[RFC][PATCH] fix gdb segv when objfile can't be opened

[Mike Gulick <mike.gulick@mathworks.com>]

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)
---
 gdb/gdb_bfd.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/gdb/gdb_bfd.c b/gdb/gdb_bfd.c
index 29080b8..229f5ae 100644
--- a/gdb/gdb_bfd.c
+++ b/gdb/gdb_bfd.c
@@ -705,9 +705,15 @@ 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));
+      /* Section is invalid -- set size to 0 and return NULL */
+      descriptor->size = 0;
+      *size = descriptor->size;
+      return (const gdb_byte *) NULL;
+    }
   descriptor->data = data;
 
  done:
-- 
2.1.4