Description

When running Vault via systemd and PrivateTmp=on, the Vault register command fails, even when setting the VAULT_PLUGIN_TMPDIR environment variable:

ubuntu@ip-172-25-19-239:~$ vault plugin register -sha256="${SHA256}" -oci_image=hashicorp/vault-plugin-secrets-kv secret my-kv-container
WARNING! VAULT_ADDR and -address unset. Defaulting to https://127.0.0.1:8200/.
Error registering plugin my-kv-container: Error making API request.

URL: PUT https://127.0.0.1:8200/v1/sys/plugins/catalog/secret/my-kv-container
Code: 400. Errors:

* unable to run plugin: 1 error occurred:
        * error creating container: Error response from daemon: invalid mount config for type "bind": bind source path does not exist: /tmp/plugin-dir3781501367
        *

This change makes it so that the register flow respects the tmpdir, if set.

With this change, the output looks like:

vault plugin register -sha256="${SHA256}" -oci_image=hashicorp/vault-plugin-secrets-kv  secret my-kv-container
WARNING! VAULT_ADDR and -address unset. Defaulting to https://127.0.0.1:8200.
Success! Registered plugin: my-kv-container

TODO only if you're a HashiCorp employee

• Backport Labels: If this fix needs to be backported, use the appropriate backport/ label that matches the desired release branch. Note that in the CE repo, the latest release branch will look like backport/x.x.x, but older release branches will be backport/ent/x.x.x+ent.
  • LTS: If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
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