loading: improve circular using error message clarity

Author: vtjnash

base/loading.jl

@@ -2178,7 +2178,6 @@ function canstart_loading(modkey::PkgId, build_id::UInt128, stalecheck::Bool)
     # load already in progress for this module on the task
     task, cond = loading
     deps = String[modkey.name]
-    pkgid = modkey
     assert_havelock(cond.lock)
     if debug_loading_deadlocks && current_task() !== task
         waiters = Dict{Task,Pair{Task,PkgId}}() # invert to track waiting tasks => loading tasks
@@ -2198,18 +2197,26 @@ function canstart_loading(modkey::PkgId, build_id::UInt128, stalecheck::Bool)
         end
     end
     if current_task() === task
-        others = String[modkey.name] # repeat this to emphasize the cycle here
+        push!(deps, modkey.name) # repeat this to emphasize the cycle here
+        others = Set{String}()
         for each in package_locks # list the rest of the packages being loaded too
             if each[2][1] === task
                 other = each[1].name
-                other == modkey.name || other == pkgid.name || push!(others, other)
+                other == modkey.name || push!(others, other)
             end
         end
+        # remove duplicates from others already in deps
+        for dep in deps
+            delete!(others, dep)
+        end
         msg = sprint(deps, others) do io, deps, others
             print(io, "deadlock detected in loading ")
-            join(io, deps, " -> ")
-            print(io, " -> ")
-            join(io, others, " && ")
+            join(io, deps, " using ")
+            if !isempty(others)
+                print(io, " (while loading ")
+                join(io, others, " and ")
+                print(io, ")")
+            end
         end
         throw(ConcurrencyViolationError(msg))
     end

test/loading.jl

@@ -1345,9 +1345,9 @@ module loaded_pkgid4 end
     end
     wait(e)
     reset(e)
-    @test_throws(ConcurrencyViolationError("deadlock detected in loading pkgid3 -> pkgid2 -> pkgid1 -> pkgid3 && pkgid4"),
+    @test_throws(ConcurrencyViolationError("deadlock detected in loading pkgid3 using pkgid2 using pkgid1 using pkgid3 (while loading pkgid4)"),
         @lock Base.require_lock Base.start_loading(pkid3, build_id, false)).value            # try using pkgid3
-    @test_throws(ConcurrencyViolationError("deadlock detected in loading pkgid4 -> pkgid4 && pkgid1"),
+    @test_throws(ConcurrencyViolationError("deadlock detected in loading pkgid4 using pkgid4 (while loading pkgid1)"),
         @lock Base.require_lock Base.start_loading(pkid4, build_id, false)).value            # try using pkgid4
     @lock Base.require_lock Base.end_loading(pkid1, loaded_pkgid1)        # end
     @lock Base.require_lock Base.end_loading(pkid4, loaded_pkgid4)        # end

test/precompile.jl

@@ -1607,7 +1607,7 @@ precompile_test_harness("Issue #26028") do load_path
         """
         module Baz26028
         using Test
-        @test_throws(ConcurrencyViolationError("deadlock detected in loading Foo26028 -> Foo26028"),
+        @test_throws(ConcurrencyViolationError("deadlock detected in loading Foo26028 using Foo26028"),
                      @eval import Foo26028.Bar26028.x)
         import ..Foo26028.Bar26028.y
         end