We might not be correctly clearing OIDC cookies when handling an OIDC callback

[matrixbot]

This issue has been migrated from #12782.

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Looking at sentry, this one is really weird. It happens when session == b"", which might be a result of the cookie clearing done before. Testing it a bit, on Safari, it looks like the cookie clearing line 201 does not clear the cookie properly, and instead sets it to a blank value, hence the issue.

So, it's a legit exception, and probably a legit bug because we're not clearing the cookie properly.

Originally posted by @sandhose in matrix-org/synapse#12723 (comment)

The lines in question:

https://github.com/matrix-org/synapse/blob/df3c432ed6e5b47f06970251024fd93038a1e6cd/synapse/handlers/oidc.py#L193-L205

And the sentry report: https://sentry.matrix.org/sentry/synapse-matrixorg/issues/219508/?query=is%3Aunresolved