Fix crash on attaching external threads

src/gc/impl/conservative/gc.d

@@ -2384,6 +2384,14 @@ struct Gcx
      */
     size_t fullcollect(bool nostack = false) nothrow
     {
+        // It is possible that `fullcollect` will be called from a thread which
+        // is not yet registered in runtime (because allocating `new Thread` is
+        // part of `thread_attachThis` implementation). In that case it is
+        // better not to try actually collecting anything
+
+        if (Thread.getThis() is null)
+            return 0;
+
         MonoTime start, stop, begin;
 
         if (config.profile)

test/thread/Makefile

@@ -1,6 +1,6 @@
 include ../common.mak
 
-TESTS:=fiber_guard_page
+TESTS:=fiber_guard_page external_threads
 
 .PHONY: all clean
 all: $(addprefix $(ROOT)/,$(addsuffix .done,$(TESTS)))
@@ -11,6 +11,11 @@ $(ROOT)/fiber_guard_page.done: $(ROOT)/%.done : $(ROOT)/%
 	$(QUIET)$(TIMELIMIT)$(ROOT)/$* $(RUN_ARGS); rc=$$?; [ $$rc -eq 139 ] || [ $$rc -eq 138 ]
 	@touch $@
 
+$(ROOT)/external_threads.done: $(ROOT)/%.done : $(ROOT)/%
+	@echo Testing $*
+	$(QUIET)$(TIMELIMIT)$(ROOT)/$*
+	@touch $@
+
 $(ROOT)/%: $(SRC)/%.d
 	$(QUIET)$(DMD) $(DFLAGS) -of$@ $<
 

test/thread/src/external_threads.d

@@ -0,0 +1,22 @@
+import core.sys.posix.pthread;
+import core.memory;
+
+extern(C)
+void* entry_point(void*)
+{
+    // try collecting - GC must ignore this call because this thread
+    // is not registered in runtime
+    GC.collect();
+    return null;
+}
+
+void main()
+{
+    // allocate some garbage
+    auto x = new int[1000];
+
+    pthread_t thread;
+    auto status = pthread_create(&thread, null, &entry_point, null);
+    assert(status == 0);
+    pthread_join(thread, null);
+}