error/internal-error printing local variable during "bt full".

error/internal-error printing local variable during "bt full".

One of our users reported an internal error using the "bt full"
command. In their situation, reproducing involved the following
scenario:

    (gdb) frame 1
    (gdb) bt full
    #0  0xf7783430 in __kernel_vsyscall ()
    No symbol table info available.
    #1  0xf5550aeb in waitpid () at ../sysdeps/unix/syscall-template.S:81
    No locals.
    [...]
    #6  0x0fe83139 in xxxx (arg=...)
    [...some locals printed, and then...]
    <S17b> =
    [...]/dwarf2loc.c:364: internal-error: dwarf_expr_frame_base: Assertion
    `framefunc != NULL' failed.

As shown above, the error happens while GDB is trying to print the value
of <S17b>, which is a local string internally generated by the compiler.
For that, it finds that the array lives in memory, and therefore tries
to create a struct value for it via:

        case DWARF_VALUE_MEMORY:
          {
            CORE_ADDR address = dwarf_expr_fetch_address (ctx, 0);
            [...]
            retval = value_at_lazy (type, address + byte_offset);

Unfortunately for us, TYPE happens to be an array whose bounds
are dynamic. More precisely, the bounds of our arrays are described
in the debugging info as being...

 <4><2c1985e>: Abbrev Number: 33 (DW_TAG_subrange_type)
    <2c1985f>   DW_AT_type        : <0x2c1989c>
    <2c19863>   DW_AT_lower_bound : <0x2c19835>
    <2c19867>   DW_AT_upper_bound : <0x2c19841>

... which are references to a pair of local variables. For instance,
the lower bound is a reference to the following DIE

 <3><2c19835>: Abbrev Number: 32 (DW_TAG_variable)
    <2c19836>   DW_AT_name        : [...]
    <2c1983a>   DW_AT_type        : <0x2c198b4>
    <2c1983e>   DW_AT_artificial  : 1
    <2c1983e>   DW_AT_location    : 2 byte block: 91 58         (DW_OP_fbreg: -40)

As a result of the above, value_at_lazy indirectly triggers
a resolution of TYPE (via value_from_contents_and_address),
which means a resolution of TYPE's bounds, and as seen in
the DW_AT_location attribute above for our bounds, computing
the bound's location requires the frame (its location expression
uses DW_OP_fbreg).

Unfortunately for us, value_at_lazy does not get passed a frame,
we've lost the relevant frame when we try to resolve the array's
bounds. Instead, resolve_dynamic_range gets calls dwarf2_evaluate_property
with NULL as the frame:

    static struct type *
    resolve_dynamic_range (struct type *dyn_range_type,
                           struct property_addr_info *addr_stack)
    {
      [...]
      if (dwarf2_evaluate_property (prop, NULL, addr_stack, &value))
                                          ^^^^

... which then handles this by using the selected frame instead:

    if (frame == NULL && has_stack_frames ())
      frame = get_selected_frame (NULL);

In our case, the selected frame happens to be frame #1, which is
a frame where we have a minimal amount of debugging info, and in
particular, no debug info for the function itself. And because of that,
when we try to determine the frame's base...

    static void
    dwarf_expr_frame_base (void *baton, const gdb_byte **start,
                           size_t * length)
    {
      struct dwarf_expr_baton *debaton = (struct dwarf_expr_baton *) baton;
      const struct block *bl = get_frame_block (debaton->frame, NULL);
      [...]
      framefunc = block_linkage_function (bl);

... framefunc ends up being NULL, which triggers the assert
in that same function:

      gdb_assert (framefunc != NULL);

This patches avoids the issue by temporarily setting the selected_frame
before printing the locals of each frames.

This patch also adds a small testcase, which reproduces the same
issue, but with a slightly different outcome:

    (gdb) bt full
    #0  0x000000000040049a in opaque_routine ()
    No symbol table info available.
    #1  0x0000000000400532 in main () at wrong_frame_bt_full-main.c:20
            my_table_size = 3
            my_table = <error reading variable my_table (frame address is not available.)>

With this patch, the output becomes:

    (gdb) bt full
    [...]
            my_table = {0, 1, 2}

gdb/ChangeLog:

        * stack.c (print_frame_local_vars): Temporarily set the selected
        frame to FRAME while printing the frame's local variables.

gdb/testsuite/ChangeLog:

        * gdb.base/wrong_frame_bt_full-main.c: New file.
        * gdb.base/wrong_frame_bt_full-opaque.c: New file.
        * gdb.base/wrong_frame_bt_full.exp: New file.