Make "set osabi none" really work (PR 22980)

I was looking for a way to reproduce easily PR 22979 by doing this:

  (gdb) set architecture i386:x86-64
  (gdb) set osabi none

However, I noticed that even though I did "set osabi none", the gdbarch
gdb created was for Linux:

  (gdb) set debug arch 1
  (gdb) set architecture i386:x86-64
  ...
  (gdb) set osabi none
  gdbarch_find_by_info: info.bfd_arch_info i386:x86-64
  gdbarch_find_by_info: info.byte_order 1 (little)
  gdbarch_find_by_info: info.osabi 4 (GNU/Linux)  <--- Wrong?
  gdbarch_find_by_info: info.abfd 0x0
  gdbarch_find_by_info: info.tdep_info 0x0
  gdbarch_find_by_info: Previous architecture 0x1e6fd30 (i386:x86-64)
  selected
  gdbarch_update_p: Architecture 0x1e6fd30 (i386:x86-64) unchanged

This is because the value GDB_OSABI_UNKNOWN has an unclear role,
sometimes meaning "no osabi" and sometimes "please selected
automatically".  Doing "set osabi none" sets the requested osabi to
GDB_OSABI_UNKNOWN, in which case gdbarch_info_fill overrides it with a
value from the target description, or the built-in default osabi.  This
means that it's impossible to force GDB not to use an osabi with "set
osabi".  Since my GDB's built-in default osabi is Linux, it always falls
back to GDB_OSABI_LINUX.

To fix it, I introduced GDB_OSABI_NONE, which really means "I don't want
any osabi".  GDB_OSABI_UNKNOWN can then be used only for "not set yet,
please auto-detect".  GDB_OSABI_UNINITIALIZED now seems unnecessary
since it overlaps with GDB_OSABI_UNKNOWN, so I think it can be removed
and gdbarch_info::osabi can be initialized to GDB_OSABI_UNKNOWN.

gdb/ChangeLog:

	PR gdb/22980
	* defs.h (enum gdb_osabi): Remove GDB_OSABI_UNINITIALIZED, add
	GDB_OSABI_NONE.
	* arch-utils.c (gdbarch_info_init): Don't set info->osabi.
	* osabi.c (gdb_osabi_names): Add "unknown" entry.

gdb/testsuite/ChangeLog:

	PR gdb/22980
	* gdb.base/osabi.exp: New file.

6 files changed, 50 insertions, 11 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index aad71e4..a5773e8 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,13 @@
 2018-04-07  Simon Marchi  <simon.marchi@ericsson.com>
 
+	PR gdb/22980
+	* defs.h (enum gdb_osabi): Remove GDB_OSABI_UNINITIALIZED, add
+	GDB_OSABI_NONE.
+	* arch-utils.c (gdbarch_info_init): Don't set info->osabi.
+	* osabi.c (gdb_osabi_names): Add "unknown" entry.
+
+2018-04-07  Simon Marchi  <simon.marchi@ericsson.com>
+
 	* common/byte-vector.h (char_vector): New type.
 	* target.h (target_read_alloc): Return
 	gdb::optional<byte_vector>.
diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
index 2ff0f4d..5986ed6 100644
--- a/gdb/arch-utils.c
+++ b/gdb/arch-utils.c
@@ -727,7 +727,6 @@ gdbarch_info_init (struct gdbarch_info *info)
   memset (info, 0, sizeof (struct gdbarch_info));
   info->byte_order = BFD_ENDIAN_UNKNOWN;
   info->byte_order_for_code = info->byte_order;
-  info->osabi = GDB_OSABI_UNINITIALIZED;
 }
 
 /* Similar to init, but this time fill in the blanks.  Information is
@@ -772,9 +771,10 @@ gdbarch_info_fill (struct gdbarch_info *info)
 
