which was [implemented slightly differently](https://cs.android.com/android/platform/superproject/+/master:frameworks/base/services/core/java/com/android/server/content/ContentService.java?q=%22int%20getSyncExemptionAndCleanUpExtrasForCaller%22&ss=android%2Fplatform%2Fsuperproject)
in Android 9, compared to Android 10 and later.
The logic is as follows:
- When the caller's procstate is `PROCESS_STATE_TOP` or above,
meaning if the caller has a foreground activity,
`SYNC_EXEMPTION_PROMOTE_BUCKET_WITH_TEMP` will be set.
- Otherwise, when the caller's procstate is `PROCESS_STATE_IMPORTANT_FOREGROUND` or above,
e.g. when the caller has a foreground service, a service bound by the system of a specific kind,
`SYNC_EXEMPTION_PROMOTE_BUCKET` will be set.
- Additionally, on Android 10 and later, when the caller is
"UID-active" (but the procstate is below `PROCESS_STATE_TOP`),
`SYNC_EXEMPTION_PROMOTE_BUCKET` will be set.
This is what happens when the app has just received a high-priority FCM, for example.
Temp-allowlist is also used in various other situations.
### Behavior of Each Exemption
The exemptions are tracked in `SyncOperation.syncExemptionFlag`.
- Behavior of `SYNC_EXEMPTION_PROMOTE_BUCKET`
- This will add `JobInfo.FLAG_EXEMPT_FROM_APP_STANDBY` to the sync job. This makes the job
subject to "ACTIVE" app quota, so minimum deferral will be applied to it.
- This also reports `AppStandbyController.reportExemptedSyncStart()`, so the package that owns
the sync adapter is temporarily put in the "ACTIVE" bucket for the
duration of `mExemptedSyncStartTimeoutMillis`, whose default is 10 minutes as of 2020-10-23.
This will allow the app to access network, even if it has been in the `RARE` bucket
(in which case, the system cuts its network access).
Note if the device is dozing or in battery saver, promoting to the "ACTIVE" bucket will still
_not_ give the app network access.
- Behavior of `SYNC_EXEMPTION_PROMOTE_BUCKET_WITH_TEMP`
- This gives all the perks given by `SYNC_EXEMPTION_PROMOTE_BUCKET`, plus puts the target app
in the temp-allowlist (by calling `DeviceIdleInternal.addPowerSaveTempWhitelistApp()`)
for the duration of `SyncManagerConstants.getKeyExemptionTempWhitelistDurationInSeconds()`,
whose default is 10 minutes.
Temp-allowlist will grant the app network access even if the device is in doze or in battery
saver.
(However, note that when the device is dozing, sync jobs will not run anyway.)
### How Retries Are Handled
- When a sync operation needs a retry, SyncManager creates a new operation (job) with a back-off
(in `SyncManager.maybeRescheduleSync()`). In this case, the new sync operation will inherit
`SyncOperation.syncExemptionFlag`, unless the number of retries (not counting the original sync
job) is equal to or greater than `SyncManagerConstants.getMaxRetriesWithAppStandbyExemption()`,
whose default is 5.
### Special-handling of Pre-installed Packages
- When a content provider is accessed, `AppStandbyController.reportContentProviderUsage()` is
triggered, which elevates the standby bucket of the associated sync adapters' packages to `ACTIVE`
for the duration of `mSyncAdapterTimeoutMillis`, whose default is 10 minutes, but _only for_
pre-installed packages. This is to help pre-installed sync adapters, which often don't have UI,
sync properly.
- Also, since Android 11, all the pre-installed apps with no activities will be kept in
the `ACTIVE` bucket, which greatly relaxes app-standby throttling. But they're still subject
to doze and battery saver.
### Summary
- When the device is dozing, no sync operations will be executed.
- Normally, sync operations are subject to App-Standby, which throttles jobs owned by background
apps. Jobs owned by foreground apps are not affected.
- A sync operation requested by a foreground activity will be executed immediately even if the
app owning the sync adapter is in RARE bucket, and the device is in battery saver.
- A sync operation requested by a foreground service (or a "bound foreground" service)
will be executed immediately even if the app owning the sync adapter is in RARE bucket,
*unless* the device is in battery saver.
Since Android 9 and later, the same thing will happen if the requester is temp-allowlisted (e.g.
when it has just received a "high-priority FCM").
- There are certain exemptions for pre-installed apps, but doze and battery saver will still
