forkables: On fork failure, retry with hardlinks.

To support in-cygwin package managers, the fork() implementation must
not rely on .exe and .dll files to stay in their original location, as
the package manager's job is to replace these files.  Instead, when the
first fork try fails, and we have NTFS, we use hardlinks to the original
binaries in /var/run/cygfork/ to create the child process during the
second fork try, along the main.exe.local file to enable the "DotLocal
Dll Redirection" feature for the dlls.

The (probably few) users that need an update-safe fork manually have to
create the /var/run/cygfork/ directory for now, using:
mkdir --mode=a=rwxt /var/run/cygfork

	* child_info.h: Bump CURR_CHILD_INFO_MAGIC.
	(enum child_status): Add _CI_SILENTFAIL flag.
	(struct child_info): Add silentfail setter and getter.
	* winsup.h (child_copy): Add bool silentfail parameter.
	* cygheap.cc: Pass silentfail parameter to child_copy.
	* dcrt0.cc: Ditto.
	* dll_init.h (struct dll): Define public inline method forkedntname.
	(struct dll_list): Declare private method find_by_forkedntname.
	* dll_init.cc (struct dll_list): Implement find_by_forkedntname.
	(dll_list::alloc): Use find_by_forkedntname when in load after fork.
	(dll_list::load_after_fork_impl): Load dlls using dll::forkedntname.
	* fork.cc (frok::parent): Set silentfail child info flag.  Pass
	silentfail parameter to child_copy.  Use forkedntname of
	dlls.main_executable.
	(fork): When first dofork run failed and did not use forkables,
	run dofork again with_forkables set to true.
	(child_copy): Use debug_printf if silentfail is true,
	system_printf otherwise.

winsup/cygwin/fork.cc

@@ -315,12 +315,14 @@ frok::parent (volatile char * volatile stack_here)
 
   *with_forkables = dlls.setup_forkables (*with_forkables);
 
+  ch.silentfail (!*with_forkables); /* fail silently without forkables */
+
   while (1)
     {
       PCWCHAR forking_progname = NULL;
       if (dlls.main_executable)
         forking_progname = dll_list::buffered_shortname
-			   (dlls.main_executable->ntname);
+			   (dlls.main_executable->forkedntname ());
       if (!forking_progname || !*forking_progname)
 	forking_progname = myself->progname;
 
@@ -444,7 +446,7 @@ frok::parent (volatile char * volatile stack_here)
       impure_beg = _impure_ptr;
       impure_end = _impure_ptr + 1;
     }
-  rc = child_copy (hchild, true,
+  rc = child_copy (hchild, true, !*with_forkables,
 		   "stack", stack_here, ch.stackbase,
 		   impure, impure_beg, impure_end,
 		   NULL);
@@ -462,7 +464,7 @@ frok::parent (volatile char * volatile stack_here)
   for (dll *d = dlls.istart (DLL_LINK); d; d = dlls.inext ())
     {
       debug_printf ("copying data/bss of a linked dll");
-      if (!child_copy (hchild, true,
+      if (!child_copy (hchild, true, !*with_forkables,
 		       "linked dll data", d->p.data_start, d->p.data_end,
 		       "linked dll bss", d->p.bss_start, d->p.bss_end,
 		       NULL))
@@ -492,7 +494,7 @@ frok::parent (volatile char * volatile stack_here)
       for (dll *d = dlls.istart (DLL_LOAD); d; d = dlls.inext ())
 	{
 	  debug_printf ("copying data/bss for a loaded dll");
-	  if (!child_copy (hchild, true,
+	  if (!child_copy (hchild, true, !*with_forkables,
 			   "loaded dll data", d->p.data_start, d->p.data_end,
 			   "loaded dll bss", d->p.bss_start, d->p.bss_end,
 			   NULL))
@@ -539,7 +541,13 @@ cleanup:
 extern "C" int
 fork ()
 {
-  bool with_forkables = true;
+  bool with_forkables = false; /* do not force hardlinks on first try */
+  int res = dofork (&with_forkables);
+  if (res >= 0)
+    return res;
+  if (with_forkables)
+    return res; /* no need for second try when already enabled */
+  with_forkables = true; /* enable hardlinks for second try */
   return dofork (&with_forkables);
 }
 
@@ -613,6 +621,9 @@ dofork (bool *with_forkables)
     {
       if (!grouped.errmsg)
 	syscall_printf ("fork failed - child pid %d, errno %d", grouped.child_pid, grouped.this_errno);
+      else if (grouped.ch.silentfail ())
+	debug_printf ("child %d - %s, errno %d", grouped.child_pid,
+		       grouped.errmsg, grouped.this_errno);
       else
 	system_printf ("child %d - %s, errno %d", grouped.child_pid,
 		       grouped.errmsg, grouped.this_errno);
@@ -640,10 +651,10 @@ vfork ()
 /* Copy memory from one process to another. */
 
 bool
-child_copy (HANDLE hp, bool write, ...)
+child_copy (HANDLE hp, bool write, bool silentfail, ...)
 {
   va_list args;
-  va_start (args, write);
+  va_start (args, silentfail);
   static const char *huh[] = {"read", "write"};
 
   char *what;
@@ -669,10 +680,14 @@ child_copy (HANDLE hp, bool write, ...)
 	    {
 	      if (!res)
 		__seterrno ();
-	      /* If this happens then there is a bug in our fork
-		 implementation somewhere. */
-	      system_printf ("%s %s copy failed, %p..%p, done %lu, windows pid %u, %E",
-			    what, huh[write], low, high, done, myself->dwProcessId);
+	      if (silentfail)
+		debug_printf ("%s %s copy failed, %p..%p, done %lu, windows pid %u, %E",
+			     what, huh[write], low, high, done, myself->dwProcessId);
+	      else
+		/* If this happens then there is a bug in our fork
+		   implementation somewhere. */
+		system_printf ("%s %s copy failed, %p..%p, done %lu, windows pid %u, %E",
+			      what, huh[write], low, high, done, myself->dwProcessId);
 	      goto err;
 	    }
 	}