bug: ensure FFI glue always passes a valid pointer

Previously, if a caller provided a `&Vec::new()` to the
search function, then the library would pass that vec's
pointer to PCRE2 directly. Since creating an empty Vec
does not allocate, this pointer is actually invalid and
cannot be dereferenced. However, PCRE2 (rightfully)
assumes the pointer it is handed is valid.

We fix this by detecting the case of an empty slice.
If it's empty, we pass a pointer to an empty slice that
we know is valid.

Author: sharksforarms

src/bytes.rs

@@ -1259,6 +1259,19 @@ mod tests {
         assert!(re.is_match(b("a")).unwrap());
     }
 
+    // This tests a regression caused a segfault in the pcre2 library
+    // https://github.com/BurntSushi/rust-pcre2/issues/10
+    #[test]
+    fn jit_test_lazy_alloc_subject() {
+        let subject: Vec<u8> = vec![];
+
+        let re = RegexBuilder::new()
+            .jit_if_available(true)
+            .build(r"xxxx|xxxx|xxxx")
+            .unwrap();
+        assert!(!re.is_match(&subject).unwrap());
+    }
+
     #[test]
     fn utf_with_invalid_data() {
         let re = RegexBuilder::new()

src/ffi.rs

@@ -402,10 +402,20 @@ impl MatchData {
     pub unsafe fn find(
         &mut self,
         code: &Code,
-        subject: &[u8],
+        mut subject: &[u8],
         start: usize,
         options: u32,
     ) -> Result<bool, Error> {
+        // When the subject is empty, we use an empty slice
+        // with a known valid pointer. Otherwise, slices derived
+        // from, e.g., an empty `Vec<u8>` may not have a valid
+        // pointer, since creating an empty `Vec` is guaranteed
+        // to not allocate.
+        const EMPTY: &[u8] = &[];
+        if subject.is_empty() {
+            subject = EMPTY;
+        }
+
         let rc = pcre2_match_8(
             code.as_ptr(),
             subject.as_ptr(),