Make REPL not slow-down if we load a valid cachefile

[vchuravy]

Fixes #51532.

We have funny series of interactions. With the REPL and its dependencies being removed from the system image,
we observed latency regressions when users create their own precompilation cache of REPL.

During the precompilation of REPL we launch a subordinate process that we send statements too.
Now we do want that process to use the existing cache of the REPL dependencies so we launch it with
--compiled-modules=existing. Otherwise precompilation of REPL is even slower than it is now.

When the user triggers recompilation of REPL due to the use of -O3 the subordinate process sees
a valid cache file for REPL itself. Thus no (or very few) precompilation statements are being
generated. Leading to the cache file compiled with -O3 to have a significant latency regression.

In this PR I work around this by replaying the precompilation statements of REPL from the subordinate
process. A bit hacky, but should be more reliable than trying to set up a "just right" depot,
or filtering the REPL cache-file out.

[vtjnash]

I don't know what it is doing currently, but the cache files should not be left to the whims of using REPL, but be getting loaded by exactly:

Base._tryrequire_from_serialized(pkg::PkgId, path::String, ocachepath::Union{Nothing, String})

If as subprocess it required. With the path and ocachepath values from Base.pkgorigins[pkg]

[vchuravy]

The code is taken pretty verbatim form `generate_precompile.jl` and is in https://github.com/JuliaLang/julia/blob/master/stdlib/REPL/src/precompile.jl The issue is that during the execution of subordinate process no valid cachefile exists?

…

On Thu, Oct 5, 2023, 13:29 Jameson Nash ***@***.***> wrote: I don't know what it is doing currently, but the cache files should not be left to the whims of using REPL, but be getting loaded by exactly: Base._tryrequire_from_serialized(pkg::PkgId, path::String, ocachepath::Union{Nothing, String}) If as subprocess it required. With the path and ocachepath values from Base.pkgorigins[pkg] — Reply to this email directly, view it on GitHub <#51565 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AABDO2QGIC5LRWZG2NHZ7JLX53U7JAVCNFSM6AAAAAA5RDAHNKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTONBZGM2TKNZRGI> . You are receiving this because you modified the open/close state.Message ID: ***@***.***>

[IanButterworth]

@vtjnash I've been trying to implement your suggestion but, to reiterate @vchuravy the issue is that this using REPL is within the subprocess and is called with --compiled-modules=existing, and REPL has no cache file yet.

Update: Found another way around it: #52783 (comment)