 					}
@@ -1284,8 +1287,8 @@ json_categorize_type(Oid typoid,
 
 	/*
 	 * We need to get the output function for everything except date and
-	 * timestamp types, array and composite types, booleans,
-	 * and non-builtin types  where there's a cast to json.
+	 * timestamp types, array and composite types, booleans, and non-builtin
+	 * types where there's a cast to json.
 	 */
 
 	switch (typoid)
@@ -1335,11 +1338,12 @@ json_categorize_type(Oid typoid,
 				/* but let's look for a cast to json, if it's not built-in */
 				if (typoid >= FirstNormalObjectId)
 				{
-					Oid castfunc;
+					Oid			castfunc;
 					CoercionPathType ctype;
 
 					ctype = find_coercion_pathway(JSONOID, typoid,
-												  COERCION_EXPLICIT, &castfunc);
+												  COERCION_EXPLICIT,
+												  &castfunc);
 					if (ctype == COERCION_PATH_FUNC && OidIsValid(castfunc))
 					{
 						*tcategory = JSONTYPE_CAST;
@@ -2382,30 +2386,7 @@ escape_json(StringInfo buf, const char *str)
 				appendStringInfoString(buf, "\\\"");
 				break;
 			case '\\':
-
-				/*
-				 * Unicode escapes are passed through as is. There is no
-				 * requirement that they denote a valid character in the
-				 * server encoding - indeed that is a big part of their
-				 * usefulness.
-				 *
-				 * All we require is that they consist of \uXXXX where the Xs
-				 * are hexadecimal digits. It is the responsibility of the
-				 * caller of, say, to_json() to make sure that the unicode
-				 * escape is valid.
-				 *
-				 * In the case of a jsonb string value being escaped, the only
-				 * unicode escape that should be present is \u0000, all the
-				 * other unicode escapes will have been resolved.
-				 */
-				if (p[1] == 'u' &&
-					isxdigit((unsigned char) p[2]) &&
-					isxdigit((unsigned char) p[3]) &&
-					isxdigit((unsigned char) p[4]) &&
-					isxdigit((unsigned char) p[5]))
-					appendStringInfoCharMacro(buf, *p);
-				else
-					appendStringInfoString(buf, "\\\\");
+				appendStringInfoString(buf, "\\\\");
 				break;
 			default:
 				if ((unsigned char) *p < ' ')

src/test/regress/expected/json.out

@@ -426,20 +426,6 @@ select to_json(timestamptz '2014-05-28 12:22:35.614298-04');
 (1 row)
 
 COMMIT;
--- unicode escape - backslash is not escaped
-select to_json(text '\uabcd');
- to_json  
-----------
- "\uabcd"
-(1 row)
-
--- any other backslash is escaped
-select to_json(text '\abcd');
- to_json  
-----------
- "\\abcd"
-(1 row)
-
 --json_agg
 SELECT json_agg(q)
   FROM ( SELECT $$a$$ || x AS b, y AS c,
@@ -1400,6 +1386,36 @@ ERROR:  invalid input syntax for type json
 DETAIL:  Unicode low surrogate must follow a high surrogate.
 CONTEXT:  JSON data, line 1: { "a":...
 --handling of simple unicode escapes
+select json '{ "a":  "the Copyright \u00a9 sign" }' as correct_in_utf8;
+            correct_in_utf8            
+---------------------------------------
+ { "a":  "the Copyright \u00a9 sign" }
+(1 row)
+
+select json '{ "a":  "dollar \u0024 character" }' as correct_everywhere;
+         correct_everywhere          
+-------------------------------------
+ { "a":  "dollar \u0024 character" }
+(1 row)
+
+select json '{ "a":  "dollar \\u0024 character" }' as not_an_escape;
+            not_an_escape             
+--------------------------------------
+ { "a":  "dollar \\u0024 character" }
+(1 row)
+
+select json '{ "a":  "null \u0000 escape" }' as not_unescaped;
+         not_unescaped          
+--------------------------------
+ { "a":  "null \u0000 escape" }
+(1 row)
+
+select json '{ "a":  "null \\u0000 escape" }' as not_an_escape;
+          not_an_escape          
+---------------------------------
+ { "a":  "null \\u0000 escape" }
+(1 row)
+
 select json '{ "a":  "the Copyright \u00a9 sign" }' ->> 'a' as correct_in_utf8;
    correct_in_utf8    
 ----------------------
@@ -1412,8 +1428,18 @@ select json '{ "a":  "dollar \u0024 character" }' ->> 'a' as correct_everywhere;
  dollar $ character
 (1 row)
 
