feat(write-flash): apply compression only if it reduces the file size

Author: jedi7

esptool/cmds.py

@@ -814,11 +814,24 @@ def write_flash(
                 "Security features enabled, so not changing any flash settings."
             )
         calcmd5 = hashlib.md5(image).hexdigest()
-        uncsize = len(image)
+        uncsize = image_size = len(image)
         if compress:
-            uncimage = image
-            image = zlib.compress(uncimage, 9)
-            compsize = len(image)
+            compressed_image = zlib.compress(image, 9)
+            compsize = len(compressed_image)
+            # Only use compression if it actually reduces the file size
+            if compsize < uncsize:
+                image = compressed_image
+                image_size = compsize
+            else:
+                # Compression didn't help, disable it for this file
+                source = "input image" if name is None else f"file '{name}'"
+                log.note(
+                    f"Cannot compress {source} more than the original size, "
+                    "will flash uncompressed. "
+                    f"Compressed size {compsize} bytes >= uncompressed {uncsize} bytes."
+                )
+                compress = False
+
         original_image = image  # Save the whole image in case retry is needed
         # Try again if reconnect was successful
         log.stage()
@@ -829,7 +842,7 @@ def write_flash(
                     # to dynamically calculate the timeout based on the real write size
                     decompress = zlib.decompressobj()
                     esp.flash_defl_begin(
-                        uncsize, compsize, address, encrypted_write=encrypted
+                        uncsize, image_size, address, encrypted_write=encrypted
                     )
                 else:
                     esp.flash_begin(uncsize, address, encrypted_write=encrypted)
@@ -839,7 +852,6 @@ def write_flash(
                 t = time.time()
 
                 timeout = DEFAULT_TIMEOUT
-                image_size = compsize if compress else uncsize
                 while len(image) >= 0:
                     if not no_progress:
                         log.progress_bar(

test/test_esptool.py

@@ -749,6 +749,36 @@ def test_compressible_non_trivial_file(self):
         finally:
             os.unlink(input_file.name)
 
+    def test_compression_auto_skipped_when_no_size_reduction(self):
+        """
+        Test that compression is automatically skipped when it doesn't reduce file size.
+        Uses truly random data which is hard to compress.
+        """
+        try:
+            input_file = tempfile.NamedTemporaryFile(delete=False)
+            file_size = 64 * 1024  # 64KB of random data
+            # Write truly random data that won't compress well
+            for _ in range(file_size):
+                input_file.write(struct.pack("B", random.randrange(0, 256)))
+            input_file.close()
+
+            output = self.run_esptool(
+                f"write-flash --compress 0x20000 {input_file.name}"
+            )
+
+            # Verify that compression was skipped by checking the output message
+            # When compression is used: "Wrote X bytes (Y compressed)"
+            # When compression is skipped: "Wrote X bytes" (no "compressed")
+            assert "Wrote" in output and "bytes" in output, (
+                "Should have 'Wrote X bytes' message"
+            )
+            assert " compressed)" not in output, (
+                f"Compression should have been skipped for random data, "
+                f"but output shows compressed format: {output}"
+            )
+        finally:
+            os.unlink(input_file.name)
+
     @pytest.mark.quick_test
     def test_zero_length(self):
         # Zero length files are skipped with a warning