Fix HTTP resume (#84)

* Fix HTTP resume

* Handle error 416 when resuming complete file

* Line break for comment

* Fix logic

* ensure err.output is digits

* amend comment

* Support retry on error 412

installinstallmacos.py

@@ -264,23 +264,40 @@ def replicate_url(full_url,
         options = '-fL'
     else:
         options = '-sfL'
-    curl_cmd = ['/usr/bin/curl', options,
-                '--create-dirs',
-                '-o', local_file_path]
-    if not full_url.endswith(".gz"):
-        # stupid hack for stupid Apple behavior where it sometimes returns
-        # compressed files even when not asked for
-        curl_cmd.append('--compressed')
-    if not ignore_cache and os.path.exists(local_file_path):
-        curl_cmd.extend(['-z', local_file_path])
-        if attempt_resume:
-            curl_cmd.extend(['-C', '-'])
-    curl_cmd.append(full_url)
-    print("Downloading %s..." % full_url)
-    try:
-        subprocess.check_call(curl_cmd)
-    except subprocess.CalledProcessError as err:
-        raise ReplicationError(err)
+    need_download = True
+    while need_download:
+        curl_cmd = ['/usr/bin/curl', options,
+                    '--create-dirs',
+                    '-o', local_file_path,
+                    '-w', '%{http_code}']
+        if not full_url.endswith(".gz"):
+            # stupid hack for stupid Apple behavior where it sometimes returns
+            # compressed files even when not asked for
+            curl_cmd.append('--compressed')
+        resumed = False
+        if not ignore_cache and os.path.exists(local_file_path):
+            if not attempt_resume:
+                curl_cmd.extend(['-z', local_file_path])
+            else:
+                resumed = True
+                curl_cmd.extend(['-z', '-' + local_file_path, '-C', '-'])
+        curl_cmd.append(full_url)
+        print("Downloading %s..." % full_url)
+        need_download = False
+        try:
+            output = subprocess.check_output(curl_cmd)
+        except subprocess.CalledProcessError as err:
+            if not resumed or not err.output.isdigit():
+                raise ReplicationError(err)
+            # HTTP error 416 on resume: the download is already complete and the
+            # file is up-to-date
+            # HTTP error 412 on resume: the file was updated server-side
+            if int(err.output) == 412:
+                print("Removing %s and retrying." % local_file_path)
+                os.unlink(local_file_path)
+                need_download = True
+            elif int(err.output) != 416:
+                raise ReplicationError(err)
     return local_file_path