[Bug 1849368] Re: apt-get update issues invalid range requests for previously completed downloads

[Julian Andres Klode]

RFC 7233 allows clients to send range request without checking whether
the server supports them.

The reason being that servers generally, if they do not support ranges,
be ignoring any range values.

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https://bugs.launchpad.net/bugs/1849368

Title:
  apt-get update issues invalid range requests for previously completed
  downloads

Status in apt package in Ubuntu:
  New

Bug description:
  When an apt-get update fails, for instance due to a mismatch between
  the Release and Packages.gz due to a mirror sync/index update, it
  leaves files in the /var/lib/apt/lists/partial for the distribution:

  15466565     16 -rw-r--r--   1 root     root        13860 Oct 22 12:50 /var/lib/apt/lists/partial/ourhost_internal_repo_dists_main_Release
   15466663      4 -rw-r--r--   1 root     root          475 Oct 22 12:50 /var/lib/apt/lists/partial/ourhost_internal_repo_dists_main_Release.gpg
   15471150      0 -rw-r--r--   1 root     root            0 Oct 22 12:52 /var/lib/apt/lists/partial/ourhost_internal_repo_dists_main_main_binary-amd64_Packages.gz.FAILED

  In this case both Release and Release.gpg were complete, however when
  running apt-get update again it issues the revalidation requests for
  these resources with Range headers that are the length of the file
  resulting in '416 Requested Range Not Satisfiable':

  GET .../Release HTTP/1.1
  Cache-Control: max-age=0
  Accept: text/*
  Range: bytes=13860-
  If-Range: Tue, 22 Oct 2019 12:50:05 GMT
  User-Agent: Debian APT-HTTP/1.3 (1.6.12)

  HTTP/1.1 416 Requested Range Not Satisfiable

  It appears:

  - There's no way for apt-get to know if the file is complete so this logic will always be hit if the file exists https://github.com/Debian/apt/blob/master/methods/http.cc#L956-L962
  - The logic for determining whether a server supports range requests is only entered if an Accept-Range header exists, however an absence of this header means support is not available - https://github.com/Debian/apt/blob/master/methods/basehttp.cc#L251

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