Fix last_ext always returning empty data

The XMP patch that landed recently did a `std::mem::take` on the extension
data - this broke the `last_ext` API.

Co-authored-by: Rasmus Piorr <piorr@google.com>

Author: 1c3t3a

src/reader/decoder.rs

@@ -773,28 +773,25 @@ impl StreamingDecoder {
             }
             ApplicationExtension => {
                 debug_assert_eq!(0, b);
-                // We can consume the extension data here as the next states won't access it anymore.
-                let mut data = std::mem::take(&mut self.ext.data);
 
                 // the parser removes sub-block lenghts, so app name and data are concatenated
-                if let Some(&[first, second, ..]) = data.strip_prefix(EXT_NAME_NETSCAPE) {
+                if let Some(&[first, second, ..]) = self.ext.data.strip_prefix(EXT_NAME_NETSCAPE) {
                     let repeat = u16::from(first) | u16::from(second) << 8;
                     goto!(BlockEnd, emit Decoded::Repetitions(if repeat == 0 { Repeat::Infinite } else { Repeat::Finite(repeat) }))
-                } else if data.starts_with(EXT_NAME_XMP) {
-                    data.drain(..EXT_NAME_XMP.len());
+                } else if let Some(mut rest) = self.ext.data.strip_prefix(EXT_NAME_XMP) {
                     // XMP adds a "ramp" of 257 bytes to the end of the metadata to let the "pascal-strings"
                     // parser converge to the null byte. The ramp looks like "0x01, 0xff, .., 0x01, 0x00".
                     // For convenience and to allow consumers to not be bothered with this implementation detail,
                     // we cut the ramp.
                     const RAMP_SIZE: usize = 257;
-                    if data.len() >= RAMP_SIZE
-                        && data.ends_with(&[0x03, 0x02, 0x01, 0x00])
-                        && data[data.len() - RAMP_SIZE..].starts_with(&[0x01, 0x0ff])
+                    if rest.len() >= RAMP_SIZE
+                        && rest.ends_with(&[0x03, 0x02, 0x01, 0x00])
+                        && rest[rest.len() - RAMP_SIZE..].starts_with(&[0x01, 0x0ff])
                     {
-                        data.truncate(data.len() - RAMP_SIZE);
+                        rest = &rest[..(rest.len() - RAMP_SIZE)];
                     }
 
-                    self.xmp_metadata = Some(data);
+                    self.xmp_metadata = Some(rest.to_vec());
                     goto!(BlockEnd)
                 } else {
                     goto!(BlockEnd)

tests/decode.rs

@@ -1,7 +1,10 @@
 #![cfg(feature = "std")]
 
-use gif::{DecodeOptions, Decoder, DisposalMethod, Encoder, Frame};
-use std::fs::File;
+use gif::{
+    streaming_decoder::{Decoded, OutputBuffer, StreamingDecoder},
+    DecodeOptions, Decoder, DisposalMethod, Encoder, Frame,
+};
+use std::{fs::File, io::BufRead};
 
 #[test]
 fn test_simple_indexed() {
@@ -269,3 +272,29 @@ fn xmp_metadata() {
 
     assert_eq!(decoder.xmp_metadata(), Some(EXPECTED_METADATA.as_bytes()))
 }
+
+#[test]
+fn check_last_extension_returns() {
+    let mut buf: &[u8] = include_bytes!("samples/beacon_xmp.gif");
+
+    let mut out_buf = Vec::new();
+    let mut output = OutputBuffer::Vec(&mut out_buf);
+
+    let mut decoder = StreamingDecoder::new();
+
+    loop {
+        let (consumed, result) = {
+            if buf.is_empty() {
+                break;
+            }
+
+            decoder.update(&mut buf, &mut output).unwrap()
+        };
+        buf.consume(consumed);
+        if let Decoded::HeaderEnd = result {
+            break;
+        }
+    }
+
+    assert_eq!(decoder.last_ext().1.len(), 3129);
+}