Allow watchers for the active project (#42731)

This allows packages (with Revise as the intended target) to register
themselves to be notified when the active project changes.
This will allow Revise to keep track of the appropriate source for
non-default environments.

For practical effect, Pkg (and perhaps packages like Pluto) will have
to switch from setting `Base.ACTIVE_PROJECT[]` directly to calling
`Base.set_active_project`.

Co-authored by: Jameson Nash <jameson@juliacomputing.com>

base/initdefs.jl

@@ -169,7 +169,11 @@ See also
 const LOAD_PATH = copy(DEFAULT_LOAD_PATH)
 # HOME_PROJECT is no longer used, here just to avoid breaking things
 const HOME_PROJECT = Ref{Union{String,Nothing}}(nothing)
-const ACTIVE_PROJECT = Ref{Union{String,Nothing}}(nothing)
+const ACTIVE_PROJECT = Ref{Union{String,Nothing}}(nothing) # Modify this only via `Base.set_active_project(proj)`
+## Watchers for when the active project changes (e.g., Revise)
+# Each should be a thunk, i.e., `f()`. To determine the current active project,
+# the thunk can query `Base.active_project()`.
+const active_project_callbacks = []
 
 function current_project(dir::AbstractString)
     # look for project file in current dir and parents
@@ -231,10 +235,11 @@ function init_active_project()
     project = (JLOptions().project != C_NULL ?
         unsafe_string(Base.JLOptions().project) :
         get(ENV, "JULIA_PROJECT", nothing))
-    ACTIVE_PROJECT[] =
+    set_active_project(
         project === nothing ? nothing :
         project == "" ? nothing :
         startswith(project, "@") ? load_path_expand(project) : abspath(expanduser(project))
+    )
 end
 
 ## load path expansion: turn LOAD_PATH entries into concrete paths ##
@@ -280,7 +285,7 @@ load_path_expand(::Nothing) = nothing
 """
     active_project()
 
-Return the path of the active `Project.toml` file.
+Return the path of the active `Project.toml` file. See also [`Base.set_active_project`](@ref).
 """
 function active_project(search_load_path::Bool=true)
     for project in (ACTIVE_PROJECT[],)
@@ -306,6 +311,23 @@ function active_project(search_load_path::Bool=true)
     end
 end
 
+"""
+    set_active_project(projfile::Union{AbstractString,Nothing})
+
+Set the active `Project.toml` file to `projfile`. See also [`Base.active_project`](@ref).
+"""
+function set_active_project(projfile::Union{AbstractString,Nothing})
+    ACTIVE_PROJECT[] = projfile
+    for f in active_project_callbacks
+        try
+            Base.invokelatest(f)
+        catch
+            @error "active project callback $f failed" maxlog=1
+        end
+    end
+end
+
+
 """
     load_path()
 

base/loading.jl

@@ -1345,7 +1345,7 @@ function load_path_setup_code(load_path::Bool=true)
         code *= """
         append!(empty!(Base.LOAD_PATH), $(repr(load_path)))
         ENV["JULIA_LOAD_PATH"] = $(repr(join(load_path, Sys.iswindows() ? ';' : ':')))
-        Base.ACTIVE_PROJECT[] = nothing
+        Base.set_active_project(nothing)
         """
     end
     return code
@@ -1358,7 +1358,7 @@ function include_package_for_output(pkg::PkgId, input::String, depot_path::Vecto
     append!(empty!(Base.DL_LOAD_PATH), dl_load_path)
     append!(empty!(Base.LOAD_PATH), load_path)
     ENV["JULIA_LOAD_PATH"] = join(load_path, Sys.iswindows() ? ';' : ':')
-    Base.ACTIVE_PROJECT[] = nothing
+    set_active_project(nothing)
     Base._track_dependencies[] = true
     get!(Base.PkgOrigin, Base.pkgorigins, pkg).path = input
     append!(empty!(Base._concrete_dependencies), concrete_deps)

doc/src/base/base.md

@@ -39,6 +39,7 @@ Base.methods
 Base.@show
 ans
 Base.active_project
+Base.set_active_project
 ```
 
 ## [Keywords](@id Keywords)

test/loading.jl

@@ -183,10 +183,15 @@ end
 saved_load_path = copy(LOAD_PATH)
 saved_depot_path = copy(DEPOT_PATH)
 saved_active_project = Base.ACTIVE_PROJECT[]
+watcher_counter = Ref(0)
+push!(Base.active_project_callbacks, () -> watcher_counter[] += 1)
+push!(Base.active_project_callbacks, () -> error("broken"))
 
 push!(empty!(LOAD_PATH), joinpath(@__DIR__, "project"))
 append!(empty!(DEPOT_PATH), [mktempdir(), joinpath(@__DIR__, "depot")])
-Base.ACTIVE_PROJECT[] = nothing
+@test watcher_counter[] == 0
+@test_logs (:error, r"active project callback .* failed") Base.set_active_project(nothing)
+@test watcher_counter[] == 1
 
 @test load_path() == [joinpath(@__DIR__, "project", "Project.toml")]
 
@@ -598,10 +603,10 @@ end == "opening file $(repr(joinpath(@__DIR__, "notarealfile.jl")))"
 old_act_proj = Base.ACTIVE_PROJECT[]
 pushfirst!(LOAD_PATH, "@")
 try
-    Base.ACTIVE_PROJECT[] = joinpath(@__DIR__, "TestPkg")
+    Base.set_active_project(joinpath(@__DIR__, "TestPkg"))
     @eval using TestPkg
 finally
-    Base.ACTIVE_PROJECT[] = old_act_proj
+    Base.set_active_project(old_act_proj)
     popfirst!(LOAD_PATH)
 end
 
@@ -684,7 +689,9 @@ end
 
 append!(empty!(LOAD_PATH), saved_load_path)
 append!(empty!(DEPOT_PATH), saved_depot_path)
-Base.ACTIVE_PROJECT[] = saved_active_project
+for _ = 1:2 pop!(Base.active_project_callbacks) end
+Base.set_active_project(saved_active_project)
+@test watcher_counter[] == 3
 
 # issue #28190
 module Foo; import Libdl; end