zlib: clear unconsumed_tail when a bounded decompress reaches stream end (#8908)

* zlib: clear unconsumed_tail when a bounded decompress reaches stream end

When a max_length-bounded Decompress.decompress() call reached the end
of the stream, the bytes after the stream went to unused_data, but
unconsumed_tail kept the previous call's tail, including input the call
had already consumed. A later flush() then moved those stale bytes into
unused_data.

Empty unconsumed_tail at end of stream, as CPython's
save_unconsumed_input() does since gh-158169. CPython releases before
that change report the bytes after the stream in unconsumed_tail, so
the snippet accepts either value when it runs under CPython.

Fixes #8906

Assisted-by: Claude Code:claude-opus-5-5

* zlib: bind the unconsumed input with an if expression

Choose between the empty slice and the rest of the input in one
expression instead of reassigning a mutable binding.

Assisted-by: Claude Code:claude-opus-5-5

* zlib: accept both valid tails in the snippet on every implementation

The snippet no longer branches on sys.implementation. RustPython and
CPython after gh-158169 give an empty tail; CPython before it gives the
bytes after the stream. The stale tail of gh-8906 fails either way.

Assisted-by: Claude Code:claude-opus-5-5

crates/common/src/compression/zlib.rs

@@ -670,17 +670,16 @@ impl Decompressor {
         };
         let consumed = (d.total_in() - prev_in) as usize;
 
-        // save unused input
-        let unconsumed = &data[consumed..];
-        if !unconsumed.is_empty() {
-            if stream_end {
-                unused_data.extend_from_slice(unconsumed);
-            } else {
-                *unconsumed_tail = unconsumed.to_vec();
-            }
-        } else if !unconsumed_tail.is_empty() {
-            unconsumed_tail.clear();
-        }
+        // save unused input: past the end of the stream it is unused_data and
+        // unconsumed_tail is emptied, otherwise it is the new unconsumed_tail
+        let unconsumed: &[u8] = if stream_end {
+            unused_data.extend_from_slice(&data[consumed..]);
+            &[]
+        } else {
+            &data[consumed..]
+        };
+        unconsumed_tail.clear();
+        unconsumed_tail.extend_from_slice(unconsumed);
 
         Ok((ret, stream_end))
     }
@@ -1005,4 +1004,22 @@ mod tests {
         assert_eq!(d.decompress(&encoded, None).unwrap(), b"later input");
         assert!(d.eof());
     }
+
+    #[test]
+    fn stream_end_clears_unconsumed_tail() {
+        let source = b"x".repeat(20000);
+        let data = [compress(&source, -1, MAX_WBITS).unwrap(), b"NEXT".to_vec()].concat();
+        let mut d = Decompressor::new(MAX_WBITS, None).unwrap();
+        let first = d.decompress(&data, Some(100)).unwrap();
+        assert!(!d.eof());
+        let tail = d.unconsumed_tail().to_vec();
+        let rest = d.decompress(&tail, Some(100_000)).unwrap();
+        assert!(d.eof());
+        assert_eq!([first, rest].concat(), source);
+        assert_eq!(d.unconsumed_tail(), b"");
+        assert_eq!(d.unused_data(), b"NEXT");
+        assert_eq!(d.flush(DEF_BUF_SIZE).unwrap(), b"");
+        assert_eq!(d.unconsumed_tail(), b"");
+        assert_eq!(d.unused_data(), b"NEXT");
+    }
 }

extra_tests/snippets/stdlib_zlib.py

@@ -62,3 +62,20 @@ for text in compressed_lorem_list:
 
 assert_raises(zlib.error, lambda: zlib.compress(b"123", -40))
 assert_raises(zlib.error, lambda: zlib.compress(b"123", 10))
+
+# A max_length-bounded call that reaches the end of the stream keeps no
+# stale input in unconsumed_tail (gh-8906). The tail is empty, or, as in
+# CPython before gh-158169, the bytes after the stream.
+source = b"x" * 20000
+d = zlib.decompressobj()
+out = d.decompress(zlib.compress(source) + b"NEXT", 100)
+assert not d.eof
+out += d.decompress(d.unconsumed_tail, 100000)
+assert d.eof
+assert out == source
+assert d.unused_data == b"NEXT"
+tail = d.unconsumed_tail
+assert tail in (b"", b"NEXT")
+assert d.flush() == b""
+assert d.unconsumed_tail == tail
+assert d.unused_data == b"NEXT" + tail