c++: Recursing header imports and other duplicates [PR 99174]

The fix for 98741 introduced two issues.  (a) recursive header units
iced because we tried to read the preprocessor state after having
failed to read the config.  (b) we could have duplicate imports of
named modules in our pending queue, and that would lead to duplicate
requests for pathnames, which coupled with the use of a null-pathname
to indicate we'd asked could lead to desynchronization with the module
mapper.  Fixed by adding a 'visited' flag to module state, so we ask
exactly once.

	PR c++/99174
	gcc/cp
	* module.cc (struct module_state): Add visited_p flag.
	(name_pending_imports): Use it to avoid duplicate requests.
	(preprocess_module): Don't read preprocessor state if we failed to
	load a module's config.
	gcc/testsuite/
	* g++.dg/modules/pr99174-1_a.C: New.
	* g++.dg/modules/pr99174-1_b.C: New.
	* g++.dg/modules/pr99174-1_c.C: New.
	* g++.dg/modules/pr99174.H: New.

gcc/cp/module.cc

@@ -3551,9 +3551,10 @@ class GTY((chain_next ("%h.parent"), for_user)) module_state {
   bool call_init_p : 1; /* This module's global initializer needs
 			   calling.  */
   bool inform_read_p : 1; /* Inform of a read.  */
+  bool visited_p : 1;    /* A walk-once flag. */
   /* Record extensions emitted or permitted.  */
   unsigned extensions : SE_BITS;
-  /* 13 bits used, 3 bits remain  */
+  /* 14 bits used, 2 bits remain  */
 
  public:
   module_state (tree name, module_state *, bool);
@@ -3787,6 +3788,7 @@ module_state::module_state (tree name, module_state *parent, bool partition)
   partition_p = partition;
 
   inform_read_p = false;
+  visited_p = false;
 
   extensions = 0;
   if (name && TREE_CODE (name) == STRING_CST)
@@ -19305,16 +19307,16 @@ name_pending_imports (cpp_reader *reader, bool at_end)
     {
       module_state *module = (*pending_imports)[ix];
       gcc_checking_assert (module->is_direct ());
-      if (!module->filename)
+      if (!module->filename
+	  && !module->visited_p
+	  && (module->is_header () || !only_headers))
 	{
-	  Cody::Flags flags
-	    = (flag_preprocess_only ? Cody::Flags::None
-	       : Cody::Flags::NameOnly);
+	  module->visited_p = true;
+	  Cody::Flags flags = (flag_preprocess_only
+			       ? Cody::Flags::None : Cody::Flags::NameOnly);
 
-	  if (only_headers && !module->is_header ())
-	    ;
-	  else if (module->module_p
-		   && (module->is_partition () || module->exported_p))
+	  if (module->module_p
+	      && (module->is_partition () || module->exported_p))
 	    mapper->ModuleExport (module->get_flatname (), flags);
 	  else
 	    mapper->ModuleImport (module->get_flatname (), flags);
@@ -19326,15 +19328,13 @@ name_pending_imports (cpp_reader *reader, bool at_end)
   for (unsigned ix = 0; ix != pending_imports->length (); ix++)
     {
       module_state *module = (*pending_imports)[ix];
-      gcc_checking_assert (module->is_direct ());
-      if (only_headers && !module->is_header ())
-	;
-      else if (!module->filename)
+      if (module->visited_p)
 	{
-	  Cody::Packet const &p = *r_iter;
-	  ++r_iter;
+	  module->visited_p = false;
+	  gcc_checking_assert (!module->filename);
 
-	  module->set_filename (p);
+	  module->set_filename (*r_iter);
+	  ++r_iter;
 	}
     }
 
@@ -19444,7 +19444,8 @@ preprocess_module (module_state *module, location_t from_loc,
 	  /* Now read the preprocessor state of this particular
 	     import.  */
 	  unsigned n = dump.push (module);
-	  if (module->read_preprocessor (true))
+	  if (module->loadedness == ML_CONFIG
+	      && module->read_preprocessor (true))
 	    module->import_macros ();
 	  dump.pop (n);
 

gcc/testsuite/g++.dg/modules/pr99174-1_a.C

@@ -0,0 +1,8 @@
+// PR 99174 what if we import the same module twice (with no
+// intervening header import)?
+// { dg-additional-options -fmodules-ts }
+
+export module Foo;
+// { dg-module-cmi Foo }
+
+export void Foo ();

gcc/testsuite/g++.dg/modules/pr99174-1_b.C

@@ -0,0 +1,6 @@
+// { dg-additional-options -fmodules-ts }
+
+export module Bar;
+// { dg-module-cmi Bar }
+
+export void Bar ();

gcc/testsuite/g++.dg/modules/pr99174-1_c.C

@@ -0,0 +1,11 @@
+// { dg-additional-options -fmodules-ts }
+
+import Foo;
+import Foo;
+import Bar;
+
+int main ()
+{
+  Foo ();
+  Bar ();
+}

gcc/testsuite/g++.dg/modules/pr99174.H

@@ -0,0 +1,5 @@
+// { dg-additional-options -fmodule-header }
+// { dg-module-cmi !{} }
+import "pr99174.H"; // { dg-error "cannot import" }
+
+// { dg-prune-output {not writing module} }