Github user som-snytt commented on the issue:
https://github.com/apache/spark/pull/21495
The Scala REPL change at startup was to read user input while the
single-threaded compiler was initializing on the main thread.
There is a `SplashReader` that collects that input; its last act is to
replace itself with the real interactive reader. (I see that `-e` is
implemented as a reader of the expression string.)
Since you can forgo the illusion of a "snappy start-up", I think your
`printWelcome` could just block on a latch, waiting for your custom init to
complete. (Your init code could either just print stuff, or it could stash a
string for your printWelcome to append.)
Never mind, that doesn't work because printWelcome is on the main thread,
not the splash thread; since printWelcome is expensive I/O, I ought to have had
the foresight to move it to the splash thread.
So, your best bet for synchronous startup is to do everything in
printWelcome: createInterpreter, your init commands that produce output.
`IMain` has a compiler that is initialized lazily, so I don't think you have to
explicitly `intp.initializeSynchronous`.
But `createInterpreter` will be called again; you'll want to detect that
and make it a no-op; the method is also called for `:replay`, I don't know if
that is useful in the spark environment?
Unfortunately, there is no option for specifying a custom splash reader
that might do these things.
As a final irony, I did some janitorial work to deprecate `-Yrepl-sync`,
which was unused (I think it was put in because the old threading was broken);
maybe I will revive it for this use case, to skip the splash reader which
prints the prompt you don't want.
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