Fix PR 21337: segfault when re-reading symbols.

Fix issue exposed by commit 3e29f34.

The basic issue is that section data referenced through an objfile
pointer can also be referenced via the program-space data pointer,
although via a separate mapping mechanism, which is set up by
update_section_map.  Thus once section data attached to an objfile
pointer is released, the section map associated with the program-space
data pointer must be marked dirty to ensure that update_section_map is
called to prevent stale data being referenced.  For the matter at hand
this marking is being done via a call to objfiles_changed.

Before commit 3e29f34 objfiles_changed could be called after all of
the objfile pointers were processed in reread_symbols since section
data references via the program-space data pointer would not occur in
the calls of read_symbols performed by reread_symbols.

With commit 3e29f34 MIPS target specific calls to find_pc_section were
added to the code for DWARF information processing, which is called
via read_symbols.  Thus in reread_symbols the call to objfiles_changed
needs to be called before calling read_symbols, otherwise stale
section data can be referenced.

Thanks to Luis Machado for providing text for the main comment
associated with the change.

gdb/
2017-06-28  Doug Gilmore  <Doug.Gilmore@imgtec.com>
    PR gdb/21337
    * symfile.c (reread_symbols): Call objfiles_changed just before
    read_symbols.

gdb/testsuite/
2017-06-28  Doug Gilmore  <Doug.Gilmore@imgtec.com>
    PR gdb/21337
    * gdb.base/reread-readsym.exp: New file.
    * gdb.base/reread-readsym.c: New file.

3 files changed, 89 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 253e229..b7462a5 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2017-06-28  Doug Gilmore  <Doug.Gilmore@imgtec.com>
+
+	PR gdb/21337
+	* gdb.base/reread-readsym.exp: New file.
+	* gdb.base/reread-readsym.c: New file.
+
 2017-06-21  Kevin Buettner  <kevinb@redhat.com>
 
 	* gdb.exp (gdb_test): Using noncapturing parens for the $pattern
diff --git a/gdb/testsuite/gdb.base/reread-readsym.c b/gdb/testsuite/gdb.base/reread-readsym.c
new file mode 100644
index 0000000..2fee696
--- /dev/null
+++ b/gdb/testsuite/gdb.base/reread-readsym.c
@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 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/>.  */
+
+int
+main (void)
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/reread-readsym.exp b/gdb/testsuite/gdb.base/reread-readsym.exp
new file mode 100644
index 0000000..b69eaad
--- /dev/null
+++ b/gdb/testsuite/gdb.base/reread-readsym.exp
@@ -0,0 +1,61 @@
+#   Copyright 2017 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/>.
+
+standard_testfile
+
+set gdbfile [standard_output_file ${testfile}.gdb]
+
+# Test rereading executable.  See PR gdb/21337.
+
+proc generate_cmd_file {gdbfile binfile} {
+    set ofd [open $gdbfile w]
+
+    puts $ofd "file ${binfile}"
+    puts $ofd "shell sleep 1; touch ${binfile}"
+    puts $ofd "run"
+    puts $ofd "file"
+    puts $ofd "file ${binfile}"
+    puts $ofd "shell sleep 1; touch ${binfile}"
+    puts $ofd "run"
+    puts $ofd "file"
+    puts $ofd "file ${binfile}"
+    puts $ofd "shell sleep 1; touch ${binfile}"
+    puts $ofd "run"
+    puts $ofd "file"
+    puts $ofd "p \"source-command-completed\""
+    close $ofd
+}
+
+if [use_gdb_stub] {
+    return 0
+}
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
+    return -1
+}
+
+# Start with a fresh gdb.
+clean_restart ${testfile}
+
+# Using the source command to read commands from a file is important,
+# otherwise section data is freed and reallocated using the same
+# memory locations and the bug is not exposed.
+generate_cmd_file $gdbfile $binfile
+
+gdb_test "source $gdbfile" ".*source-command-completed.*" \
+    "source $testfile.gdb 1"
+# Sometimes the failure only occurs on the second invocation.
+gdb_test "source $gdbfile" ".*source-command-completed.*" \
+    "source $testfile.gdb 2"