LibRegex: Restore checkpoints when restoring the state post-fork

Fixes the lockup/OOM in #968.

Tests/LibRegex/Regex.cpp

@@ -705,6 +705,8 @@ TEST_CASE(ECMA262_match)
         { "a$"sv, "a\r\n"sv, true, global_multiline.value() }, // $ should accept all LineTerminators in ECMA262 mode with Multiline.
         { "^a"sv, "\ra"sv, true, global_multiline.value() },
         { "^(.*?):[ \\t]*([^\\r\\n]*)$"sv, "content-length: 488\r\ncontent-type: application/json; charset=utf-8\r\n"sv, true, global_multiline.value() },
+        { "^\\?((&?category=[0-9]+)?(&?shippable=1)?(&?ad_type=demand)?(&?page=[0-9]+)?(&?locations=(r|d)_[0-9]+)?)+$"sv,
+            "?category=54&shippable=1&baby_age=p,0,1,3"sv, false }, // ladybird#968, ?+ should not loop forever.
     };
     // clang-format on
 
@@ -981,6 +983,7 @@ TEST_CASE(theoretically_infinite_loop)
         "(a*?)*"sv, // Infinitely matching empty substrings, the outer loop should short-circuit.
         "(a*)*?"sv, // Should match exactly nothing.
         "(?:)*?"sv, // Should not generate an infinite fork loop.
+        "(a?)+$"sv, // Infinitely matching empty strings, but with '+' instead of '*'.
     };
     for (auto& pattern : patterns) {
         Regex<ECMA262> re(pattern);

Userland/Libraries/LibRegex/RegexByteCode.cpp

@@ -1072,20 +1072,20 @@ ALWAYS_INLINE ExecutionResult OpCode_ResetRepeat::execute(MatchInput const&, Mat
     return ExecutionResult::Continue;
 }
 
-ALWAYS_INLINE ExecutionResult OpCode_Checkpoint::execute(MatchInput const& input, MatchState& state) const
+ALWAYS_INLINE ExecutionResult OpCode_Checkpoint::execute(MatchInput const&, MatchState& state) const
 {
     auto id = this->id();
-    if (id >= input.checkpoints.size())
-        input.checkpoints.resize(id + 1);
+    if (id >= state.checkpoints.size())
+        state.checkpoints.resize(id + 1);
 
-    input.checkpoints[id] = state.string_position + 1;
+    state.checkpoints[id] = state.string_position + 1;
     return ExecutionResult::Continue;
 }
 
 ALWAYS_INLINE ExecutionResult OpCode_JumpNonEmpty::execute(MatchInput const& input, MatchState& state) const
 {
     u64 current_position = state.string_position;
-    auto checkpoint_position = input.checkpoints[checkpoint()];
+    auto checkpoint_position = state.checkpoints[checkpoint()];
 
     if (checkpoint_position != 0 && checkpoint_position != current_position + 1) {
         auto form = this->form();

Userland/Libraries/LibRegex/RegexMatch.h

@@ -547,7 +547,6 @@ struct MatchInput {
     mutable Vector<size_t> saved_positions;
     mutable Vector<size_t> saved_code_unit_positions;
     mutable Vector<size_t> saved_forks_since_last_save;
-    mutable Vector<u64, 64> checkpoints;
     mutable Optional<size_t> fork_to_replace;
 };
 
@@ -562,6 +561,7 @@ struct MatchState {
     COWVector<Match> matches;
     COWVector<Vector<Match>> capture_group_matches;
     COWVector<u64> repetition_marks;
+    Vector<u64, 64> checkpoints;
 };
 
 }