Linux/ptrace: don't convert ptids when asking inf-ptrace layer to resume LWP

Linux/ptrace: don't convert ptids when asking inf-ptrace layer to resume LWP

Ref: https://sourceware.org/ml/gdb-patches/2015-03/msg00060.html

The record-btrace target can hit an assertion here:

 Breakpoint 1, record_btrace_fetch_registers (ops=0x974bfc0 <record_btrace_ops>,
     regcache=0x9a0a798, regno=8) at gdb/record-btrace.c:1202
 1202	  gdb_assert (tp != NULL);

 (gdb) p regcache->ptid
 $3 = {pid = 23856, lwp = 0, tid = 0}

The problem is that the linux-nat layer converts the ptid to a
single-process ptid before passing the request down to the inf-ptrace
layer, which loses information, and then record-btrace can't find the
corresponding thread in GDB's thread list:

 (gdb) bt
 #0  record_btrace_fetch_registers (ops=0x974bfc0 <record_btrace_ops>, regcache=0x9a0a798, regno=8)
     at gdb/record-btrace.c:1202
 #1  0x083f4ee2 in delegate_fetch_registers (self=0x974bfc0 <record_btrace_ops>, arg1=0x9a0a798,
     arg2=8) at gdb/target-delegates.c:149
 #2  0x08406562 in target_fetch_registers (regcache=0x9a0a798, regno=8)
     at gdb/target.c:3279
 #3  0x08355255 in regcache_raw_read (regcache=0x9a0a798, regnum=8,
     buf=0xbfffe6c0 "¨\003\222\tÀ8kIøæÿ¿HO5\b\035]")
     at gdb/regcache.c:643
 #4  0x083558a7 in regcache_cooked_read (regcache=0x9a0a798, regnum=8,
     buf=0xbfffe6c0 "¨\003\222\tÀ8kIøæÿ¿HO5\b\035]")
     at gdb/regcache.c:734
 #5  0x08355de3 in regcache_cooked_read_unsigned (regcache=0x9a0a798, regnum=8, val=0xbfffe738)
     at gdb/regcache.c:838
 #6  0x0827a106 in i386_linux_resume (ops=0x9737ca0 <linux_ops_saved>, ptid=..., step=1,
     signal=GDB_SIGNAL_0) at gdb/i386-linux-nat.c:670
 #7  0x08280c12 in linux_resume_one_lwp (lp=0x9a0a5b8, step=1, signo=GDB_SIGNAL_0)
     at gdb/linux-nat.c:1529
 #8  0x08281281 in linux_nat_resume (ops=0x98da608, ptid=..., step=1, signo=GDB_SIGNAL_0)
     at gdb/linux-nat.c:1708
 #9  0x0850738e in record_btrace_resume (ops=0x98da608, ptid=..., step=1, signal=GDB_SIGNAL_0)
     at gdb/record-btrace.c:1760
 ...

The fix is just to not lose information, and let the intact ptid reach
record-btrace.c.

Tested on x86-64 Fedora 20, -m32.

gdb/ChangeLog:
2015-03-03  Pedro Alves  <palves@redhat.com>

	* i386-linux-nat.c (i386_linux_resume): Get the ptrace PID out of
	the lwp field of ptid.  Pass the full ptid to get_thread_regcache.
	* inf-ptrace.c (get_ptrace_pid): New function.
	(inf_ptrace_resume): Use it.
	* linux-nat.c (linux_resume_one_lwp): Pass the LWP's ptid ummodified
	to the lower layer.