I hit a bug where the Matrix sync loop treated a passing 502 from
Umbrel's app proxy as a permanent auth failure and stopped syncing for
good. The old check did a naive "403" in str(exc) substring match, and
the 502 HTML error body embedded an SVG path with the coordinate
40.4302, which contains the digit sequence 403.
I replaced the substring check with a layered classifier. Transport
exceptions like TimeoutError, ConnectionError, and OSError are always
treated as transient regardless of their message text. Structured
signals take priority next: the errcode attribute against a known set
of permanent Matrix error codes, then the http_status attribute
against 401/403 specifically (not status, status_code, or code, which
belong to unrelated exception shapes and risk coincidental integer
matches). Only when none of those are present does it fall back to a
bounded, word-boundary-safe text scan on the first 200 characters.
Added tests covering the attribute narrowing, the transient exception
types, and two loop-level tests exercising _sync_loop directly to
confirm it retries on a transient error and stops on a genuine 401/403.
(cherry picked from commit 96d3363e45a63e08d9f07518ed949df334a33b3c)
104889ace6