When a call through the content frame proxy is interrupted by the
dialogueObserver, the synchronous promise that is meant to wait for a
response from the frame script is resolved immediately with an undefined
return value.
When an undefined value is assigned to the response body, it gets dropped
during JSON serialisation. To ensure the "value" field expected from
the Execute Script and Execute Async Script commands is populated,
we need to assign a null value to resp.body.value.
We can treat undefined as null by calling evaluate.toJSON again on the
return value from the proxied frame script call. This effectively means
we serialise it twice, since it first needs to be serialised to cross
the IPC border, though the second computation only looks at primitives
and no known web element store is required.
It would be nicer if the content frame script itself would be able to
return early with null by installing a user prompt notification event,
but this is not possible because the tabmodal dialogue that appears
blocks script execution. This means we need to rely on the
dialogueObserver in testing/marionette/proxy.js to take care of the
dialogue for us.
MozReview-Commit-ID: D14TA2TYYXI
--HG--
extra : rebase_source : 5fc17a1f0786e2fc9a2d054ef7c1f8b0e3336906
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fec9a028f3