Backport of connect: rewrite envoy bootstrap on every restart into release/1.6.x #19811
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Backport
This PR is auto-generated from #19787 to be assessed for backporting due to the inclusion of the label backport/1.6.x.
The below text is copied from the body of the original PR.
Fixes #19781
Do not mark the envoy bootstrap hook as done after successfully running once. Since the bootstrap file is written to /secrets, which is a tmpfs on supported platforms, it is not persisted across reboots. This causes the task and allocation to fail on reboot (see #19781).
This fixes it by always rewriting the envoy bootstrap file every time the Nomad agent starts. This does mean we may write a new bootstrap file to an already running Envoy task, but in my testing that doesn't have any impact.
Alternative 1: Use a regular file
An alternative approach would be to write the bootstrap file somewhere other than the tmpfs, but this is unsafe as when Consul ACLs are enabled the file will contain a secret token:
https://developer.hashicorp.com/consul/commands/connect/envoy#bootstrap
Alternative 2: Detect if file is already written
An alternative approach would be to detect if the bootstrap file exists, and only write it if it doesn't.
This is just a more complicated form of the current fix. I think in general in the absence of other factors task hooks should be idempotent and therefore able to rerun on any agent startup. This simplifies the code and our ability to reason about task restarts vs agent restarts vs node reboots by making them all take the same code path.
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