fix #41546, make using thread-safe

base/loading.jl

@@ -263,6 +263,7 @@ function parsed_toml(project_file::AbstractString, toml_cache::TOMLCache, toml_l
 end
 
 ## package identification: determine unique identity of package to be loaded ##
+const require_lock = ReentrantLock()
 
 # Used by Pkg but not used in loading itself
 function find_package(arg)
@@ -873,7 +874,7 @@ function _require_search_from_serialized(pkg::PkgId, sourcepath::String, depth::
 end
 
 # to synchronize multiple tasks trying to import/using something
-const package_locks = Dict{PkgId,Condition}()
+const package_locks = Dict{PkgId,Threads.Condition}()
 
 # to notify downstream consumers that a module was successfully loaded
 # Callbacks take the form (mod::Base.PkgId) -> nothing.
@@ -968,6 +969,7 @@ For more details regarding code loading, see the manual sections on [modules](@r
 [parallel computing](@ref code-availability).
 """
 function require(into::Module, mod::Symbol)
+    @lock require_lock begin
     LOADING_CACHE[] = LoadingCache()
     try
         uuidkey = identify_package(into, String(mod))
@@ -1019,6 +1021,7 @@ function require(into::Module, mod::Symbol)
     finally
         LOADING_CACHE[] = nothing
     end
+    end
 end
 
 mutable struct PkgOrigin
@@ -1030,6 +1033,7 @@ PkgOrigin() = PkgOrigin(nothing, nothing)
 const pkgorigins = Dict{PkgId,PkgOrigin}()
 
 function require(uuidkey::PkgId)
+    @lock require_lock begin
     if !root_module_exists(uuidkey)
         cachefile = _require(uuidkey)
         if cachefile !== nothing
@@ -1041,13 +1045,14 @@ function require(uuidkey::PkgId)
         end
     end
     return root_module(uuidkey)
+    end
 end
 
 const loaded_modules = Dict{PkgId,Module}()
 const module_keys = IdDict{Module,PkgId}() # the reverse
 
-is_root_module(m::Module) = haskey(module_keys, m)
-root_module_key(m::Module) = module_keys[m]
+is_root_module(m::Module) = @lock require_lock haskey(module_keys, m)
+root_module_key(m::Module) = @lock require_lock module_keys[m]
 
 function register_root_module(m::Module)
     key = PkgId(m, String(nameof(m)))
@@ -1074,12 +1079,13 @@ using Base
 end
 
 # get a top-level Module from the given key
-root_module(key::PkgId) = loaded_modules[key]
+root_module(key::PkgId) = @lock require_lock loaded_modules[key]
 root_module(where::Module, name::Symbol) =
     root_module(identify_package(where, String(name)))
+maybe_root_module(key::PkgId) = @lock require_lock get(loaded_modules, key, nothing)
 
-root_module_exists(key::PkgId) = haskey(loaded_modules, key)
-loaded_modules_array() = collect(values(loaded_modules))
+root_module_exists(key::PkgId) = @lock require_lock haskey(loaded_modules, key)
+loaded_modules_array() = @lock require_lock collect(values(loaded_modules))
 
 function unreference_module(key::PkgId)
     if haskey(loaded_modules, key)
@@ -1098,7 +1104,7 @@ function _require(pkg::PkgId)
         wait(loading)
         return
     end
-    package_locks[pkg] = Condition()
+    package_locks[pkg] = Threads.Condition(require_lock)
 
     last = toplevel_load[]
     try

base/toml_parser.jl

@@ -104,7 +104,7 @@ function Parser(str::String; filepath=nothing)
             IdSet{TOMLDict}(),    # defined_tables
             root,
             filepath,
-            isdefined(Base, :loaded_modules) ? get(Base.loaded_modules, DATES_PKGID, nothing) : nothing,
+            isdefined(Base, :loaded_modules) ? Base.maybe_root_module(DATES_PKGID) : nothing,
         )
     startup(l)
     return l

test/threads_exec.jl

@@ -912,3 +912,31 @@ end
         @test reproducible_rand(r, 10) == val
     end
 end
+
+# issue #41546, thread-safe package loading
+@testset "package loading" begin
+    ch = Channel{Bool}(nthreads())
+    barrier = Base.Event()
+    old_act_proj = Base.ACTIVE_PROJECT[]
+    try
+        pushfirst!(LOAD_PATH, "@")
+        Base.ACTIVE_PROJECT[] = joinpath(@__DIR__, "TestPkg")
+        @sync begin
+            for _ in 1:nthreads()
+                Threads.@spawn begin
+                    put!(ch, true)
+                    wait(barrier)
+                    @eval using TestPkg
+                end
+            end
+            for _ in 1:nthreads()
+                take!(ch)
+            end
+            notify(barrier)
+        end
+        @test Base.root_module(@__MODULE__, :TestPkg) isa Module
+    finally
+        Base.ACTIVE_PROJECT[] = old_act_proj
+        popfirst!(LOAD_PATH)
+    end
+end