ensureGatewayProfile used to activate the target gateway and set
$activeGatewayProfile while the connection descriptor fetch was still
in flight, so during that window $gateway already targeted the new
backend while $connection still described the previous one, and any
request or plugin mode-listener firing then announced the wrong mode to
the new backend. A failed descriptor fetch made the mismatch permanent.
prepareGatewayForProfile (new gateway-store seam) opens the socket and
returns a synchronous activation thunk without publishing anything;
ensureGatewayForProfile now delegates to it. The switch resolves the
descriptor and opens the socket first, then flips the active gateway,
the profile atom, and $connection in one synchronous frame. A
descriptor failure aborts the switch as a unit: nothing is published and
every atom still consistently describes the previous profile.
The deferred-descriptor test holds the fetch open and asserts the public
atoms never disagree, then releases it and asserts all three flipped
together; the failure test asserts no partial publication.
Jack Lau
d57f94a330