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diff --git a/third_party/rust/regex-automata/tests/fuzz/sparse.rs b/third_party/rust/regex-automata/tests/fuzz/sparse.rs
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+// This is a regression test for a bug in how special states are handled. The
+// fuzzer found a case where a state returned true for 'is_special_state' but
+// *didn't* return true for 'is_dead_state', 'is_quit_state', 'is_match_state',
+// 'is_start_state' or 'is_accel_state'. This in turn tripped a debug assertion
+// in the core matching loop that requires 'is_special_state' being true to
+// imply that one of the other routines returns true.
+//
+// We fixed this by adding some validation to both dense and sparse DFAs that
+// checks that this property is true for every state ID in the DFA.
+#[test]
+fn invalid_special_state() {
+    let data = include_bytes!(
+        "testdata/deserialize_sparse_crash-a1b839d899ced76d5d7d0f78f9edb7a421505838",
+    );
+    let _ = fuzz_run(data);
+}
+
+// This is an interesting case where a fuzzer generated a DFA with
+// a transition to a state ID that decoded as a valid state, but
+// where the ID itself did not point to one of the two existing
+// states for this particular DFA. This combined with marking this
+// transition's state ID as special but without actually making one of the
+// 'is_{dead,quit,match,start,accel}_state' predicates return true ended up
+// tripping the 'debug_assert(dfa.is_quit_state(sid))' code in the search
+// routine.
+//
+// We fixed this in alloc mode by checking that every transition points to a
+// valid state ID. Technically this bug still exists in core-only mode, but
+// it's not clear how to fix it. And it's worth pointing out that the search
+// routine won't panic in production. It will just provide invalid results. And
+// that's acceptable within the contract of DFA::from_bytes.
+#[test]
+fn transition_to_invalid_but_valid_state() {
+    let data = include_bytes!(
+        "testdata/deserialize_sparse_crash-dbb8172d3984e7e7d03f4b5f8bb86ecd1460eff9",
+    );
+    let _ = fuzz_run(data);
+}
+
+// Another one caught by the fuzzer where it generated a DFA that reported a
+// start state as a match state. Since matches are always delayed by one byte,
+// start states specifically cannot be match states. And indeed, the search
+// code relies on this.
+#[test]
+fn start_state_is_not_match_state() {
+    let data = include_bytes!(
+        "testdata/deserialize_sparse_crash-0da59c0434eaf35e5a6b470fa9244bb79c72b000",
+    );
+    let _ = fuzz_run(data);
+}
+
+// This is variation on 'transition_to_invalid_but_valid_state', but happens
+// to a start state. Namely, the fuzz data here builds a DFA with a start
+// state ID that is incorrect but points to a sequence of bytes that satisfies
+// state decoding validation. This errant state in turn has a non-zero number
+// of transitions, and its those transitions that point to a state that does
+// *not* satisfy state decoding validation. But we never checked those. So the
+// fix here was to add validation of the transitions off of the start state.
+#[test]
+fn start_state_has_valid_transitions() {
+    let data = include_bytes!(
+        "testdata/deserialize_sparse_crash-61fd8e3003bf9d99f6c1e5a8488727eefd234b98",
+    );
+    let _ = fuzz_run(data);
+}
+
+// This fuzz input generated a DFA with a state whose ID was in the match state
+// ID range, but where the state itself was encoded with zero pattern IDs. We
+// added validation code to check this case.
+#[test]
+fn match_state_inconsistency() {
+    let data = include_bytes!(
+        "testdata/deserialize_sparse_crash-c383ae07ec5e191422eadc492117439011816570",
+    );
+    let _ = fuzz_run(data);
+}
+
+// This fuzz input generated a DFA with a state whose ID was in the accelerator
+// range, but who didn't have any accelerators. This violated an invariant that
+// assumes that if 'dfa.is_accel_state(sid)' returns true, then the state must
+// have some accelerators.
+#[test]
+fn invalid_accelerators() {
+    let data = include_bytes!(
+        "testdata/deserialize_sparse_crash-d07703ceb94b10dcd9e4acb809f2051420449e2b",
+    );
+    let _ = fuzz_run(data);
+}
+
+// This fuzz input generated a DFA with a state whose EOI transition led to
+// a quit state, which is generally considered illegal. Why? Because the EOI
+// transition is defined over a special sentinel alphabet element and one
+// cannot configure a DFA to "quit" on that sentinel.
+#[test]
+fn eoi_transition_to_quit_state() {
+    let data = include_bytes!(
+        "testdata/deserialize_sparse_crash-18cfc246f2ddfc3dfc92b0c7893178c7cf65efa9",
+    );
+    let _ = fuzz_run(data);
+}
+
+// This is the code from the fuzz target. Kind of sucks to duplicate it here,
+// but this is fundamentally how we interpret the date.
+fn fuzz_run(given_data: &[u8]) -> Option<()> {
+    use regex_automata::dfa::Automaton;
+
+    if given_data.len() < 2 {
+        return None;
+    }
+    let haystack_len = usize::from(given_data[0]);
+    let haystack = given_data.get(1..1 + haystack_len)?;
+    let given_dfa_bytes = given_data.get(1 + haystack_len..)?;
+
+    // We help the fuzzer along by adding a preamble to the bytes that should
+    // at least make these first parts valid. The preamble expects a very
+    // specific sequence of bytes, so it makes sense to just force this.
+    let label = "rust-regex-automata-dfa-sparse\x00\x00";
+    assert_eq!(0, label.len() % 4);
+    let endianness_check = 0xFEFFu32.to_ne_bytes().to_vec();
+    let version_check = 2u32.to_ne_bytes().to_vec();
+    let mut dfa_bytes: Vec<u8> = vec![];
+    dfa_bytes.extend(label.as_bytes());
+    dfa_bytes.extend(&endianness_check);
+    dfa_bytes.extend(&version_check);
+    dfa_bytes.extend(given_dfa_bytes);
+    // This is the real test: checking that any input we give to
+    // DFA::from_bytes will never result in a panic.
+    let (dfa, _) =
+        regex_automata::dfa::sparse::DFA::from_bytes(&dfa_bytes).ok()?;
+    let _ = dfa.try_search_fwd(&regex_automata::Input::new(haystack));
+    Some(())
+}