make WeaselJson_OVERFLOW a terminal state like WeaselJson_REJECT

When a parse step returned WeaselJson_OVERFLOW, the pushdown stack was
left corrupted (frames had been popped before the failing push), but the
overflow status was not made sticky the way WeaselJson_REJECT is. A
subsequent end-of-data call WeaselJsonParser_parse(parser, nullptr, 0)
could then dispatch through the corrupted stack and return WeaselJson_OK,
accepting an incomplete, invalid too-deeply-nested document as valid JSON.

Add an `overflowed` flag, set it whenever a continuation returns
WeaselJson_OVERFLOW, and short-circuit parse() to return
WeaselJson_OVERFLOW on every subsequent call. reset() clears the flag so a
reused parser can accept a different document. This mirrors the existing
stickiness handling for WeaselJson_REJECT.

Closes #52

src/parser3.h

@@ -140,6 +140,7 @@ struct Parser3 {
     std::ignore = push({N_VALUE, N_WHITESPACE, T_EOF});
     inKey = false;
     rejected = false;
+    overflowed = false;
     utf8Codepoint = 0;
     utf16Surrogate = 0;
     minCodepoint = 0;
@@ -163,6 +164,7 @@ struct Parser3 {
   Utf8Dfa strDfa;
   bool inKey = false;
   bool rejected = false;
+  bool overflowed = false;
 
 #ifndef HAS_MUSTTAIL
   char *stashBufForTrampoline;
@@ -1074,6 +1076,10 @@ inline WeaselJsonStatus Parser3::parse(char *buf, int len) {
     return WeaselJson_REJECT;
   }
 
+  if (this->overflowed) [[unlikely]] {
+    return WeaselJson_OVERFLOW;
+  }
+
   if (len < 0) [[unlikely]] {
     this->rejected = true;
     return WeaselJson_REJECT;
@@ -1087,6 +1093,8 @@ inline WeaselJsonStatus Parser3::parse(char *buf, int len) {
       symbolTables.continuations[top()](this, buf, buf + len);
   if (status == WeaselJson_REJECT) {
     this->rejected = true;
+  } else if (status == WeaselJson_OVERFLOW) {
+    this->overflowed = true;
   }
   return WeaselJsonStatus(status);
 #else
@@ -1097,6 +1105,8 @@ inline WeaselJsonStatus Parser3::parse(char *buf, int len) {
     ;
   if (result == WeaselJson_REJECT) {
     this->rejected = true;
+  } else if (result == WeaselJson_OVERFLOW) {
+    this->overflowed = true;
   }
   return WeaselJsonStatus(result);
 #endif

src/test.cpp

@@ -202,6 +202,62 @@ TEST_CASE("parser3") {
   }
 }
 
+TEST_CASE("overflow state is sticky") {
+  auto c = noopCallbacks();
+  // stackSize 3 is exactly big enough to hold reset()'s bootstrap, but too
+  // small for nested arrays. Overflows must be terminal like rejects: a later
+  // end-of-data call must never report OK for an incomplete document.
+  auto *parser = WeaselJsonParser_create(3, &c, nullptr, 0);
+  REQUIRE(parser != nullptr);
+
+  std::string doc = "[[";
+  REQUIRE(WeaselJsonParser_parse(parser, doc.data(), doc.size()) ==
+          WeaselJson_OVERFLOW);
+
+  // After overflow, the end-of-data call must not return OK.
+  REQUIRE(WeaselJsonParser_parse(parser, nullptr, 0) != WeaselJson_OK);
+  // It should keep reporting a terminal failure.
+  REQUIRE(WeaselJsonParser_parse(parser, nullptr, 0) != WeaselJson_OK);
+
+  // Further data chunks must also stay terminal.
+  std::string more = "]]";
+  REQUIRE(WeaselJsonParser_parse(parser, more.data(), more.size()) !=
+          WeaselJson_OK);
+
+  WeaselJsonParser_destroy(parser);
+}
+
+TEST_CASE("overflow is sticky for nested objects") {
+  auto c = noopCallbacks();
+  auto *parser = WeaselJsonParser_create(4, &c, nullptr, 0);
+  REQUIRE(parser != nullptr);
+
+  std::string doc = "{\"a\":{ \"a\":";
+  REQUIRE(WeaselJsonParser_parse(parser, doc.data(), doc.size()) ==
+          WeaselJson_OVERFLOW);
+  REQUIRE(WeaselJsonParser_parse(parser, nullptr, 0) != WeaselJson_OK);
+
+  WeaselJsonParser_destroy(parser);
+}
+
+TEST_CASE("reset clears overflow state") {
+  auto c = noopCallbacks();
+  auto *parser = WeaselJsonParser_create(3, &c, nullptr, 0);
+  REQUIRE(parser != nullptr);
+
+  std::string doc = "[[";
+  REQUIRE(WeaselJsonParser_parse(parser, doc.data(), doc.size()) ==
+          WeaselJson_OVERFLOW);
+  // After reset the parser should accept a minimal document again.
+  WeaselJsonParser_reset(parser);
+  std::string copy = "1";
+  REQUIRE(WeaselJsonParser_parse(parser, copy.data(), copy.size()) ==
+          WeaselJson_AGAIN);
+  REQUIRE(WeaselJsonParser_parse(parser, nullptr, 0) == WeaselJson_OK);
+
+  WeaselJsonParser_destroy(parser);
+}
+
 TEST_CASE("rejected state is sticky") {
   auto c = noopCallbacks();
   auto *parser = WeaselJsonParser_create(1024, &c, nullptr, 0);