[3.0.0 prep] Lossy error messages

CHANGELOG.md

@@ -11,6 +11,10 @@
   `LOOPBACK | UP`.
 - `errno` updated from 0.2 to 0.3. `Error::ErrnoError` carries an `errno::Errno`, so crates that
   construct or match on it have to move to 0.3 as well.
+- Error messages coming from libpcap are decoded lossily. libpcap truncates them at
+  `PCAP_ERRBUF_SIZE` without regard for character boundaries, so one quoting a long non-ASCII
+  path used to arrive as `Error::MalformedError` with the message thrown away. It now arrives as
+  `Error::PcapError`. Device and link-layer type names are still rejected when malformed.
 - `windows-sys` updated from 0.36 to 0.61. `HANDLE` is a raw pointer there rather than an `isize`,
   which changes the signature of `Capture::get_event` on Windows. A raw pointer is not `Send`, so
   a type of your own that stores the returned `HANDLE` no longer derives `Send` and can no longer

src/lib.rs

@@ -135,10 +135,19 @@ pub enum Error {
 
 impl Error {
     unsafe fn new(ptr: *const libc::c_char) -> Error {
-        match unsafe { cstr_to_string(ptr) } {
-            Err(e) => e as Error,
-            Ok(string) => PcapError(string.unwrap_or_default()),
+        if ptr.is_null() {
+            return PcapError(String::new());
         }
+
+        // libpcap truncates its messages at PCAP_ERRBUF_SIZE without regard for character
+        // boundaries, so one quoting a long path can end in the middle of a UTF-8 sequence.
+        // Take such a message lossily rather than lose it. Strings that are not error messages
+        // still go through cstr_to_string, which rejects the malformed ones.
+        PcapError(
+            unsafe { CStr::from_ptr(ptr as _) }
+                .to_string_lossy()
+                .into_owned(),
+        )
     }
 
     fn with_errbuf<T, F>(func: F) -> Result<T, Error>
@@ -252,7 +261,8 @@ mod tests {
     fn test_error_invalid_utf8() {
         let bytes: [u8; 8] = [0x78, 0xfe, 0xe9, 0x89, 0x00, 0x00, 0xed, 0x4f];
         let error = unsafe { Error::new(&bytes as *const _ as _) };
-        assert!(matches!(error, Error::MalformedError(_)));
+        // The message is kept, with the bytes that are not valid UTF-8 replaced.
+        assert_eq!(error, Error::PcapError("x\u{fffd}\u{fffd}".to_string()));
     }
 
     #[test]

src/linktype.rs

@@ -186,6 +186,13 @@ mod tests {
 
         let linktype_name = Linktype::ARCNET_LINUX.get_name().unwrap();
         assert_eq!(&linktype_name, name);
+
+        let ctx = raw::pcap_datalink_val_to_name_context();
+        ctx.checkpoint();
+        ctx.expect().return_once(|_| std::ptr::null());
+
+        let err = Linktype::ARCNET_LINUX.get_name().unwrap_err();
+        assert_eq!(err, Error::InvalidLinktype);
     }
 
     #[test]