[Bug 1794957] Re: pipelining on archive.u.c aborts after 101 packages

[Julian Andres Klode]

Same for xenial; 1.2.27 shows the symptoms, and the 1.2.29 SRU fixes
them. Not going to attach more images or text as it basically looks
identical.

** Tags removed: verification-needed verification-needed-bionic verification-needed-xenial
** Tags added: verification-done verification-done-bionic verification-done-xenial

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Title:
  pipelining on archive.u.c aborts after 101 packages

Status in apt package in Ubuntu:
  Fix Released
Status in apt source package in Xenial:
  Fix Committed
Status in apt source package in Bionic:
  Fix Committed

Bug description:
  [Impact]
  Downloading many packages on archive.ubuntu.com or some other mirrors seems to close the connection after every 100 or so packages. APT prior to 1.7.0~rc1 (commit df696650b7a8c58bbd92e0e1619e956f21010a96), treats a connection closure with a 200 response as meaning that the server does not support pipelining, hence disabling it for any further downloads.

  With high speed connections at higher latency, this can cause a severe
  reduction in usable bandwidth. For example, I saw speeds drop from 40
  MB/s to 15 MB/s due to this.

  The fix ensures that we continue pipelining if the previous connection
  to the server successfully retrieved at least 3 files with pipelining
  enabled.

  [Test case]
  Pick a package that would cause a large number (200/300 packages of packages to be installed). I used plasma-desktop and xubuntu-desktop, for example. Run apt install -d $package. Ensure that after the first 101 packages the progress does not slow down - you should not see a lot "working" in the progress output. The speed should be substantially higher.

  Don't run in a container setup by cloud-init, as cloud-init disables
  pipelining; or remove /etc/apt/apt.conf.d/90cloud-init-pipelining (see
  bug 1794982).

  Requirements:
  * High speed, medium-high latency connection (e.g. 400 Mbit/s at 30 ms RTT is enough); or just increase latency, e.g. 
     sudo tc qdisc add dev wlp61s0 root netem delay 300ms
   until you see the slow down
  * Not a terribly slow CPU, as we'd get slowed down by hashing otherwise

  [Regression potential]
  This fix is isolated to code enabling/disabling pipelining on subsequent connections. It could cause more pipelining to be tried on servers who are not particularly good at it, but can deal with 3 items correctly.

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