Fix Packet::write* rejecting zero-sized packets that carry side data

src/codec/packet/packet.rs

@@ -17,6 +17,11 @@ impl Packet {
     pub unsafe fn is_empty(&self) -> bool {
         self.0.size == 0
     }
+
+    #[inline(always)]
+    fn lacks_payload_and_side_data(&self) -> bool {
+        self.0.size == 0 && self.0.side_data_elems == 0
+    }
 }
 
 impl Packet {
@@ -226,7 +231,9 @@ impl Packet {
     #[inline]
     pub fn write(&self, format: &mut format::context::Output) -> Result<bool, Error> {
         unsafe {
-            if self.is_empty() {
+            // FFmpeg allows zero-size packets that still carry side data, such as
+            // FLAC encoder flush packets with AV_PKT_DATA_NEW_EXTRADATA.
+            if self.lacks_payload_and_side_data() {
                 return Err(Error::InvalidData);
             }
 
@@ -241,7 +248,9 @@ impl Packet {
     #[inline]
     pub fn write_interleaved(&self, format: &mut format::context::Output) -> Result<(), Error> {
         unsafe {
-            if self.is_empty() {
+            // Keep wrapper behavior aligned with av_interleaved_write_frame():
+            // zero-size packets are still valid when they only carry side data.
+            if self.lacks_payload_and_side_data() {
                 return Err(Error::InvalidData);
             }
 
@@ -338,4 +347,26 @@ impl<'a> Iterator for SideDataIter<'a> {
     }
 }
 
+#[cfg(test)]
+mod tests {
+    use super::Packet;
+
+    #[test]
+    fn empty_packet_without_side_data_is_still_empty_for_write_checks() {
+        let packet = Packet::empty();
+
+        assert!(unsafe { packet.is_empty() });
+        assert!(packet.lacks_payload_and_side_data());
+    }
+
+    #[test]
+    fn zero_sized_packet_with_side_data_is_not_treated_as_invalid_empty_packet() {
+        let mut packet = Packet::empty();
+        packet.0.side_data_elems = 1;
+
+        assert!(unsafe { packet.is_empty() });
+        assert!(!packet.lacks_payload_and_side_data());
+    }
+}
+
 impl<'a> ExactSizeIterator for SideDataIter<'a> {}