Fix gdb segv when objfile can't be opened

This fixes PR 16577.

This patch changes gdb_bfd_map_section to issue a warning rather than an error
if it is unable to read the object file, and sets the size of the section/frame
that it attempted to read to 0 on error.

The description of gdb_bfd_map_section states that it will try to read or map
the contents of the section SECT, and if successful, the section data is
returned and *SIZE is set to the size of the section data.  This function was
throwing an error and leaving *SIZE as-is.  Setting the section size to 0
indicates to dwarf2_build_frame_info that there is no data to read, otherwise
it will try to read from an invalid frame pointer.

Changing the error to a warning allows this to be handled gracefully.
Additionally, the error was clobbering the breakpoint output indicating the
current frame (function name, arguments, source file, and line number).  E.g.

Thread 3 "foo" hit Breakpoint 1, BFD: reopening /tmp/jna-1013829440/jna2973250704389291330.tmp: No such file or directory
BFD: reopening /tmp/jna-1013829440/jna2973250704389291330.tmp: No such file or directory
(gdb)

While the "BFD: reopening ..." messages will still appear interspersed in the
breakpoint output, the current frame info is now displayed:

Thread 3 "foo" hit Breakpoint 1, BFD: reopening /tmp/jna-1013829440/jna1875755897659885075.tmp: No such file or directory
BFD: reopening /tmp/jna-1013829440/jna1875755897659885075.tmp: No such file or directory
warning: Can't read data for section '.eh_frame' in file '/tmp/jna-1013829440/jna1875755897659885075.tmp'
do_something () at file.cpp:80
80	{
(gdb)

8 files changed, 261 insertions, 12 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index bfd3492..76e6548 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,11 @@
+2017-10-30  Mike Gulick  <mgulick@mathworks.com>
+
+	PR gdb/16577
+	* gdb_bfd.c (gdb_bfd_map_section): If unable to read object file, issue
+	a warning instead of throwing an error, set section size to 0 and return
+	NULL.
+	* gdb_bfd.h (gdb_bfd_map_section): Update description.
+
 2018-01-17  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* nat/linux-ptrace.h (linux_ptrace_attach_fail_reason): Return
diff --git a/gdb/gdb_bfd.c b/gdb/gdb_bfd.c
index 80ddd42..02b1375 100644
--- a/gdb/gdb_bfd.c
+++ b/gdb/gdb_bfd.c
@@ -702,9 +702,15 @@ gdb_bfd_map_section (asection *sectp, bfd_size_type *size)
 
   data = NULL;
   if (!bfd_get_full_section_contents (abfd, sectp, &data))
-    error (_("Can't read data for section '%s' in file '%s'"),
-	   bfd_get_section_name (abfd, sectp),
-	   bfd_get_filename (abfd));
+    {
+      warning (_("Can't read data for section '%s' in file '%s'"),
+	       bfd_get_section_name (abfd, sectp),
+	       bfd_get_filename (abfd));
+      /* Set size to 0 to prevent further attempts to read the invalid
+	 section.  */
+      *size = 0;
+      return (const gdb_byte *) NULL;
+    }
   descriptor->data = data;
 
  done:
diff --git a/gdb/gdb_bfd.h b/gdb/gdb_bfd.h
index ea3fdeb..2e4ac2c 100644
--- a/gdb/gdb_bfd.h
+++ b/gdb/gdb_bfd.h
@@ -115,15 +115,14 @@ void gdb_bfd_mark_parent (bfd *child, bfd *parent);
 
 void gdb_bfd_record_inclusion (bfd *includer, bfd *includee);
 
-/* Try to read or map the contents of the section SECT.  If
-   successful, the section data is returned and *SIZE is set to the
-   size of the section data; this may not be the same as the size
-   according to bfd_get_section_size if the section was compressed.
-   The returned section data is associated with the BFD and will be
-   destroyed when the BFD is destroyed.  There is no other way to free
-   it; for temporary uses of section data, see
-   bfd_malloc_and_get_section.  SECT may not have relocations.  This
-   function will throw on error.  */
+/* Try to read or map the contents of the section SECT.  If successful, the
+   section data is returned and *SIZE is set to the size of the section data;
+   this may not be the same as the size according to bfd_get_section_size if the
+   section was compressed.  The returned section data is associated with the BFD
+   and will be destroyed when the BFD is destroyed.  There is no other way to
+   free it; for temporary uses of section data, see bfd_malloc_and_get_section.
+   SECT may not have relocations.  If there is an error reading the section,
+   this issues a warning, sets *SIZE to 0, and returns NULL.  */
 
 const gdb_byte *gdb_bfd_map_section (asection *section, bfd_size_type *size);
 
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index cd99b84..2eaec1c 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,11 @@
+2017-10-30  Mike Gulick  <mgulick@mathworks.com>
+
+	PR gdb/16577
+	* gdb.base/solib-vanish.exp: New.
+	* gdb.base/solib-vanish-main.c: New.
+	* gdb.base/solib-vanish-lib1.c: New.
+	* gdb.base/solib-vanish-lib2.c: New.
+
 2018-01-17  Yao Qi  <yao.qi@linaro.org>
 
