[gdb/symtab] Fix .debug_aranges duplicate offset warning

The function read_addrmap_from_aranges contains code to issue a warning:
...
      if (!insertpair.second)
       {
         warning (_("Section .debug_aranges in %s has duplicate "
                    "debug_info_offset %s, ignoring .debug_aranges."),
                  objfile_name (objfile), sect_offset_str (per_cu->sect_off));
         return false;
       }
...
but the warning is in fact activated when all_comp_units has duplicate
entries, which is very misleading.

Fix this by:
- adding a test-case that should trigger the warning,
- replacing the current implementation of the warning with an
  assert that all_comp_units should not contain duplicates, and
- properly re-implementing the warning, such that it is triggered
  by the test-case.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29381

gdb/dwarf2/read.c

@@ -2347,15 +2347,13 @@ read_addrmap_from_aranges (dwarf2_per_objfile *per_objfile,
       const auto insertpair
 	= debug_info_offset_to_per_cu.emplace (per_cu->sect_off,
 					       per_cu.get ());
-      if (!insertpair.second)
-	{
-	  warning (_("Section .debug_aranges in %s has duplicate "
-		     "debug_info_offset %s, ignoring .debug_aranges."),
-		   objfile_name (objfile), sect_offset_str (per_cu->sect_off));
-	  return false;
-	}
+
+      /* Assume no duplicate offsets in all_comp_units.  */
+      gdb_assert (insertpair.second);
     }
 
+  std::set<sect_offset> debug_info_offset_seen;
+
   section->read (objfile);
 
   const bfd_endian dwarf5_byte_order = gdbarch_byte_order (gdbarch);
@@ -2413,6 +2411,16 @@ read_addrmap_from_aranges (dwarf2_per_objfile *per_objfile,
 		   pulongest (debug_info_offset));
 	  return false;
 	}
+      const auto insertpair
+	= debug_info_offset_seen.insert (sect_offset (debug_info_offset));
+      if (!insertpair.second)
+	{
+	  warning (_("Section .debug_aranges in %s has duplicate "
+		     "debug_info_offset %s, ignoring .debug_aranges."),
+		   objfile_name (objfile),
+		   sect_offset_str (sect_offset (debug_info_offset)));
+	  return false;
+	}
       dwarf2_per_cu_data *const per_cu = per_cu_it->second;
 
       const uint8_t address_size = *addr++;

gdb/testsuite/gdb.dwarf2/debug-aranges-duplicate-offset-warning.c

@@ -0,0 +1,28 @@
+/*
+   Copyright 2022 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/>.  */
+
+void
+frame2 (void)
+{
+  asm ("frame2_label: .globl frame2_label");
+}
+
+int
+main (void)
+{
+  asm ("main_label: .globl main_label");
+  frame2 ();
+}

gdb/testsuite/gdb.dwarf2/debug-aranges-duplicate-offset-warning.exp

@@ -0,0 +1,81 @@
+# Copyright 2022 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/>.
+load_lib dwarf.exp
+
+# Test with two aranges entries referring to the same CU.
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    verbose "Skipping $gdb_test_file_name."
+    return 0
+}
+
+standard_testfile .c -dw.S
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile srcfile2
+    declare_labels ranges_label
+
+    get_func_info main
+    get_func_info frame2
+
+    cu { label cu_label } {
+	compile_unit {
+	    {language @DW_LANG_C}
+	    {name $srcfile}
+	} {
+	    subprogram {
+		{external 1 flag}
+		{name main}
+		{low_pc $main_start addr}
+		{high_pc $main_len DW_FORM_data4}
+	    }
+	    subprogram {
+		{external 1 flag}
+		{name frame2}
+		{low_pc $frame2_start addr}
+		{high_pc $frame2_len DW_FORM_data4}
+	    }
+	}
+    }
+
+    aranges {} cu_label {
+	arange {} $main_start $main_len
+    }
+    aranges {} cu_label {
+	arange {} $frame2_start $frame2_len
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+set readnow_p [readnow]
+
+set test "file command warnings"
+if { $readnow_p } {
+    set re "warning:"
+    gdb_assert { ![regexp $re $gdb_file_cmd_msg] } $test
+} else {
+    set re \
+	[concat \
+	     "warning: Section .debug_aranges in \[^\r\n\]* has" \
+	     "duplicate debug_info_offset $hex," \
+	     "ignoring \\.debug_aranges\\."]
+    gdb_assert { [regexp $re $gdb_file_cmd_msg] } $test
+}