Coalesce concurrent access token requests

[dlon]

Currently, multiple concurrent API calls will all send an auth request for the current account, if there is no valid access token. This PR fixes this by waiting for the in-flight request to complete if there already is one.

Fix DES-311.

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This change is 

[linear]

DES-311 Bashing auth endpoints

Gregoire said:

Hej app-team!
We're getting reports from users being throttled on the app. Support posted a log that looks like this:

[2023-07-10 02:28:21.743][mullvad_api::access][DEBUG] Replacing expired access token
[2023-07-10 02:28:21.744][mullvad_api::access][DEBUG] Fetching access token for an account
[2023-07-10 02:28:21.745][mullvad_api::access][DEBUG] Replacing expired access token
[2023-07-10 02:28:21.745][mullvad_api::access][DEBUG] Fetching access token for an account
[2023-07-10 02:28:22.751][mullvad_api::access][DEBUG] Replacing expired access token
[2023-07-10 02:28:22.751][mullvad_api::access][DEBUG] Fetching access token for an account
[2023-07-10 02:28:23.148][mullvad_api::rest][ERROR] Unexpected HTTP status code 429 Too Many Requests, expected codes [200 OK]
[2023-07-10 02:28:23.148][mullvad_api::access][ERROR] Error: Failed to obtain access token
Caused by: Unexpected response status code 429 Too Many Requests - THROTTLED
[2023-07-10 02:28:23.149][mullvad_jni][ERROR] Error: Failed to get account data
Caused by: Error performing RPC with the remote API
Caused by: Unexpected response status code 429 Too Many Requests - THROTTLED

This seems to indicate that the app is sending three requests to /auth/v1/token within a second. Could that be a bug? (Concurrency issue perhaps?)

When logging in, the desktop app will retry 3 times in quick succession to get the account token. This makes it fail. There are two issues to investigate:

• maybe bump RETRY_ACTION_INTERVAL from Duration::ZERO to 1 or 3 seconds.
• why is the app seemingly failing to fetch the auth token in quick succession? The account seemingly is valid.

bashing-auth-99c2cac5-fbf3-48e5-aeec-081c3e769ee7.log

bashing-auth-d24c2bf0-fb5e-45b9-bb2b-84d28f9dcb9e.log

bashing-auth-19FE9F66-F8F8-4BC0-A23F-C33C75DB814E.log

bashing-auth-1a16003a-b1ff-46b5-9964-81c5bf5944c2.log

[MarkusPettersson98]

Reviewed 13 of 13 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

[MarkusPettersson98]

🔥

Reviewed 15 of 15 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved