Move vgdb special case into remote_filesystem_is_local

Valgrind GDB (vgdb) presents itself as a remote target but works on
the local filesystem.  gdb_bfd_open contained a special case to make
vgdb work with "target:" sysroots, but the implementation meant that
GDB would fall back to the local filesystem if *any* to_fileio_open
method failed with ENOSYS for *any* reason.  This commit moves the
vgdb special case to remote_filesystem_is_local to allow the fallback
to be restricted only to the specific case that remote file transfer
is unsupported.  This commit also adds a warning which is displayed
the first time the fallback is used.

gdb/ChangeLog:

	* gdb_bfd.c (gdb_bfd_open): Move vgdb special case to...
	* remote.c (remote_filesystem_is_local): ...here.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2015-06-05  Gary Benson <gbenson@redhat.com>
+
+	* gdb_bfd.c (gdb_bfd_open): Move vgdb special case to...
+	* remote.c (remote_filesystem_is_local): ...here.
+
 2015-06-04  Yao Qi  <yao.qi@linaro.org>
 
 	* gdbarch.c: Regenerate it.

gdb/gdb_bfd.c

@@ -326,25 +326,11 @@ gdb_bfd_open (const char *name, const char *target, int fd)
 	{
 	  gdb_assert (fd == -1);
 
-	  abfd = gdb_bfd_openr_iovec (name, target,
+	  return gdb_bfd_openr_iovec (name, target,
 				      gdb_bfd_iovec_fileio_open, NULL,
 				      gdb_bfd_iovec_fileio_pread,
 				      gdb_bfd_iovec_fileio_close,
 				      gdb_bfd_iovec_fileio_fstat);
-
-	  if (abfd != NULL || errno != ENOSYS)
-	    return abfd;
-
-	  /* gdb_bfd_iovec_fileio_open failed with ENOSYS.  This can
-	     happen, for example, with vgdb (Valgrind GDB), which
-	     presents itself as a remote target but works on the local
-	     filesystem: it does not implement remote get and users
-	     are not expected to set gdb_sysroot.  To handle this case
-	     we fall back to trying the local filesystem iff
-	     gdb_sysroot is exactly TARGET_SYSROOT_PREFIX.  */
-	  if (gdb_sysroot == NULL
-	      || strcmp (gdb_sysroot, TARGET_SYSROOT_PREFIX) != 0)
-	    return NULL;
 	}
 
       name += strlen (TARGET_SYSROOT_PREFIX);

gdb/remote.c

@@ -10213,15 +10213,6 @@ remote_hostio_send_command (int command_bytes, int which_packet,
   return ret;
 }
 
-/* Return nonzero if the filesystem accessed by the target_fileio_*
-   methods is the local filesystem, zero otherwise.  */
-
-static int
-remote_filesystem_is_local (struct target_ops *self)
-{
-  return 0;
-}
-
 /* Open FILENAME on the remote target, using FLAGS and MODE.  Return a
    remote file descriptor, or -1 if an error occurs (and set
    *REMOTE_ERRNO).  */
@@ -10459,6 +10450,59 @@ remote_hostio_fstat (struct target_ops *self,
   return 0;
 }
 
+/* Return nonzero if the filesystem accessed by the target_fileio_*
+   methods is the local filesystem, zero otherwise.  */
+
+static int
+remote_filesystem_is_local (struct target_ops *self)
+{
+  /* Valgrind GDB presents itself as a remote target but works
+     on the local filesystem: it does not implement remote get
+     and users are not expected to set a sysroot.  To handle
+     this case we treat the remote filesystem as local if the
+     sysroot is exactly TARGET_SYSROOT_PREFIX and if the stub
+     does not support vFile:open.  */
+  if (gdb_sysroot != NULL
+      && strcmp (gdb_sysroot, TARGET_SYSROOT_PREFIX) == 0)
+    {
+      enum packet_support ps = packet_support (PACKET_vFile_open);
+
+      if (ps == PACKET_SUPPORT_UNKNOWN)
+	{
+	  int fd, remote_errno;
+
+	  /* Try opening a file to probe support.  The supplied
+	     filename is irrelevant, we only care about whether
+	     the stub recognizes the packet or not.  */
+	  fd = remote_hostio_open (self, "just probing",
+				   FILEIO_O_RDONLY, 0700,
+				   &remote_errno);
+
+	  if (fd >= 0)
+	    remote_hostio_close (self, fd, &remote_errno);
+
+	  ps = packet_support (PACKET_vFile_open);
+	}
+
+      if (ps == PACKET_DISABLE)
+	{
+	  static int warning_issued = 0;
+
+	  if (!warning_issued)
+	    {
+	      warning (_("remote target does not support file"
+			 " transfer, attempting to access files"
+			 " from local filesystem."));
+	      warning_issued = 1;
+	    }
+
+	  return 1;
+	}
+    }
+
+  return 0;
+}
+
 static int
 remote_fileio_errno_to_host (int errnum)
 {