tree aff23954e46997eab2d9c1bc48bc3407d1f63046
parent 04f44b94eaaf31c70dffb488eef961b4f5454bbf
author Oliver Newman <olivernewman@google.com> 1579055531 -0800
committer Oliver Newman <olivernewman@google.com> 1579110335 -0800

[fuchsia_cipd_roller] Handle concurrent updates during rolls

The fuchsia_cipd_roller breaks down when rolling multiple packages,
whose latest instances are expected to share a tag so that the packages
can all be pinned to the same source code version.

However, this breaks down when new versions of the packages are being
published at the same time as the roller is trying to find a common tag
to pin the packages to.

For example, consider an attempt to roll packages A, B, and C that are
published from the same source and should always have the same version
tag. The instances of A, B, and C with the "latest" ref all have the tag
"version:1"

Then suppose a concurrent (non-fuchsia) roller starts pushing new
versions of the packages with the tag "version:2". It pushes new
versions of packages A and B and updates the "latest" ref for those
packages to point to the "version:2" instances.

Then our roller tries to roll all three packages to the "latest"
versions. This will fail, because the "latest" versions do not share a
common version tag that we can pin them to (A and B have "version:2",
while C still has "version:1" on its "latest" instance).

We get around this by "backtracking" when we hit package C. We see that
its "latest" instance doesn't share a tag with the "latest" instances of
A and B. So we go back and query for instances of packages A and B that
share C's "version:1" tag, and find the "version:1" instances of A
and B. We then pin those, even though they don't have the "latest" refs,
because at least they give us a consistent version across all packages.

Bug: 42879
Change-Id: I8658ccf7dc8efeba311ca20893ab642eae93a00e