-select json '{ "a":  "null \u0000 escape" }' ->> 'a' as not_unescaped;
-   not_unescaped    
+select json '{ "a":  "dollar \\u0024 character" }' ->> 'a' as not_an_escape;
+      not_an_escape      
+-------------------------
+ dollar \u0024 character
+(1 row)
+
+select json '{ "a":  "null \u0000 escape" }' ->> 'a' as fails;
+ERROR:  unsupported Unicode escape sequence
+DETAIL:  \u0000 cannot be converted to text.
+CONTEXT:  JSON data, line 1: { "a":...
+select json '{ "a":  "null \\u0000 escape" }' ->> 'a' as not_an_escape;
+   not_an_escape    
 --------------------
  null \u0000 escape
 (1 row)

src/test/regress/expected/json_1.out

@@ -426,20 +426,6 @@ select to_json(timestamptz '2014-05-28 12:22:35.614298-04');
 (1 row)
 
 COMMIT;
--- unicode escape - backslash is not escaped
-select to_json(text '\uabcd');
- to_json  
-----------
- "\uabcd"
-(1 row)
-
--- any other backslash is escaped
-select to_json(text '\abcd');
- to_json  
-----------
- "\\abcd"
-(1 row)
-
 --json_agg
 SELECT json_agg(q)
   FROM ( SELECT $$a$$ || x AS b, y AS c,
@@ -1378,7 +1364,7 @@ select * from json_populate_recordset(row('def',99,null)::jpop,'[{"a":[100,200,3
 
 -- handling of unicode surrogate pairs
 select json '{ "a":  "\ud83d\ude04\ud83d\udc36" }' -> 'a' as correct_in_utf8;
-ERROR:  invalid input syntax for type json
+ERROR:  unsupported Unicode escape sequence
 DETAIL:  Unicode escape values cannot be used for code point values above 007F when the server encoding is not UTF8.
 CONTEXT:  JSON data, line 1: { "a":...
 select json '{ "a":  "\ud83d\ud83d" }' -> 'a'; -- 2 high surrogates in a row
@@ -1398,8 +1384,38 @@ ERROR:  invalid input syntax for type json
 DETAIL:  Unicode low surrogate must follow a high surrogate.
 CONTEXT:  JSON data, line 1: { "a":...
 --handling of simple unicode escapes
+select json '{ "a":  "the Copyright \u00a9 sign" }' as correct_in_utf8;
+            correct_in_utf8            
+---------------------------------------
+ { "a":  "the Copyright \u00a9 sign" }
+(1 row)
+
+select json '{ "a":  "dollar \u0024 character" }' as correct_everywhere;
+         correct_everywhere          
+-------------------------------------
+ { "a":  "dollar \u0024 character" }
+(1 row)
+
+select json '{ "a":  "dollar \\u0024 character" }' as not_an_escape;
+            not_an_escape             
+--------------------------------------
+ { "a":  "dollar \\u0024 character" }
+(1 row)
+
+select json '{ "a":  "null \u0000 escape" }' as not_unescaped;
+         not_unescaped          
+--------------------------------
+ { "a":  "null \u0000 escape" }
+(1 row)
+
+select json '{ "a":  "null \\u0000 escape" }' as not_an_escape;
+          not_an_escape          
+---------------------------------
+ { "a":  "null \\u0000 escape" }
+(1 row)
+
 select json '{ "a":  "the Copyright \u00a9 sign" }' ->> 'a' as correct_in_utf8;
-ERROR:  invalid input syntax for type json
+ERROR:  unsupported Unicode escape sequence
 DETAIL:  Unicode escape values cannot be used for code point values above 007F when the server encoding is not UTF8.
 CONTEXT:  JSON data, line 1: { "a":...
 select json '{ "a":  "dollar \u0024 character" }' ->> 'a' as correct_everywhere;
@@ -1408,8 +1424,18 @@ select json '{ "a":  "dollar \u0024 character" }' ->> 'a' as correct_everywhere;
  dollar $ character
 (1 row)
 
-select json '{ "a":  "null \u0000 escape" }' ->> 'a' as not_unescaped;
-   not_unescaped    
+select json '{ "a":  "dollar \\u0024 character" }' ->> 'a' as not_an_escape;
+      not_an_escape      
+-------------------------
+ dollar \u0024 character
+(1 row)
+
+select json '{ "a":  "null \u0000 escape" }' ->> 'a' as fails;
+ERROR:  unsupported Unicode escape sequence
+DETAIL:  \u0000 cannot be converted to text.
+CONTEXT:  JSON data, line 1: { "a":...
+select json '{ "a":  "null \\u0000 escape" }' ->> 'a' as not_an_escape;
+   not_an_escape    
 --------------------
  null \u0000 escape
 (1 row)