[Payment due @huult] Coalesce concurrent SAML reauthentication redirects to fix SSO sign-in flashing

[allgandalf]

Explanation of Change

SSO users on a SAML-required domain are sent back to their identity provider when their session token expires. The bug is that the sign-in page flashes and the SSO prompt fires several times in a row instead of once.

When the token expires the app sends a burst of requests at the same time (reconnectApp, openApp, AuthenticatePusher and so on) and they all come back 407 in the same tick. In reauthenticate() a SAML-required account cannot silently reauthenticate because there is no stored password, so every one of those 407s takes the account.isSAMLRequired branch and calls redirectToSignIn() on its own. Each call clears the session and re-mounts the sign-in page, and each mount re-initiates SAML, so the user sees the screen flash and the IdP prompt open several times.

Production logs from an affected account (anonymized) show the pattern. On reconnect, three commands hit the expired token and 407 inside the same half second:

15:14:32.979  ReconnectApp        ->  407 Unauthorized - Invalid token   (AuthTokenExpired: "Your session has expired")
15:14:33.368  OpenApp             ->  407 Unauthorized - Invalid token   (AuthTokenExpired)
15:14:33.523  AuthenticatePusher  ->  407 Unauthorized - Invalid token   (AuthTokenExpired)

Each of those 407s then drives its own reauthenticate() -> redirectToSignIn(), firing in the same instant (this is the flash):

12:20:59.258  [info] [Reauthenticate] Redirecting to Sign In because SAML is required
12:20:59.375  [info] [Reauthenticate] Redirecting to Sign In because SAML is required
12:20:59.376  [info] [Reauthenticate] Redirecting to Sign In because SAML is required

This coalesces those redirects. The first expired request in a burst queues the redirect and the rest are skipped, until the next SAML sign-in begins. The reset is keyed on the existing RAM_ONLY_IS_AUTHENTICATING_WITH_SHORT_LIVED_TOKEN flag, which flips on when a new short-lived-token exchange starts, so a later token expiry is still allowed to redirect. The result is one clean redirect and one SSO prompt per expiry instead of a flashing burst.

This does not change the redirect behaviour for non-SAML accounts, and it does not change how often a session expires.

Fixed Issues

$ https://github.com/Expensify/Expensify/issues/659015
PROPOSAL: N/A (internal fix)

Tests

1. On the native app, sign in as a user whose domain has SAML/SSO set to required.
2. Use the app until the session auth token expires, or force the next reconnect to receive a 407.
3. Bring the app back to the foreground so it reconnects.
4. Verify you are sent to the SSO sign-in once, with a single IdP prompt and no repeated screen flashing.
5. Complete SSO and verify you land back in the app signed in.
6. Let the new session expire and repeat, and verify each expiry produces a single prompt rather than a burst.

• Verify that no errors appear in the JS console

Offline tests

SAML sign-in needs a network connection. While offline the requests never reach the server, so no 407 and no reauthentication redirect is triggered. There is no offline-specific behaviour for this change.

QA Steps

Same as the Tests section above. Requires a test account on a domain with SAML/SSO set to required, since the flow cannot be reproduced without one.

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
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  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
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• I followed the guidelines as stated in the Review Guidelines
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• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.

[melvin-bot]

@eh2077 Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]

[mountiny]

@huult can you please test this thoroughly?

[huult]

we need to release it to production to test. Could you help me after I've reviewed the code quality?

[huult]

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[huult]

Reviewer Checklist

• I have verified the author checklist is complete (all boxes are checked off).
• I verified the correct issue is linked in the ### Fixed Issues section above
• I verified testing steps are clear and they cover the changes made in this PR
  • I verified the steps for local testing are in the Tests section
  • I verified the steps for Staging and/or Production testing are in the QA steps section
  • I verified the steps cover any possible failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
• I checked that screenshots or videos are included for tests on all platforms
• I included screenshots or videos for tests on all platforms
• I verified that the composer does not automatically focus or open the keyboard on mobile unless explicitly intended. This includes checking that returning the app from the background does not unexpectedly open the keyboard.
• I verified tests pass on all platforms & I tested again on:
  • Android: HybridApp
  • Android: mWeb Chrome
  • iOS: HybridApp
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• If there are any errors in the console that are unrelated to this PR, I either fixed them (preferred) or linked to where I reported them in Slack
• I verified proper code patterns were followed (see Reviewing the code)
  • I verified that comments were added to code that is not self explanatory
  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
  • I verified any copy / text that was added to the app is grammatically correct in English. It adheres to proper capitalization guidelines (note: only the first word of header/labels should be capitalized), and is either coming verbatim from figma or has been approved by marketing (in order to get marketing approval, ask the Bug Zero team member to add the Waiting for copy label to the issue)
• If a new code pattern is added I verified it was agreed to be used by multiple Expensify engineers
• I verified that this PR follows the guidelines as stated in the Review Guidelines
• I verified other components that can be impacted by these changes have been tested, and I retested again (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar have been tested & I retested again)
• If a new component is created I verified that:
  • A similar component doesn't exist in the codebase
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  • The component has the minimum amount of code necessary for its purpose, and it is broken down into smaller components in order to separate concerns and functions
• If a new CSS style is added I verified that:
  • A similar style doesn't already exist
  • The style can't be created with an existing StyleUtils function (i.e. StyleUtils.getBackgroundAndBorderStyle(theme.componentBG)
• If the PR modifies code that runs when editing or sending messages, I tested and verified there is no unexpected behavior for all supported markdown - URLs, single line code, code blocks, quotes, headings, bold, strikethrough, and italic.
• If the PR modifies a generic component, I tested and verified that those changes do not break usages of that component in the rest of the App (i.e. if a shared library or component like Avatar is modified, I verified that Avatar is working as expected in all cases)
• If the PR modifies a component related to any of the existing Storybook stories, I tested and verified all stories for that component are still working as expected.
• If the PR modifies a component or page that can be accessed by a direct deeplink, I verified that the code functions as expected when the deeplink is used - from a logged in and logged out account.
• If the PR modifies the UI (e.g. new buttons, new UI components, changing the padding/spacing/sizing, moving components, etc) or modifies the form input styles:
  • I verified that all the inputs inside a form are aligned with each other.
  • I added Design label and/or tagged @Expensify/design so the design team can review the changes.
• For any bug fix or new feature in this PR, I verified that sufficient unit tests are included to prevent regressions in this flow.
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.
• I have checked off every checkbox in the PR reviewer checklist, including those that don't apply to this PR.

Screenshots/Videos

Android: HybridApp

Android: mWeb Chrome

iOS: HybridApp

iOS: mWeb Safari

MacOS: Chrome / Safari

[huult]

LGTM.

[melvin-bot]

🎯 @huult, thanks for reviewing and testing this PR! 🎉

A payment issue will be created for your review once this PR is deployed to production.
E/E issue linked to the PR - https://www.github.com/Expensify/Expensify/issues/659015.

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[huult]

@mountiny We need to cherry-pick this to production for manual testing. Please let me know when it's ready in prod, and I will test it.