[gdb/symtab] Remove superfluous end-of-sequence marker

While working on PR26935 I noticed that when running test-case
gdb.base/morestack.exp with target board unix/-m32/-fPIE/-pie and ld linker,
I get this linetable fragment for morestack.S using readelf -wL:
...
CU: ../../../../libgcc/config/i386/morestack.S:
Line number    Starting address    View    Stmt
109               0xc9c               x
  ...
838               0xe03               x
  -               0xe04

636                   0               x
637                 0x3               x
  -                 0x4
...
but with "maint info line-table" I get:
...
INDEX  LINE   ADDRESS            IS-STMT
0      END    0x00000004         Y
1      109    0x00000c9c         Y
  ...
110    838    0x00000e03         Y
111    END    0x00000e04         Y
...

So, apparently the entries with addresses 0x0 and 0x3 are filtered out
because the addresses are out of range, but the same doesn't happen with the
end-of-seq terminator.

Fix this by filtering out end-of-seq terminators that do not actually
terminate anything.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-01-04  Tom de Vries  <tdevries@suse.de>

	* buildsym.c (buildsym_compunit::record_line): Filter out end-of-seq
	terminators that do not terminate anything.

gdb/testsuite/ChangeLog:

2021-01-04  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-out-of-range-end-of-seq.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2021-01-04  Tom de Vries  <tdevries@suse.de>
+
+	* buildsym.c (buildsym_compunit::record_line): Filter out end-of-seq
+	terminators that do not terminate anything.
+
 2021-01-04  Simon Marchi  <simon.marchi@efficios.com>
 
 	* debug.c (debug_print_depth): New.

gdb/buildsym.c

@@ -707,13 +707,18 @@ buildsym_compunit::record_line (struct subfile *subfile, int line,
      anyway.  */
   if (line == 0)
     {
+      struct linetable_entry *last = nullptr;
       while (subfile->line_vector->nitems > 0)
 	{
-	  e = subfile->line_vector->item + subfile->line_vector->nitems - 1;
-	  if (e->pc != pc)
+	  last = subfile->line_vector->item + subfile->line_vector->nitems - 1;
+	  if (last->pc != pc)
 	    break;
 	  subfile->line_vector->nitems--;
 	}
+
+      /* Ignore an end-of-sequence marker marking an empty sequence.  */
+      if (last == nullptr || last->line == 0)
+	return;
     }
 
   e = subfile->line_vector->item + subfile->line_vector->nitems++;

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2021-01-04  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.dwarf2/dw2-out-of-range-end-of-seq.exp: New file.
+
 2021-01-04  Simon Marchi  <simon.marchi@efficios.com>
 
 

gdb/testsuite/gdb.dwarf2/dw2-out-of-range-end-of-seq.exp

@@ -0,0 +1,94 @@
+# Copyright 2021 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/>.
+
+# Check that an out-of-range DW_LNE_end_sequence is removed from the
+# line table.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    verbose "Skipping $gdb_test_file_name."
+    return 0
+}
+
+# The .c files use __attribute__.
+if [get_compiler_info] {
+    return -1
+}
+if !$gcc_compiled {
+    verbose "Skipping $gdb_test_file_name."
+    return 0
+}
+
+standard_testfile main.c -dw.S
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    declare_labels Llines
+    global srcdir subdir srcfile
+
+    cu {} {
+	compile_unit {
+	    {language @DW_LANG_C}
+	    {name $srcfile}
+	    {stmt_list $Llines DW_FORM_sec_offset}
+	} {
+	    subprogram {
+		{external 1 flag}
+		{MACRO_AT_func {main}}
+	    }
+	}
+    }
+
+    lines {version 2} Llines {
+	include_dir "${srcdir}/${subdir}"
+	file_name "$srcfile" 1
+
+	program {
+	    {DW_LNE_set_address main_label}
+	    {line 1}
+	    {DW_LNS_copy}
+
+	    {DW_LNS_advance_pc 1}
+	    {DW_LNE_end_sequence}
+
+	    {DW_LNE_set_address 0}
+	    {line 2}
+	    {DW_LNS_copy}
+
+	    {DW_LNS_advance_pc 1}
+	    {DW_LNE_end_sequence}
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+gdb_test_multiple "maint info line-table" "END with address 1 eliminated" {
+    -re -wrap "END *0x0*1 Y \r\n.*" {
+	fail $gdb_test_name
+    }
+    -re -wrap "" {
+	pass $gdb_test_name
+    }
+}