[GDBserver] Check input interrupt after reading in a packet

[GDBserver] Check input interrupt after reading in a packet

GDBserver may read some packet together with '\003' in one go.  We've
already checked '\003' first when reading packet by my patch,

  Check input interrupt first when reading packet
  https://sourceware.org/ml/gdb-patches/2016-01/msg00057.html

but if we don't check '\003' *after* each packet, the interrupt will
be processed next time GDBserver reads from the buffer, so that the
interrupt isn't processed in time.  For example, GDB sends vCont;c and
interrupt (see gdb.base/interrupt-noterm.exp), we'll resume the
inferior and wait once packet vCont;c is seen.  If we don't check the
interrupt character after vCont;c packet, interrupt character will stay
in the buffer unattended until GDBserver returns from the wait, which
may take a while.  Note that since we've read '\003' from file
descriptor, SIGIO signal handler input_interrupt doesn't help either.

This issue can be exposed by hacking the end of getpkt like
@@ -1041,6 +1050,9 @@ getpkt (char *buf)
        }
     }

+  if (readchar_bufcnt > 0)
+    gdb_assert (*readchar_bufp != '\003');
+
   return bp - buf;
 }

and this can trigger internal error,
(gdb) PASS: gdb.base/interrupt-noterm.exp: interrupt
Remote connection closed^M
(gdb) FAIL: gdb.base/interrupt-noterm.exp: inferior received SIGINT
Remote debugging from host 10.2.206.40^M
/home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/remote-utils.c:1054: A problem internal to GDBserver has been detected.^M
getpkt: Assertion `*readchar_bufp != '\003'' failed.^M

This patch is to peek the buffer, if it is '\003', consume it and call
*the_target->request_interrupt.

gdb/gdbserver:

2016-01-26  Yao Qi  <yao.qi@linaro.org>

	* remote-utils.c (getpkt): If the buffer isn't empty, and the
	first character is '\003', call *the_target->request_interrupt.