Resume incomplete download

src/pip/_internal/network/download.py

@@ -26,9 +26,36 @@ def _get_http_response_size(resp: Response) -> Optional[int]:
     except (ValueError, KeyError, TypeError):
         return None
 
+def _get_http_accept_ranges(resp: Response) -> Optional[str]:
+    return resp.headers.get("accept-ranges", None)
+
+def _resume_if_incomplete(
+    resp: Response,
+    session: PipSession,
+    link: Link
+) -> Iterable[bytes]:
+    chunks = response_chunks(resp, CONTENT_CHUNK_SIZE)
+    received_length = 0
+
+    for chunk in chunks:
+        received_length += len(chunk)
+        yield chunk
+
+    total_length = _get_http_response_size(resp)
+    accept_ranges = _get_http_accept_ranges(resp)
+
+    if total_length is not None and accept_ranges == 'bytes':
+        while received_length < total_length:
+            logger.info("Resuming incomplete download (%s received)", format_size(received_length))
+            resume_resp = _http_get_download(session, link, range_start=received_length)
+            resume_chunks = response_chunks(resume_resp, CONTENT_CHUNK_SIZE)
+            for chunk in resume_chunks:
+                received_length += len(chunk)
+                yield chunk
 
 def _prepare_download(
     resp: Response,
+    session: PipSession,
     link: Link,
     progress_bar: str,
 ) -> Iterable[bytes]:
@@ -60,7 +87,7 @@ def _prepare_download(
     else:
         show_progress = False
 
-    chunks = response_chunks(resp, CONTENT_CHUNK_SIZE)
+    chunks = _resume_if_incomplete(resp, session, link)
 
     if not show_progress:
         return chunks
@@ -112,9 +139,13 @@ def _get_http_response_filename(resp: Response, link: Link) -> str:
     return filename
 
 
-def _http_get_download(session: PipSession, link: Link) -> Response:
+def _http_get_download(session: PipSession, link: Link, range_start: Optional[int] = None) -> Response:
     target_url = link.url.split("#", 1)[0]
-    resp = session.get(target_url, headers=HEADERS, stream=True)
+    if range_start is not None:
+        headers = {**HEADERS, "Range": "bytes={}-".format(range_start)}
+    else:
+        headers = HEADERS
+    resp = session.get(target_url, headers=headers, stream=True)
     raise_for_status(resp)
     return resp
 
@@ -142,7 +173,7 @@ def __call__(self, link: Link, location: str) -> Tuple[str, str]:
         filename = _get_http_response_filename(resp, link)
         filepath = os.path.join(location, filename)
 
-        chunks = _prepare_download(resp, link, self._progress_bar)
+        chunks = _prepare_download(resp, self._session, link, self._progress_bar)
         with open(filepath, "wb") as content_file:
             for chunk in chunks:
                 content_file.write(chunk)
@@ -178,7 +209,7 @@ def __call__(
             filename = _get_http_response_filename(resp, link)
             filepath = os.path.join(location, filename)
 
-            chunks = _prepare_download(resp, link, self._progress_bar)
+            chunks = _prepare_download(resp, self._session, link, self._progress_bar)
             with open(filepath, "wb") as content_file:
                 for chunk in chunks:
                     content_file.write(chunk)