forkables: Create forkable hardlinks, yet unused.

In preparation to protect fork() against dll- and exe-updates, create
hardlinks to the main executable and each loaded dll in subdirectories
of /var/run/cygfork/, if that one exists on the NTFS file system.

The directory names consist of the user sid, the main executable's NTFS
IndexNumber, and the most recent LastWriteTime of all involved binaries
(dlls and main executable).  Next to the main.exe hardlink we create the
empty file main.exe.local to enable dll redirection.

The name of the mutex to synchronize hardlink creation/cleanup also is
assembled from these directory names, to allow for synchronized cleanup
of even orphaned hardlink directories.

The hardlink to each dynamically loaded dll goes into another directory,
named using the NTFS IndexNumber of the dll's original directory.

	* Makefile.in (DLL_OFILES): Add forkable.o.
	* dll_init.h (struct dll): Declare member variables fbi, fii,
	forkable_ntname.  Declare methods nominate_forkable,
	create_forkable.
	(struct dll_list): Declare enum forkables_needs.  Declare member
	variables forkables_dirx_size, forkables_dirx_ntname,
	forkables_mutex_name, forkables_mutex.  Declare private methods
	forkable_ntnamesize, prepare_forkables_nomination,
	update_forkables_needs, update_forkables, create_forkables,
	denominate_forkables, close_mutex, try_remove_forkables,
	set_forkables_inheritance, request_forkables.  Declare public
	static methods ntopenfile, read_fii, read_fbi.  Declare public
	methods release_forkables, cleanup_forkables.  Define public
	inline method setup_forkables.
	* dll_init.cc (dll_list::alloc): Allocate memory to hold the
	name of the hardlink in struct dll member forkable_ntname.
	Initialize struct dll members fbi, fii.
	(dll_list::load_after_fork): Call release_forkables method.
	* fork.cc: Rename public fork function to static dofork, add
	with_forkables as bool pointer parameter.  Add new fork function
	calling dofork.  (struct frok): Add bool pointer member
	with_forkables, add as constructor parameter.
	(frok::parent): Call dlls.setup_forkables before CreateProcessW,
	dlls.release_forkables afterwards.
	* pinfo.cc (pinfo::exit): Call dlls.cleanup_forkables.
	* syscalls.cc (_unlink_nt): Rename public unlink_nt function to
	static _unlink_nt, with 'shareable' as additional argument.
	(unlink_nt): New, wrap _unlink_nt for original behaviour.
	(unlink_nt_shareable): New, wrap _unlink_nt to keep a binary
	file still loadable while removing one of its hardlinks.
	* forkable.cc: New file.
	Implement static functions mkdirs, rmdirs, rmdirs_synchronized,
	stat_real_file_once, format_IndexNumber, rootname, sidname,
	exename, lwtimename.  Define static array forkable_nameparts.
	(struct dll): Implement nominate_forkable, create_forkable.
	(struct dll_list): Implement static methods ntopenfile,
	read_fii, read_fbi.  Implement forkable_ntnamesize,

winsup/cygwin/dll_init.cc

@@ -300,6 +300,7 @@ dll_list::alloc (HINSTANCE h, per_process *p, dll_type type)
   PWCHAR ntname = nt_max_path_buf ();
   GetModuleFileNameW (h, ntname, NT_MAX_PATH);
   PWCHAR modname = form_ntname (ntname, NT_MAX_PATH, ntname);
+  DWORD ntnamelen = modname - ntname;
   while (modname > ntname && *(modname - 1) != L'\\')
     --modname;
 
@@ -334,6 +335,12 @@ dll_list::alloc (HINSTANCE h, per_process *p, dll_type type)
     }
   else
     {
+      size_t forkntsize = forkable_ntnamesize (type, ntname, modname);
+
+      /* FIXME: Change this to new at some point. */
+      d = (dll *) cmalloc (HEAP_2_DLL, sizeof (*d)
+			   + ((ntnamelen + forkntsize) * sizeof (*ntname)));
+
       /* Now we've allocated a block of information.  Fill it in with the
 	 supplied info about this DLL. */
       wcscpy (d->ntname, ntname);
@@ -348,6 +355,15 @@ dll_list::alloc (HINSTANCE h, per_process *p, dll_type type)
       d->image_size = ((pefile*)h)->optional_hdr ()->SizeOfImage;
       d->preferred_base = (void*) ((pefile*)h)->optional_hdr()->ImageBase;
       d->type = type;
+      d->fbi.FileAttributes = INVALID_FILE_ATTRIBUTES;
+      d->fii.IndexNumber.QuadPart = -1LL;
+      if (!forkntsize)
+	d->forkable_ntname = NULL;
+      else
+	{
+	  d->forkable_ntname = d->ntname + ntnamelen + 1;
+	  *d->forkable_ntname = L'\0';
+	}
       append (d);
       if (type == DLL_LOAD)
 	loaded_dlls++;
@@ -654,6 +670,8 @@ dll_list::reserve_space ()
 void
 dll_list::load_after_fork (HANDLE parent)
 {
+  release_forkables ();
+
   // moved to frok::child for performance reasons:
   // dll_list::reserve_space();