1492624 - PreallocatedProcessManager doesn't preallocate when descending from the content process limit

Status: NEW

(Core :: DOM: Content Processes, defect, P3)

Description

[Jed Davis [:jld] ⟨⏰|UTC-7⟩ ⟦he/him⟧]

In theory, PreallocatedProcessManager will launch a process (eventually, when the main thread is idle and after at least a 1s delay) if a few conditions are all false:

* The browser is shutting down
* There's already a preallocated content process
* A content process is starting up (AddBlocker/RemoveBlocker)
* The content process limit (dom.ipc.processCount) has been reached

But there's one case where this doesn't happen: when the process limit had been reached, but then one or more processes exit and we're no longer at the limit.

It would be simple to fix this (just check and queue a launch runnable in the ipc:process-shutdown observer), if that seems reasonable.

However, it will cause a regression on the AWSY closed-tabs metric, because it opens a bunch of tabs and then closes all but one.  My opinion is that that regression would be justified — the preallocated process is always a tradeoff of memory vs. future responsiveness.