 	* gdb.compile/compile.exp: Match the address printed for
diff --git a/gdb/testsuite/gdb.base/solib-vanish-lib1.c b/gdb/testsuite/gdb.base/solib-vanish-lib1.c
new file mode 100644
index 0000000..68c78ec
--- /dev/null
+++ b/gdb/testsuite/gdb.base/solib-vanish-lib1.c
@@ -0,0 +1,24 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017-2018 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/>.  */
+
+extern int bar (int);
+
+int
+foo (int x)
+{
+  return bar (x);
+}
diff --git a/gdb/testsuite/gdb.base/solib-vanish-lib2.c b/gdb/testsuite/gdb.base/solib-vanish-lib2.c
new file mode 100644
index 0000000..78fb4e2
--- /dev/null
+++ b/gdb/testsuite/gdb.base/solib-vanish-lib2.c
@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017-2018 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
+bar (int y)
+{
+  return y + 1;			/* break here */
+}
diff --git a/gdb/testsuite/gdb.base/solib-vanish-main.c b/gdb/testsuite/gdb.base/solib-vanish-main.c
new file mode 100644
index 0000000..a00ee17
--- /dev/null
+++ b/gdb/testsuite/gdb.base/solib-vanish-main.c
@@ -0,0 +1,75 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017-2018 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/>.  */
+
+#include <dlfcn.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+int
+main ()
+{
+  void *handle;
+  int (*foo) (int);
+  char *error;
+  char *dest = VANISH_LIB ".renamed";
+
+  /* Open library.  */
+  handle = dlopen (VANISH_LIB, RTLD_NOW);
+  if (!handle)
+    {
+      fprintf (stderr, "%s\n", dlerror ());
+      exit (EXIT_FAILURE);
+    }
+
+  /* Clear any existing error */
+  dlerror ();
+
+  /* Simulate deleting file by renaming it.  */
+  if (rename (VANISH_LIB, dest) == -1)
+    {
+      error = strerror (errno);
+      fprintf (stderr, "rename %s -> %s: %s\n", VANISH_LIB, dest, error);
+      exit (EXIT_FAILURE);
+    }
+
+  /* Get function pointer.  */
+  foo = dlsym (handle, "foo");
+  error = dlerror ();
+  if (error != NULL)
+    {
+      fprintf (stderr, "%s\n", error);
+      exit (EXIT_FAILURE);
+    }
+
+  /* Call function.  */
+  (*foo) (1);
+
+  /* Close and exit.  */
+  dlclose (handle);
+
+  /* Put VANISH_LIB back where we found it.  */
+  if (rename (dest, VANISH_LIB) == -1)
+    {
+      error = strerror (errno);
+      fprintf (stderr, "rename %s -> %s: %s\n", dest, VANISH_LIB, error);
+      exit (EXIT_FAILURE);
+    }
+
+  exit (EXIT_SUCCESS);
+}
diff --git a/gdb/testsuite/gdb.base/solib-vanish.exp b/gdb/testsuite/gdb.base/solib-vanish.exp
new file mode 100644
index 0000000..928ca4e
--- /dev/null
+++ b/gdb/testsuite/gdb.base/solib-vanish.exp
@@ -0,0 +1,107 @@
+# Copyright 2017-2018 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/>.
+#
+
+# This test case verifies that GDB gracefully handles a shared library file
+# vanishing after being dlopen'ed.  This consists of three nested function calls:
+#
+# main() -> foo() -> bar()
+#
+# where:
+# - foo exists in solib-vanish-lib1.so, which is dlopen'ed by main()
+# - bar exists in solib-vanish-lib2.so, which is dynamically linked into
+#   solib-vanish-lib1.so
+#
+# Immediately after dlopen'ing solib-vanish-lib1.so, the so file is moved aside
+# by renaming.  The main executable and solib-vanish-lib2.so are still
+# accessible.
+#
+# If a breakpoint is set on bar(), gdb throws an error when this breakpoint is
+# hit:
+#
+#   (gdb) r
+#   Starting program: /local/gdb/git/pr_16577_repro/simple/solib-vanish-main
+#
+#   Breakpoint 1, BFD: reopening ./solib-vanish-lib1.so: No such file or directory
+#
+#   BFD: reopening ./solib-vanish-lib1.so: No such file or directory
+#
+#   (gdb)
+#
+# Notice that this does not print the current frame, i.e.:
+#   bar (y=1) at solib-vanish-lib2.c:19
+#   19	  return y + 1; /* break here */
+#   (gdb)
+#
+# The current gdb git tip segfaults if we then try to step:
+#   (gdb) n
+#   Segmentation fault
+
+# This test verifies that:
+# 1) GDB does not segfault when stepping
+# 2) The stack frame is printed
+
+if { [skip_shlib_tests] } {
+    return 0
+}
+
+# Library 2
+set lib2name "solib-vanish-lib2"
+set srcfile_lib2 ${srcdir}/${subdir}/${lib2name}.c
+set binfile_lib2 [standard_output_file ${lib2name}.so]
+set lib2_flags {debug}
+
+# Library 1
+set lib1name "solib-vanish-lib1"
+set srcfile_lib1 ${srcdir}/${subdir}/${lib1name}.c
+set binfile_lib1 [standard_output_file ${lib1name}.so]
+set lib1_flags [list debug shlib=${binfile_lib2}]
+
+# Main program
+set testfile "solib-vanish-main"
+set srcfile ${srcdir}/${subdir}/${testfile}.c
+set executable ${testfile}
+set binfile [standard_output_file ${executable}]
+set bin_flags [list debug shlib_load additional_flags=-DVANISH_LIB=\"${binfile_lib1}\"]
+
+if { [gdb_compile_shlib ${srcfile_lib2} ${binfile_lib2} $lib2_flags] != ""
+     || [gdb_compile_shlib ${srcfile_lib1} ${binfile_lib1} $lib1_flags] != ""
+     || [gdb_compile ${srcfile} ${binfile} executable $bin_flags] != "" } {
+    untested "failed to compile"
+    return -1
+}
+
+clean_restart $testfile
+
+if { ![runto_main] } {
+    fail "can't run to main"
+    return
+}
+
+delete_breakpoints
+
+set lib2_lineno [gdb_get_line_number "break here" ${srcfile_lib2}]
+
+gdb_breakpoint "${lib2name}.c:${lib2_lineno}" {allow-pending}
+
+# Verify that both the location and source code are displayed
+gdb_continue_to_breakpoint "bar" \
+    ".*/${lib2name}.c:${lib2_lineno}.*break here.*"
+
+# This should not segfault
+gdb_test "next" \
+    "" \
+    "next succeeds"
+