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Moved to #69505

Problem

The Ingest Manager public start method is causing timeout errors for people not even directly using the plugin #66125

Root cause

Doing an await in the start lifecycle method blocks all of Kibana from progressing until it's complete

Proposed solution

1. Do not await in the public start lifecycle method. Fixes [Ingest][EPM] Ingest Manager start lifecycle didn't finish in 30 secs #66125
   PR based on [Ingest][EPM] Ingest Manager start lifecycle didn't finish in 30 secs #66125 (comment) & [Ingest][EPM] Ingest Manager start lifecycle didn't finish in 30 secs #66125 (comment)
2. Change success to be Promise

-  success: boolean;
-  error?: {
-    message: string;
-  };
+  success: Promise<boolean>;

Why this way

re (1): while great gains were made in setup time, error reports are still coming. Dropping the await prevents the possibility of any timeout errors. It's also the Platform team's preference.

re (2): the Ingest Manager plugin currently returns

1. one property to express if the plugin successfully initialized success: boolean
2. one property which gives details if an error occurred error?

We can avoid defining our own by using things already available on the Promise interface

1. .then for the success case
2. .catch for the error case which includes and an Error instance

I think we should limit the scope of the changes we make in the PRs backported to 7.8. This PR gives the most impact (makes it impossible to block/fail Kibana if the setup takes too long) with the least change.

The change to the return value is technically a breaking change for our consumers but any client side code that depended on setup completing, can now await and still get that guarantee. I believe this is the change required:

-    if (!ingestManager.success) {
-      if (ingestManager.error) {
-        displayToastWithModal(ingestManager.error.message);
-      } else {
-        displayToast();
-      }
-    }
+    ingestManager.success
+      .catch((error: Error) => displayToastWithModal(error.message));

This is only client-side, what about the server

Any server side timing issues are present in master already and should be discussed and addressed in follow up PRs. I did some exploration of that in #68631

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser
• This was checked for cross-browser compatibility, including a check against IE11

For maintainers

• This was checked for breaking API changes and was labeled appropriately