Fix crash when reading dummy CUs.

Dummy CUs are used by the incremental linker to pre-allocate space
in the output file. They have a DWARF header but no contents.

gdb/ChangeLog:

	* dwarf2read.c (dwarf2_per_cu_data): Add comment.
	(load_cu): Handle dummy CUs.
	(dw2_do_instantiate_symtab, process_queuef): Ditto.
	(dwarf2_fetch_die_loc_sect_off, dwarf2_fetch_constant_bytes): Ditto.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-dummy-cu.S: New file.
	* gdb.dwarf2/dw2-dummy-cu.exp: New file.

5 files changed, 108 insertions, 5 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index ecc7f55..ed9e302 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,10 @@
+2015-07-23  Doug Evans  <dje@google.com>
+
+	* dwarf2read.c (dwarf2_per_cu_data): Add comment.
+	(load_cu): Handle dummy CUs.
+	(dw2_do_instantiate_symtab, process_queuef): Ditto.
+	(dwarf2_fetch_die_loc_sect_off, dwarf2_fetch_constant_bytes): Ditto.
+
 2015-07-23  Ciro Santilli  <ciro.santilli@gmail.com>  (tiny patch)
 
 	* py-linetable.c (ltpy_get_all_source_lines): Adjust function
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index f4409568..24a4022 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -610,7 +610,8 @@ struct dwarf2_per_cu_data
   struct dwarf2_section_info *section;
 
   /* Set to non-NULL iff this CU is currently loaded.  When it gets freed out
-     of the CU cache it gets reset to NULL again.  */
+     of the CU cache it gets reset to NULL again.  This is left as NULL for
+     dummy CUs (a CU header, but nothing else).  */
   struct dwarf2_cu *cu;
 
   /* The corresponding objfile.
@@ -2655,7 +2656,8 @@ load_cu (struct dwarf2_per_cu_data *per_cu)
   else
     load_full_comp_unit (per_cu, language_minimal);
 
-  gdb_assert (per_cu->cu != NULL);
+  if (per_cu->cu == NULL)
+    return;  /* Dummy CU.  */
 
   dwarf2_find_base_address (per_cu->cu->dies, per_cu->cu);
 }
@@ -2685,6 +2687,7 @@ dw2_do_instantiate_symtab (struct dwarf2_per_cu_data *per_cu)
 	 that may badly handle TUs, load all the TUs in that DWO as well.
 	 http://sourceware.org/bugzilla/show_bug.cgi?id=15021  */
       if (!per_cu->is_debug_types
+	  && per_cu->cu != NULL
 	  && per_cu->cu->dwo_unit != NULL
 	  && dwarf2_per_objfile->index_table != NULL
 	  && dwarf2_per_objfile->index_table->version <= 7
@@ -7544,9 +7547,11 @@ process_queue (void)
      may load a new CU, adding it to the end of the queue.  */
   for (item = dwarf2_queue; item != NULL; dwarf2_queue = item = next_item)
     {
-      if (dwarf2_per_objfile->using_index
-	  ? !item->per_cu->v.quick->compunit_symtab
-	  : (item->per_cu->v.psymtab && !item->per_cu->v.psymtab->readin))
+      if ((dwarf2_per_objfile->using_index
+	   ? !item->per_cu->v.quick->compunit_symtab
+	   : (item->per_cu->v.psymtab && !item->per_cu->v.psymtab->readin))
+	  /* Skip dummy CUs.  */
+	  && item->per_cu->cu != NULL)
 	{
 	  struct dwarf2_per_cu_data *per_cu = item->per_cu;
 	  unsigned int debug_print_threshold;
@@ -20017,6 +20022,13 @@ dwarf2_fetch_die_loc_sect_off (sect_offset offset,
   if (per_cu->cu == NULL)
     load_cu (per_cu);
   cu = per_cu->cu;
+  if (cu == NULL)
+    {
+      /* We shouldn't get here for a dummy CU, but don't crash on the user.
+	 Instead just throw an error, not much else we can do.  */
+      error (_("Dwarf Error: Dummy CU at 0x%x referenced in module %s"),
+	     offset.sect_off, objfile_name (per_cu->objfile));
+    }
 
   die = follow_die_offset (offset, per_cu->is_dwz, &cu);
   if (!die)
@@ -20118,6 +20130,13 @@ dwarf2_fetch_constant_bytes (sect_offset offset,
   if (per_cu->cu == NULL)
     load_cu (per_cu);
   cu = per_cu->cu;
+  if (cu == NULL)
+    {
+      /* We shouldn't get here for a dummy CU, but don't crash on the user.
+	 Instead just throw an error, not much else we can do.  */
+      error (_("Dwarf Error: Dummy CU at 0x%x referenced in module %s"),
+	     offset.sect_off, objfile_name (per_cu->objfile));
+    }
 
   die = follow_die_offset (offset, per_cu->is_dwz, &cu);
   if (!die)
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index b2c1635..282810a 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2015-07-23  Doug Evans  <dje@google.com>
+
+	* gdb.dwarf2/dw2-dummy-cu.S: New file.
+	* gdb.dwarf2/dw2-dummy-cu.exp: New file.
+
 2015-07-23  Pierre-Marie de Rodat  <derodat@adacore.com>
 
 	* gdb.ada/var_arr_typedef.exp: New testcase.
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.S b/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.S
new file mode 100644
index 0000000..fa31561
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.S
@@ -0,0 +1,33 @@
+/* Copyright 2015 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/>.  */
+
+	.text
+main:
+	.4byte	0
+main_end:
+
+	.section	.debug_info
+	.4byte	.Ldebug_info_end - 1f	/* Length of Compilation Unit Info */
+1:
+	.2byte	0x2	/* DWARF version number */
+	.4byte	.Ldebug_abbrev0	/* Offset Into Abbrev. Section */
+	.byte	0x4	/* Pointer Size (in bytes) */
+
+	/* Nothing else, this is a dummy die.  */
+.Ldebug_info_end:
+
+	.section	.debug_abbrev
+.Ldebug_abbrev0:
+	.byte	0x0
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp b/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp
new file mode 100644
index 0000000..a5f6ca5
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp
@@ -0,0 +1,39 @@
+# Copyright 2015 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
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile .S
+set executable ${testfile}
+
+if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" object {}] != "" } {
+    return -1
+}
+
+set saved_gdbflags $GDBFLAGS
+set GDBFLAGS "$GDBFLAGS -readnow"
+
+clean_restart $executable
+
+# Something simple to verify gdb didn't crash, and has read in whatever symbol
+# info is there.
+gdb_test "info fun main" "main_end"
+
+set GDBFLAGS $saved_gdbflags