Fix bug where wrong data is emitted for gzip fixed blocks

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fix a bug where the gzip compressor may output incorrect data when emitting
+  "fixed blocks" (which are emitted when compressing high-entropy data).
+
 ## [0.5.0] - 2024-06-04
 
 ### Added

lib/src/gzip.rs

@@ -1022,16 +1022,34 @@ pub fn compress(bytes: &[u8], level: usize, small_mem: bool) -> Result<Box<[u8]>
             pos += prev_match_len - 1;
             lookahead -= prev_match_len - 1;
 
+            if should_flush {
+                if pos >= block_length {
+                    writer.flush_block(&mut output, Some(&window[pos - block_length..pos]), false);
+                } else {
+                    writer.flush_block(&mut output, None, false);
+                }
+                block_length = 0;
+            }
+
             has_prev_char = false;
             prev_match_len = MIN_MATCH - 1;
             prev_match_dist = 0;
         } else {
-            // Remember current match and emit previous character as literal if it exists
+            // Emit previous character as literal (if it exists) and remember current match
             if has_prev_char {
                 writer.add_literal(window[pos - 1]);
                 should_flush = writer.should_flush_block(block_length);
             }
 
+            if should_flush {
+                if pos >= block_length {
+                    writer.flush_block(&mut output, Some(&window[pos - block_length..pos]), false);
+                } else {
+                    writer.flush_block(&mut output, None, false);
+                }
+                block_length = 0;
+            }
+
             block_length += 1;
             pos += 1;
             lookahead -= 1;
@@ -1041,15 +1059,6 @@ pub fn compress(bytes: &[u8], level: usize, small_mem: bool) -> Result<Box<[u8]>
             prev_match_dist = pos - 1 - best_pos;
         }
 
-        if should_flush {
-            if pos >= block_length {
-                writer.flush_block(&mut output, Some(&window[pos - block_length..pos]), false);
-            } else {
-                writer.flush_block(&mut output, None, false);
-            }
-            block_length = 0;
-        }
-
         // Refill window
         if lookahead < MIN_LOOKAHEAD && !eof && pos >= WINDOW_SIZE + MAX_DIST {
             window.copy_within(WINDOW_SIZE..2 * WINDOW_SIZE, 0);