RFA: fix error when remote TCP connection dropped

[Jim Blandy <jimb at codesourcery dot com>]

If GDB is talking to a remote target over TCP, and the remote target
exits unexpectedly while the program is running, GDB prints a
mysterious error message about a watchdog timer having expired.

I have a gdbserver running 'getc' (a program that reads a character
from stdin and prints it) in another window.  Then:

  $ ~/gdb/exp/nat/gdb/gdb getc
  GNU gdb 6.7.50-20070926-cvs
  Copyright (C) 2007 Free Software Foundation, Inc.
  License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
  This is free software: you are free to change and redistribute it.
  There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
  and "show warranty" for details.
  This GDB was configured as "i686-pc-linux-gnu"...
  Using host libthread_db library "/lib/libthread_db.so.1".
  (gdb) target remote :1729
  Remote debugging using :1729
  0x00360810 in ?? ()
  (gdb) b main
  Breakpoint 1 at 0x80483c5: file getc.c, line 6.
  (gdb) c
  Continuing.

  Breakpoint 1, main () at getc.c:6
  6         int c = getchar ();
  (gdb) next

At this point, if I kill the gdbserver, GDB says:

  Watchdog has expired.  Target detached.
  (gdb) quit

There is no watchdog timer involved.

The fix is pretty trivial.  It seems the SERIAL_TIMEOUT case is
already handled above, and all existing read_prim functions do return
0 on EOF (including those based on Windows and Winsock functions).

Okay for trunk?  (Not worth it for branch.)

gdb/ChangeLog:
2007-09-26  Jim Blandy  <jimb@codesourcery.com>

	* serial.h (struct serial_ops): Document read_prim to return zero
	at EOF.
	* ser-base.c (do_ser_base_readchar): Return SERIAL_EOF when
	read_prim returns zero, not SERIAL_TIMEOUT.

Index: gdb/ser-base.c
===================================================================
RCS file: /cvs/src/src/gdb/ser-base.c,v
retrieving revision 1.11
diff -u -r1.11 ser-base.c
--- gdb/ser-base.c	23 Aug 2007 18:08:37 -0000	1.11
+++ gdb/ser-base.c	26 Sep 2007 18:34:29 -0000
@@ -279,9 +279,7 @@
   if (status <= 0)
     {
       if (status == 0)
-	/* 0 chars means timeout.  (We may need to distinguish between EOF
-	   & timeouts someday.)  */
-	return SERIAL_TIMEOUT;	
+        return SERIAL_EOF;
       else
 	/* Got an error from read.  */
 	return SERIAL_ERROR;	
Index: gdb/serial.h
===================================================================
RCS file: /cvs/src/src/gdb/serial.h,v
retrieving revision 1.17
diff -u -r1.17 serial.h
--- gdb/serial.h	23 Aug 2007 18:08:37 -0000	1.17
+++ gdb/serial.h	26 Sep 2007 18:34:29 -0000
@@ -244,7 +244,7 @@
        interesting.  */
     void (*async) (struct serial *scb, int async_p);
     /* Perform a low-level read operation, reading (at most) COUNT
-       bytes into SCB->BUF.  */
+       bytes into SCB->BUF.  Return zero at end of file.  */
     int (*read_prim)(struct serial *scb, size_t count);
     /* Perform a low-level write operation, writing (at most) COUNT
        bytes from BUF.  */