[gdb/symtab] Error out for .debug_types section in dwz file

There are two methods to factor out type information in a dwarf4 executable:
- use -fdebug-info-types to generate type units in a .debug_types section, and
- use dwz to create partial units.

The dwz method has an extra benefit: it also allows to factor out information
between executables into a newly created .dwz file, pointed to by a
.gnu_debugaltlink section.

There is nothing prohibiting a .gnu_debugaltlink file to contain a
.debug_types section.

It's just not generated by dwz or any other tool atm, and consequently gdb has
no support for it.  Enhancement PR symtab/30838 is open about the lack of
support.

Make the current situation explicit by emitting a dwarf error:
...
(gdb) file struct-with-sig-2^M
Reading symbols from struct-with-sig-2...^M
Dwarf Error: .debug_types section not supported in dwz file^M
...
and add an assert in write_gdbindex:
...
+      /* See enhancement PR symtab/30838.  */
+      gdb_assert (!(per_cu->is_dwz && per_cu->is_debug_types));
...
to clarify why we can use:
...
      data_buf &cu_list = (per_cu->is_debug_types
                           ? types_cu_list
                           : per_cu->is_dwz ? dwz_cu_list : objfile_cu_list);
...

The test-case is a modified copy from gdb.dwarf2/struct-with-sig.exp, so it
keeps the copyright years range.

Tested on x86_64-linux.

Tested-By: Guinevere Larsen <blarsen@redhat.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30838

gdb/dwarf2/dwz.c

@@ -91,6 +91,11 @@ locate_dwz_sections (bfd *abfd, asection *sectp, dwz_file *dwz_file)
       dwz_file->debug_names.s.section = sectp;
       dwz_file->debug_names.size = bfd_section_size (sectp);
     }
+  else if (dwarf2_elf_names.types.matches (sectp->name))
+    {
+      dwz_file->types.s.section = sectp;
+      dwz_file->types.size = bfd_section_size (sectp);
+    }
 }
 
 /* Attempt to find a .dwz file (whose full path is represented by

gdb/dwarf2/dwz.h

@@ -48,6 +48,7 @@ struct dwz_file
   struct dwarf2_section_info macro {};
   struct dwarf2_section_info gdb_index {};
   struct dwarf2_section_info debug_names {};
+  struct dwarf2_section_info types {};
 
   /* The dwz's BFD.  */
   gdb_bfd_ref_ptr dwz_bfd;

gdb/dwarf2/index-write.c

@@ -1221,6 +1221,9 @@ write_gdbindex (dwarf2_per_bfd *per_bfd, cooked_index *table,
       const auto insertpair = cu_index_htab.emplace (per_cu, counter);
       gdb_assert (insertpair.second);
 
+      /* See enhancement PR symtab/30838.  */
+      gdb_assert (!(per_cu->is_dwz && per_cu->is_debug_types));
+
       /* The all_units list contains CUs read from the objfile as well as
 	 from the eventual dwz file.  We need to place the entry in the
 	 corresponding index.  */

gdb/dwarf2/read.c

@@ -5314,6 +5314,12 @@ create_all_units (dwarf2_per_objfile *per_objfile)
       dwz->line.read (objfile);
       read_comp_units_from_section (per_objfile, &dwz->info, &dwz->abbrev, 1,
 				    types_htab, rcuh_kind::COMPILE);
+
+      if (!dwz->types.empty ())
+	{
+	  /* See enhancement PR symtab/30838.  */
+	  error (_("Dwarf Error: .debug_types section not supported in dwz file"));
+	}
     }
 
   per_objfile->per_bfd->signatured_types = std::move (types_htab);

gdb/testsuite/gdb.dwarf2/struct-with-sig-2.exp

@@ -0,0 +1,128 @@
+# Copyright 2020-2023 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/>.
+
+# Variant of struct-with-sig.exp where TUs are in .gnu_debugaltlink file.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+require dwarf2_support
+
+standard_testfile main-foo.c .S .S
+
+set build_id [string repeat ff 20]
+
+# Make some DWARF for the dwz file.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    build_id $::build_id
+
+    tu {} 0x0000000000000001 the_type_i {
+	type_unit {} {
+	    declare_labels int_type
+
+	    the_type_i: structure_type {
+		{name s}
+		{byte_size 4 sdata}
+	    } {
+		member {
+		    {name i}
+		    {type :$int_type}
+		}
+	    }
+	    int_type: base_type {
+		{name int}
+		{encoding @DW_ATE_signed}
+		{byte_size 4 sdata}
+	    }
+	}
+    }
+
+    tu {} 0x0000000000000002 the_type_j {
+	type_unit {} {
+	    declare_labels int_type
+
+	    the_type_j: structure_type {
+		{name s}
+		{byte_size 4 sdata}
+	    } {
+		member {
+		    {name j}
+		    {type :$int_type}
+		}
+	    }
+	    int_type: base_type {
+		{name int}
+		{encoding @DW_ATE_signed}
+		{byte_size 4 sdata}
+	    }
+	}
+    }
+}
+
+# We're using an object instead of executable here to keep cc-with-tweaks.sh
+# from modifying it.  It also means we can set the build-id, rather than
+# having to extract it.
+if { [gdb_compile $asm_file $binfile.dwz object {nodebug}] != "" } {
+    return -1
+}
+
+set host_dwz_file [gdb_remote_download host $binfile.dwz]
+
+# Make some DWARF for the executable.
+set asm_file [standard_output_file $srcfile3]
+Dwarf::assemble $asm_file {
+    gnu_debugaltlink $::host_dwz_file $::build_id
+
+    cu {} {
+	compile_unit {
+	    {DW_AT_language @DW_LANG_C}
+	    {DW_AT_name main.c}
+	} {
+	    structure_type {
+		{name s}
+		{signature 0x0000000000000001 ref_sig8}
+		{declaration 1 flag}
+	    }
+	    DW_TAG_subprogram {
+		{MACRO_AT_func {main}}
+	    }
+	}
+    }
+
+    cu {} {
+	compile_unit {
+	    {DW_AT_language @DW_LANG_C}
+	    {DW_AT_name     foo.c}
+	} {
+	    structure_type {
+		{name s}
+		{signature 0x0000000000000002 ref_sig8}
+		{declaration 1 flag}
+	    }
+	    DW_TAG_subprogram {
+		{MACRO_AT_func {foo}}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+set re "Dwarf Error: .debug_types section not supported in dwz file"
+gdb_assert { [regexp $re $gdb_file_cmd_msg] } "Dwarf Error message"