Workaround gdbserver<7.7 for setfs

With current FSF GDB HEAD and old FSF gdbserver I expected I could do:
	gdb -ex 'file target:/root/redhat/threadit' -ex 'target remote :1234'
(supplying that unsupported qXfer:exec-file:read by "file")
But that does not work because:
	Sending packet: $vFile:setfs:0#bf...Packet received: OK
	Packet vFile:setfs (hostio-setfs) is supported
	...
	Sending packet: $vFile:setfs:104#24...Packet received: OK
	"target:/root/redhat/threadit": could not open as an executable file: Invalid argument

GDB documentation says:
	The valid responses to Host I/O packets are:
	An empty response indicates that this operation is not recognized.

This "empty response" vs. "OK" was a bug in gdbserver < 7.7.  It was fixed by:
	commit e7f0d979dd5cc4f8b658df892e93db69d6d660b7
	Author: Yao Qi <yao@codesourcery.com>
	Date:   Tue Dec 10 21:59:20 2013 +0800
	    Fix a bug in matching notifications.
	Message-ID: <1386684626-11415-1-git-send-email-yao@codesourcery.com>
	https://sourceware.org/ml/gdb-patches/2013-12/msg00373.html
	2013-12-10  Yao Qi  <yao@codesourcery.com>
		* notif.c (handle_notif_ack): Return 0 if no notification
		matches.

with unpatched old FSF gdbserver and patched FSF GDB HEAD:
	gdb -ex 'file target:/root/redhat/threadit' -ex 'target remote :1234'
	Sending packet: $vFile:setfs:0#bf...Packet received: OK
	Packet vFile:setfs (hostio-setfs) is NOT supported
	...
	(gdb) info sharedlibrary
	From                To                  Syms Read   Shared Object Library
	0x00007ffff7ddbae0  0x00007ffff7df627a  Yes (*)     target:/lib64/ld-linux-x86-64.so.2
	0x00007ffff7bc48a0  0x00007ffff7bcf514  Yes (*)     target:/lib64/libpthread.so.0

gdb/ChangeLog
2016-04-06  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* remote.c (struct remote_state): New field unknown_v_replies_ok.
	(packet_config_support): Read it.
	(remote_start_remote): Set it.

2 files changed, 37 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index e362360..e3848ec 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,11 @@
 2016-04-06  Jan Kratochvil  <jan.kratochvil@redhat.com>
 
+	* remote.c (struct remote_state): New field unknown_v_replies_ok.
+	(packet_config_support): Read it.
+	(remote_start_remote): Set it.
+
+2016-04-06  Jan Kratochvil  <jan.kratochvil@redhat.com>
+
 	* remote.c: Revert check-in by a mistake in the previous commit.
 
 2016-04-06  Jan Kratochvil  <jan.kratochvil@redhat.com>
diff --git a/gdb/remote.c b/gdb/remote.c
index 5c407b6..ea7f5b8 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -426,6 +426,15 @@ struct remote_state
      request/reply nature of the RSP.  We only cache data for a single
      file descriptor at a time.  */
   struct readahead_cache readahead_cache;
+
+  /* gdbserver < 7.7 (before its fix from 2013-12-11) did reply to any
+     unknown 'v' packet with string "OK".  "OK" gets interpreted by GDB
+     as a reply to known packet.  For packet "vFile:setfs:" it is an
+     invalid reply and GDB would return error in
+     remote_hostio_set_filesystem, making remote files access impossible.
+     If this variable is non-zero it means the remote gdbserver is buggy
+     and any not yet detected packets are assumed as unsupported.  */
+  int unknown_v_replies_ok;
 };
 
 /* Private data that we'll store in (struct thread_info)->private.  */
@@ -1519,6 +1528,13 @@ packet_config_support (struct packet_config *config)
     case AUTO_BOOLEAN_FALSE:
       return PACKET_DISABLE;
     case AUTO_BOOLEAN_AUTO:
+      {
+	struct remote_state *rs = get_remote_state ();
+
+	if (rs->unknown_v_replies_ok && config->name != NULL
+	    && config->name[0] == 'v')
+	  return PACKET_DISABLE;
+      }
       return config->support;
     default:
       gdb_assert_not_reached (_("bad switch"));
@@ -4023,6 +4039,21 @@ remote_start_remote (int from_tty, struct target_ops *target, int extended_p)
   if (packet_support (PACKET_QAllow) != PACKET_DISABLE)
     remote_set_permissions (target);
 
+  /* See unknown_v_replies_ok description.  */
+  {
+    const char v_mustreplyempty[] = "vMustReplyEmpty";
+
+    putpkt (v_mustreplyempty);
+    getpkt (&rs->buf, &rs->buf_size, 0);
+    if (strcmp (rs->buf, "OK") == 0)
+      rs->unknown_v_replies_ok = 1;
+    else if (strcmp (rs->buf, "") == 0)
+      rs->unknown_v_replies_ok = 0;
+    else
+      error (_("Remote replied unexpectedly to '%s': %s"), v_mustreplyempty,
+	     rs->buf);
+  }
+
   /* Next, we possibly activate noack mode.
 
      If the QStartNoAckMode packet configuration is set to AUTO,