Switch to Pkg mode prompt immediately and load Pkg in the background (#54594)

Because Pkg is now a pkgimage it can load slowly on slower machines,
which is a bit frustrating in the first repl switch.

This makes the repl immediately switch to a dummy prompt that looks like
Pkg mode to allow the user to keep typing while Pkg loads. During which
the keymap is disabled.

It works best if julia has >1 thread, otherwise typing stalls during Pkg
load.

If Pkg takes longer to load than the user to type the command and press
return, then the UX isn't great as it won't do anything.

https://github.com/JuliaLang/julia/assets/1694067/1bf17323-441a-4db2-8a3b-4d571eac622f
(cherry picked from commit 13635e1)

Author: IanButterworth

stdlib/REPL/src/REPL.jl

@@ -1041,9 +1041,6 @@ setup_interface(
     extra_repl_keymap::Any = repl.options.extra_keymap
 ) = setup_interface(repl, hascolor, extra_repl_keymap)
 
-# we have to grab this after Pkg is loaded so cache it
-pkg_mode::Union{Nothing,LineEdit.Prompt} = nothing
-
 # This non keyword method can be precompiled which is important
 function setup_interface(
     repl::LineEditREPL,
@@ -1189,11 +1186,17 @@ function setup_interface(
         end,
         ']' => function (s::MIState,o...)
             if isempty(s) || position(LineEdit.buffer(s)) == 0
-                global pkg_mode
-                if pkg_mode === nothing
+                # print a dummy pkg prompt while Pkg loads
+                LineEdit.clear_line(LineEdit.terminal(s))
+                # use 6 .'s here because its the same width as the most likely `@v1.xx` env name
+                print(LineEdit.terminal(s), styled"{blue,bold:({gray:......}) pkg> }")
+                pkg_mode = nothing
+                transition_finished = false
+                iolock = Base.ReentrantLock() # to avoid race between tasks reading stdin & input buffer
+                # spawn Pkg load to avoid blocking typing during loading. Typing will block if only 1 thread
+                t_replswitch = Threads.@spawn begin
                     pkgid = Base.PkgId(Base.UUID("44cfe95a-1eb2-52ea-b672-e2afdf69b78f"), "Pkg")
                     REPLExt = Base.require_stdlib(pkgid, "REPLExt")
-                    pkg_mode = nothing
                     if REPLExt isa Module && isdefined(REPLExt, :PkgCompletionProvider)
                         for mode in repl.interface.modes
                             if mode isa LineEdit.Prompt && mode.complete isa REPLExt.PkgCompletionProvider
@@ -1202,16 +1205,31 @@ function setup_interface(
                             end
                         end
                     end
+                    if pkg_mode !== nothing
+                        @lock iolock begin
+                            buf = copy(LineEdit.buffer(s))
+                            transition(s, pkg_mode) do
+                                LineEdit.state(s, pkg_mode).input_buffer = buf
+                            end
+                            if !isempty(s)
+                                @invokelatest(LineEdit.check_for_hint(s)) && @invokelatest(LineEdit.refresh_line(s))
+                            end
+                            transition_finished = true
+                        end
+                    end
                 end
-                if pkg_mode !== nothing
-                    buf = copy(LineEdit.buffer(s))
-                    transition(s, pkg_mode) do
-                        LineEdit.state(s, pkg_mode).input_buffer = buf
+                Base.errormonitor(t_replswitch)
+                # while loading just accept all keys, no keymap functionality
+                while !istaskdone(t_replswitch)
+                    # wait but only take if task is still running
+                    peek(stdin, Char)
+                    @lock iolock begin
+                        transition_finished || edit_insert(s, read(stdin, Char))
                     end
-                    return
                 end
+            else
+                edit_insert(s, ']')
             end
-            edit_insert(s, ']')
         end,
 
         # Bracketed Paste Mode

stdlib/REPL/src/precompile.jl

@@ -220,4 +220,14 @@ end
 generate_precompile_statements()
 
 precompile(Tuple{typeof(getproperty), REPL.REPLBackend, Symbol})
+
+# Helps Pkg repl mode switch
+precompile(Tuple{typeof(REPL.Terminals.clear_line), REPL.Terminals.TTYTerminal})
+precompile(Tuple{typeof(Base.print), REPL.Terminals.TTYTerminal, Base.AnnotatedString{String}})
+precompile(Tuple{typeof(Base.peek), Base.TTY, Type{Char}})
+precompile(Tuple{typeof(Base.similar), Array{String, 1}})
+precompile(Tuple{typeof(Base.Iterators.enumerate), Array{String, 1}})
+precompile(Tuple{typeof(Base.setindex!), Array{String, 1}, String, Int64})
+precompile(Tuple{typeof(Base.convert), Type{Base.Dict{String, Union{Array{String, 1}, String}}}, Base.Dict{String, Any}})
+
 end # Precompile