add nested precompile worker debug

Author: IanButterworth

base/loading.jl

@@ -2501,6 +2501,7 @@ function create_expr_cache(pkg::PkgId, input::String, output::String, output_o::
     # write data over stdin to avoid the (unlikely) case of exceeding max command line size
     write(io.in, """
         empty!(Base.EXT_DORMITORY) # If we have a custom sysimage with `EXT_DORMITORY` prepopulated
+        Base.track_nested_precomp($(repr(vcat(Base.precompilation_stack, pkg.name))))
         Base.precompiling_extension = $(loading_extension)
         Base.include_package_for_output($(pkg_str(pkg)), $(repr(abspath(input))), $(repr(depot_path)), $(repr(dl_load_path)),
             $(repr(load_path)), $deps, $(repr(source_path(nothing))))
@@ -2509,6 +2510,17 @@ function create_expr_cache(pkg::PkgId, input::String, output::String, output_o::
     return io
 end
 
+const precompilation_stack = Vector{String}()
+# Helpful for debugging when precompilation is unexpectedly nested.
+# Enable with `JULIA_DEBUG=nested_precomp`. Note that it expected to be nested in classical code-load precompilation
+# TODO: Add detection if extension precompilation is nested and error / return early?
+function track_nested_precomp(pkg_names::Vector{String})
+    append!(Base.precompilation_stack, pkg_names)
+    if length(Base.precompilation_stack) > 1
+        @debug "Nested precompilation: $(join(Base.precompilation_stack, " > "))" _group=:nested_precomp
+    end
+end
+
 function compilecache_dir(pkg::PkgId)
     entrypath, entryfile = cache_file_entry(pkg)
     return joinpath(DEPOT_PATH[1], entrypath)

pkgimage.mk

@@ -56,6 +56,9 @@ cache-debug-$1: $$(BUILDDIR)/stdlib/$1.debug.image
 .SECONDARY: $$(BUILDDIR)/stdlib/$1.release.image $$(BUILDDIR)/stdlib/$1.debug.image
 endef
 
+# Note: you can check for the correctness of this tree by running `JULIA_DEBUG=nested_precomp make` and looking
+# out for `Debug: Nested precompilation` logs.
+
 # no dependencies
 $(eval $(call stdlib_builder,MozillaCACerts_jll,))
 $(eval $(call stdlib_builder,ArgTools,))