Avoid re-decoding an incomplete number on every ResumableParser chunk

ext/json/ext/parser/parser.c

@@ -1457,7 +1457,7 @@ static inline int json_parse_digits(JSON_ParserState *state, uint64_t *accumulat
     return (int)(state->cursor - start);
 }
 
-static inline VALUE json_parse_number(JSON_ParserState *state, JSON_ParserConfig *config, bool negative, const char *start)
+static inline VALUE json_parse_number(JSON_ParserState *state, JSON_ParserConfig *config, bool negative, const char *start, bool resumable)
 {
     bool integer = true;
     const char first_digit = *state->cursor;
@@ -1514,6 +1514,16 @@ static inline VALUE json_parse_number(JSON_ParserState *state, JSON_ParserConfig
         }
     }
 
+    // A number touching the end of the buffer may still grow in a later chunk,
+    // so the caller will rewind and wait. Decoding it now would build a value
+    // -- for a long run of digits, an expensive bignum -- only to discard it,
+    // and repeating that on every resumed chunk is quadratic in the number's
+    // length. The digit scan above already advanced the cursor, which is all
+    // the caller needs to detect the incomplete number.
+    if (RB_UNLIKELY(resumable && eos(state))) {
+        return Qundef;
+    }
+
     if (integer) {
         return json_decode_integer(mantissa, mantissa_digits, negative, start, state->cursor);
     }
@@ -1638,7 +1648,7 @@ ALWAYS_INLINE(static) bool json_parse_any(JSON_ParserState *state, JSON_ParserCo
             case '-': {
                 state->cursor++;
 
-                value = json_parse_number(state, config, true, value_start);
+                value = json_parse_number(state, config, true, value_start, resumable);
 
                 if (RB_UNLIKELY(UNDEF_P(value) && config->allow_nan && peek(state) == 'I')) {
                     state->cursor = value_start;
@@ -1661,7 +1671,7 @@ ALWAYS_INLINE(static) bool json_parse_any(JSON_ParserState *state, JSON_ParserCo
             }
 
             case '0': case '1': case '2': case '3': case '4': case '5': case '6': case '7': case '8': case '9': {
-                value = json_parse_number(state, config, false, value_start);
+                value = json_parse_number(state, config, false, value_start, resumable);
 
                 // Top level numbers are ambiguous when parsing streams, we can't
                 // know if we parsed all the digits if we hit EOS.

test/json/resumable_parser_test.rb

@@ -175,6 +175,23 @@ def test_parse_byte_by_byte_numbers
     assert_resumed_parsing('123 ', trailing_bytes: 1)
   end
 
+  def test_large_numbers_split_across_feeds_are_decoded_correctly
+    {
+      '12345678901234567890123456789012345678901234567890 ' => 12345678901234567890123456789012345678901234567890,
+      '-98765432109876543210987654321 ' => -98765432109876543210987654321,
+      '3.14159265358979323846264338327950288 ' => 3.14159265358979323846264338327950288,
+      '-1.5e-300 ' => -1.5e-300,
+    }.each do |doc, expected|
+      parser = new_parser
+      value = nil
+      doc.each_char do |char|
+        parser << char
+        value = parser.value if parser.parse
+      end
+      assert_equal expected, value, doc.inspect
+    end
+  end
+
   def test_nul_byte_is_a_syntax_error
     # A NUL byte in a structural position must raise, not stall forever waiting for more input
     # (peek() returns 0 both at EOS and for a literal NUL byte).