download_cef: add HTTP Range resume support and increase retries

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: linesight

tools/download_cef.py

@@ -37,6 +37,7 @@
 import hashlib
 import json
 import os
+import time
 import sys
 import tarfile
 import zipfile
@@ -195,41 +196,96 @@ def find_in_index(cef_version, cef_postfix2):
     sys.exit(1)
 
 
-def download(url, dest_path, expected_size=0):
-    """Download url to dest_path with a progress bar."""
+def download(url, dest_path, expected_size=0, max_retries=50):
+    """Download url to dest_path with a progress bar and resume support."""
     try:
-        from urllib.request import urlopen
+        from urllib.request import Request, urlopen
     except ImportError:
-        from urllib2 import urlopen
+        from urllib2 import Request, urlopen
 
     log("Downloading: {}".format(url))
-    try:
-        response = urlopen(url, timeout=120)
-    except Exception as exc:
-        log("ERROR: Download failed: {}".format(exc))
-        sys.exit(1)
 
-    total = int(response.headers.get("Content-Length") or expected_size or 0)
+    total = expected_size
     downloaded = 0
     chunk_size = 1024 * 1024  # 1 MB
 
-    try:
-        with open(dest_path, "wb") as fp:
-            while True:
-                chunk = response.read(chunk_size)
-                if not chunk:
-                    break
-                fp.write(chunk)
-                downloaded += len(chunk)
-                _print_progress(downloaded, total)
-    except Exception as exc:
-        if os.path.isfile(dest_path):
-            os.remove(dest_path)
-        log("\nERROR: Download failed: {}".format(exc))
-        sys.exit(1)
+    # Resume from an existing partial file if present
+    if os.path.isfile(dest_path):
+        downloaded = os.path.getsize(dest_path)
+        if total and downloaded >= total:
+            log("Already fully downloaded: {}".format(os.path.basename(dest_path)))
+            return
+        if downloaded > 0:
+            log("Resuming from {:.1f} MB...".format(downloaded / (1024 * 1024)))
+
+    for attempt in range(1, max_retries + 1):
+        if attempt > 1:
+            # Re-check how much we have on disk after a failed attempt
+            if os.path.isfile(dest_path):
+                downloaded = os.path.getsize(dest_path)
+            log("Retry {}/{} (resuming from {:.1f} MB): {}".format(
+                attempt, max_retries, downloaded / (1024 * 1024), url))
+
+        req = Request(url)
+        if downloaded > 0:
+            req.add_header("Range", "bytes={}-".format(downloaded))
+
+        try:
+            response = urlopen(req, timeout=120)
+        except Exception as exc:
+            if attempt == max_retries:
+                log("ERROR: Download failed: {}".format(exc))
+                sys.exit(1)
+            log("WARNING: Connection error (attempt {}): {}".format(attempt, exc))
+            time.sleep(2)
+            continue
 
-    print()  # end progress line
-    log("Saved: {}".format(os.path.basename(dest_path)))
+        # Determine total size from response
+        content_range = response.headers.get("Content-Range")
+        if content_range:
+            # e.g. "bytes 100-200/614600000"
+            try:
+                total = int(content_range.rsplit("/", 1)[-1])
+            except (ValueError, IndexError):
+                pass
+        else:
+            total = int(response.headers.get("Content-Length") or total or 0)
+            if downloaded > 0:
+                # Server didn't honour Range; start over
+                log("WARNING: Server ignored Range header, restarting download.")
+                downloaded = 0
+
+        error = None
+
+        try:
+            mode = "ab" if downloaded > 0 else "wb"
+            with open(dest_path, mode) as fp:
+                while True:
+                    chunk = response.read(chunk_size)
+                    if not chunk:
+                        break
+                    fp.write(chunk)
+                    downloaded += len(chunk)
+                    _print_progress(downloaded, total)
+        except Exception as exc:
+            error = exc
+
+        print()  # end progress line
+
+        if error is None and total and downloaded < total:
+            error = "short read: got {:.1f} MB of {:.1f} MB".format(
+                downloaded / (1024 * 1024), total / (1024 * 1024))
+
+        if error is not None:
+            if attempt == max_retries:
+                log("ERROR: Download failed: {}".format(error))
+                sys.exit(1)
+            log("WARNING: Download incomplete (attempt {}): {}".format(attempt, error))
+            time.sleep(2)
+            continue
+
+        log("Saved: {}".format(os.path.basename(dest_path)))
+        return
 
 
 def _print_progress(downloaded, total):