Fix Solaris regression (PR tdep/30252)

Fix Solaris regression (PR tdep/30252)

PR tdep/30252 reports that using GDB on Solaris fails an assertion in
target_resume:

 target.c:2648: internal-error: target_resume: Assertion `inferior_ptid != null_ptid' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n)

The backtrace, after running it through c++filt, looks like:

 ----- Backtrace -----
 0xa18914 gdb_internal_backtrace_1
	 /root/binutils-gdb/gdb/bt-utils.c:122
 0xa18914 gdb_internal_backtrace()
	 /root/binutils-gdb/gdb/bt-utils.c:168
 0xdec834 internal_vproblem
	 /root/binutils-gdb/gdb/utils.c:401
 0xdecad8 internal_verror(char const*, int, char const*, __va_list_tag*)
	 /root/binutils-gdb/gdb/utils.c:481
 0xf3638c internal_error_loc(char const*, int, char const*, ...)
	 /root/binutils-gdb/gdbsupport/errors.cc:58
 0xd70580 target_resume(ptid_t, int, gdb_signal)
	 /root/binutils-gdb/gdb/target.c:2648
 0xc59e85 procfs_target::wait(ptid_t, target_waitstatus*, enum_flags<target_wait_flag>)
	 /root/binutils-gdb/gdb/procfs.c:2187
 0xcf6da7 sol_thread_target::wait(ptid_t, target_waitstatus*, enum_flags<target_wait_flag>)
	 /root/binutils-gdb/gdb/sol-thread.c:442
 0xd73711 target_wait(ptid_t, target_waitstatus*, enum_flags<target_wait_flag>)
	 /root/binutils-gdb/gdb/target.c:2586
 ...

The problem is that the procfs backend, while inside target_wait,
called target_resume without switching to the leader thread of that
resumption.

The target_resume interface is:

 /* Resume execution (or prepare for execution) of the current thread
    (INFERIOR_PTID), while optionally letting other threads of the
    current process or all processes run free.
    ...

Thus calling target_resume with inferior_ptid == null_ptid is bogus.

target_wait (which leads to procfs_target::wait on Solaris) is called
with inferior_ptid == null_ptid on entry exactly to help catch such
bogus uses.

From the backtrace, it seems that the relevant line in question is
procfs.c:2187:

2186  /* How to keep going without returning to wfi: */
2187  target_continue_no_signal (ptid);
2188  goto wait_again;

target_continue_no_signal is a small wrapper around target_resume,
which would make sense.

The fix is to not call target_resume or go via the target stack at
all.  Instead, factor out a new proc_resume function out of
procfs_target::resume, and call that.  The new function does not rely
on inferior_ptid.

I've not been able to test it myself, but Petr confirmed it fixes the
assertion failure with his test case, and Marcel Telka also confirmed
it solves the problem.

Tested-By: Petr Šumbera <petr.sumbera@oracle.com>
Tested-By: Marcel Telka <marcel@telka.sk>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30252
Change-Id: I6213c59b081d400a22e799ee621c2eff6dcafbf3