[RFC datacenter-manager 0/4] add generic, per-remote (and global) cache for remote API responses

[Lukas Wagner <l.wagner@proxmox.com>]

The main intention is to avoid a sprawl of different caching approaches by
establishing a simple, easy to use cache implementation that can be used to
persistently cache API responses from remotes (and derived aggregations).

Open questions: 
  - is the per-namespace lock too coarse? Should we rather lock
    per key? Anyways, one should not hold the lock during longer periods of time
    (e.g. when doing API requests), so the namespace-level lock seemed fine to me.
    A per-namespace (so, per-remote) lock is nicer when one wants to update several
    keys in one go.

  - Base directory for cache, currently it is
    /var/cache/proxmox-datacenter-manager/cache
    But this seems both redundant and generic to me, so maybe
    'api-cache'?

  - Went with a max_age param on `get` instead of a `set` with an expiry time,
    I think it's quite common to have cache readers with different requirements
    to value freshness, so this might be a better fit.
    Also, we use the max-age mechanism in the API already, so this
    is a seamless fit then. Does this make sense? Or this we rather have
    redis-style `set` with expiry time/TTL?


The `namespaced_cache` module is pretty generic and can be moved to proxmox.git
(maybe in proxmox-shared-cache) once it has sufficiently stabilized.


proxmox-datacenter-manager:

Lukas Wagner (4):
  add persistent, generic, namespaced key-value cache implementation
  add pdm_cache cache as a specialized wrapper around the namespaced
    cache
  api: resources: subscriptions: switch over to pdm_cache
  remote-updates: switch over to pdm_cache

 Cargo.toml                                    |   1 +
 server/Cargo.toml                             |   2 +
 server/src/api/resources.rs                   |  82 ++---
 .../bin/proxmox-datacenter-privileged-api.rs  |   7 +
 server/src/lib.rs                             |   2 +
 server/src/namespaced_cache.rs                | 324 ++++++++++++++++++
 server/src/pdm_cache.rs                       |  69 ++++
 server/src/remote_updates.rs                  |  52 +--
 8 files changed, 452 insertions(+), 87 deletions(-)
 create mode 100644 server/src/namespaced_cache.rs
 create mode 100644 server/src/pdm_cache.rs


Summary over all repositories:
  8 files changed, 452 insertions(+), 87 deletions(-)

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