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80522a1b9d
Control Plane currently [^1] polls for `has_in_progress_downloads ==
false` after /attach to determine that an attach operation succeeded.
As pointed out in the OpenAPI spec as of neon#4151, polling for
`has_in_progress_downloads` is incorrect.
This patch changes the situation by
- removing `has_in_progress_downloads`
- adding a new field `attachment_status.`
- changing instructions for `/attach` to poll for `attachment_status ==
attached`.
This makes the instructions in `/attach` actionable for Control Plane.
NB that we don't expose the TenantState in the OpenAPI docs, even though
we expose it in the endpoint. That is with good reason because we don't
want to commit to a fixed set of tenant states forever. Hence, the
separate `attachment_status` field that exposes the bare minimum
required to make /attach + subsequent polling 100% safe wrt split brain.
It would have been nice to report failures explicitly, but the problem
is that we lose that state when we restart. So, we return `attached`
upon attach failure. The tenant is Broken in that case, causing Control
Plane's subsequent health check will fail. Control Plane can roll back
the relocation operation then.
NB: the reliance on the subsequent health check is no change to what we
had before this patch!
NB: we can always add additional TenantAttachmentStatus'es in the future
to communicate failure.
This PR also moves the attach-marker file's creation to the API
handler's synchronous part. That was done to avoid the need to
distinguish
* `Attaching but marker not yet written => AttachmentStatus::Maybe` from
* `Attaching, marker written, but attach failed for other reason =>
AttachmentStatus::Attached`
Coincidentally, this also adds more transactionality to the /attach API
because we only return 202 once we've written the marker file. But, in
the end, it doesn't affect how the control plane interacts with us or
how it needs to do retries. So, we don't mention any of this in the API
docs.
[^1]: The one-click tenant relocation PR cloud#4740, currently WIP, is
the first real user.