ENH: respect/log separately Retry-After + support DANDI_DEVEL_AGGRESS…

…IVE_RETRY mode

This is all to address that odd case with 000026 where connection keeps interrupting.
Unclear why so adding more specific cases handling and allowing for such an aggressive
retrying where we would proceed as long as we are getting something (but sleep would also increase)

dandi/download.py

@@ -693,14 +693,18 @@ def _download_file(
     # TODO: how do we discover the total size????
     # TODO: do not do it in-place, but rather into some "hidden" file
     resuming = False
-    for attempt in range(3):
+    attempt = 0
+    nattempts = 3  # number to do, could be incremented if we downloaded a little
+    while attempt <= nattempts:
+        attempt += 1
         try:
             if digester:
                 downloaded_digest = digester()  # start empty
             warned = False
             # I wonder if we could make writing async with downloader
             with DownloadDirectory(path, digests or {}) as dldir:
                 assert dldir.offset is not None
+                downloaded_in_attempt = 0
                 downloaded = dldir.offset
                 resuming = downloaded > 0
                 if size is not None and downloaded == size:
@@ -719,6 +723,7 @@ def _download_file(
                         assert downloaded_digest is not None
                         downloaded_digest.update(block)
                     downloaded += len(block)
+                    downloaded_in_attempt += len(block)
                     # TODO: yield progress etc
                     out: dict[str, Any] = {"done": downloaded}
                     if size:
@@ -744,30 +749,83 @@ def _download_file(
         # Catching RequestException lets us retry on timeout & connection
         # errors (among others) in addition to HTTP status errors.
         except requests.RequestException as exc:
+            sleep_amount = random.random() * 5 * attempt
+            if os.environ.get("DANDI_DEVEL_AGGRESSIVE_RETRY"):
+                # in such a case if we downloaded a little more --
+                # consider it a successful attempt
+                if downloaded_in_attempt > 0:
+                    lgr.debug(
+                        "%s - download failed on attempt #%d: %s, "
+                        "but did download %d bytes, so considering "
+                        "it a success and incrementing number of allowed attempts.",
+                        path,
+                        attempt,
+                        exc,
+                        downloaded_in_attempt,
+                    )
+                    nattempts += 1
             # TODO: actually we should probably retry only on selected codes,
-            # and also respect Retry-After
-            if attempt >= 2 or (
-                exc.response is not None
-                and exc.response.status_code
-                not in (
+            if exc.response is not None:
+                if exc.response.status_code not in (
                     400,  # Bad Request, but happened with gider:
                     # https://github.com/dandi/dandi-cli/issues/87
                     *RETRY_STATUSES,
+                ):
+                    lgr.debug(
+                        "%s - download failed due to response %d: %s",
+                        path,
+                        exc.response.status_code,
+                        exc,
+                    )
+                    yield {"status": "error", "message": str(exc)}
+                    return
+                elif retry_after := exc.response.headers.get("Retry-After"):
+                    # playing safe
+                    if not str(retry_after).isdigit():
+                        # our code is wrong, do not crash but issue warning so
+                        # we might get report/fix it up
+                        lgr.warning(
+                            "%s - download failed due to response %d with non-integer"
+                            " Retry-After=%r: %s",
+                            path,
+                            exc.response.status_code,
+                            retry_after,
+                            exc,
+                        )
+                        yield {"status": "error", "message": str(exc)}
+                        return
+                    sleep_amount = int(retry_after)
+                    lgr.debug(
+                        "%s - download failed due to response %d with "
+                        "Retry-After=%d: %s, will sleep and retry",
+                        path,
+                        exc.response.status_code,
+                        sleep_amount,
+                        exc,
+                    )
+                else:
+                    lgr.debug("%s - download failed: %s", path, exc)
+                    yield {"status": "error", "message": str(exc)}
+                    return
+            elif attempt >= nattempts:
+                lgr.debug(
+                    "%s - download failed after %d attempts: %s", path, attempt, exc
                 )
-            ):
-                lgr.debug("%s - download failed: %s", path, exc)
                 yield {"status": "error", "message": str(exc)}
                 return
             # if is_access_denied(exc) or attempt >= 2:
             #     raise
             # sleep a little and retry
-            lgr.debug(
-                "%s - failed to download on attempt #%d: %s, will sleep a bit and retry",
-                path,
-                attempt,
-                exc,
-            )
-            time.sleep(random.random() * 5)
+            else:
+                lgr.debug(
+                    "%s - download failed on attempt #%d: %s, will sleep a bit and retry",
+                    path,
+                    attempt,
+                    exc,
+                )
+            time.sleep(sleep_amount)
+    else:
+        lgr.warning("downloader logic: We should not be here!")
 
     if downloaded_digest and not resuming:
         assert downloaded_digest is not None