   /* "(gdb) set osabi ...".  Handled by gdbarch_lookup_osabi.  */
   /* From the manual override, or from file.  */
-  if (info->osabi == GDB_OSABI_UNINITIALIZED)
+  if (info->osabi == GDB_OSABI_UNKNOWN)
     info->osabi = gdbarch_lookup_osabi (info->abfd);
   /* From the target.  */
+
   if (info->osabi == GDB_OSABI_UNKNOWN && info->target_desc != NULL)
     info->osabi = tdesc_osabi (info->target_desc);
   /* From the configured default.  */
@@ -782,6 +782,9 @@ gdbarch_info_fill (struct gdbarch_info *info)
   if (info->osabi == GDB_OSABI_UNKNOWN)
     info->osabi = GDB_OSABI_DEFAULT;
 #endif
+  /* If we still don't know which osabi to pick, pick none.  */
+  if (info->osabi == GDB_OSABI_UNKNOWN)
+    info->osabi = GDB_OSABI_NONE;
 
   /* Must have at least filled in the architecture.  */
   gdb_assert (info->bfd_arch_info != NULL);
diff --git a/gdb/defs.h b/gdb/defs.h
index ff3599d..dc38a28 100644
--- a/gdb/defs.h
+++ b/gdb/defs.h
@@ -482,9 +482,8 @@ extern int longest_to_int (LONGEST);
    table in osabi.c.  */
 enum gdb_osabi
 {
-  GDB_OSABI_UNINITIALIZED = -1, /* For struct gdbarch_info.  */
-
   GDB_OSABI_UNKNOWN = 0,	/* keep this zero */
+  GDB_OSABI_NONE,
 
   GDB_OSABI_SVR4,
   GDB_OSABI_HURD,
diff --git a/gdb/osabi.c b/gdb/osabi.c
index fd44deb..ef1d993 100644
--- a/gdb/osabi.c
+++ b/gdb/osabi.c
@@ -58,6 +58,7 @@ struct osabi_names
    them in sync.  */
 static const struct osabi_names gdb_osabi_names[] =
 {
+  { "unknown", NULL },
   { "none", NULL },
 
   { "SVR4", NULL },
@@ -335,9 +336,11 @@ gdbarch_init_osabi (struct gdbarch_info info, struct gdbarch *gdbarch)
 {
   struct gdb_osabi_handler *handler;
 
-  if (info.osabi == GDB_OSABI_UNKNOWN)
+  gdb_assert (info.osabi != GDB_OSABI_UNKNOWN);
+
+  if (info.osabi == GDB_OSABI_NONE)
     {
-      /* Don't complain about an unknown OSABI.  Assume the user knows
+      /* Don't complain about no OSABI.  Assume the user knows
 	 what they are doing.  */
       return;
     }
@@ -603,11 +606,6 @@ set_osabi (const char *args, int from_tty, struct cmd_list_element *c)
       user_selected_osabi = GDB_OSABI_DEFAULT;
       user_osabi_state = osabi_user;
     }
-  else if (strcmp (set_osabi_string, "none") == 0)
-    {
-      user_selected_osabi = GDB_OSABI_UNKNOWN;
-      user_osabi_state = osabi_user;
-    }
   else
     {
       int i;
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 5536e5b..5a885bf 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2018-04-07  Simon Marchi  <simon.marchi@ericsson.com>
+
+	PR gdb/22980
+	* gdb.base/osabi.exp: New file.
+
 2018-04-02  Weimin Pan  <weimin.pan@oracle.com>
 
 	* gdb.cp/static-typedef-print.exp: New file.
diff --git a/gdb/testsuite/gdb.base/osabi.exp b/gdb/testsuite/gdb.base/osabi.exp
new file mode 100644
index 0000000..8d1ab5b
--- /dev/null
+++ b/gdb/testsuite/gdb.base/osabi.exp
@@ -0,0 +1,26 @@
+# Copyright (C) 2018 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the gdb testsuite.
+
+# Test that choosing "set osabi none" really requests a gdbarch with no osabi.
+
+proc test_set_osabi_none { } {
+    clean_restart
+    gdb_test_no_output "set debug arch 1"
+    gdb_test "set osabi none" ".*gdbarch_find_by_info: info.osabi 1 \\(none\\).*"
+}
+
+test_set_osabi_none