Extend "set cwd" to work on gdbserver

This is the "natural" extension necessary for the "set cwd" command
(and the whole "set the inferior's cwd" logic) to work on gdbserver.

The idea here is to have a new remote packet, QSetWorkingDir (name
adopted from LLDB's extension to the RSP, as can be seen at
<https://raw.githubusercontent.com/llvm-mirror/lldb/master/docs/lldb-gdb-remote.txt>),
which sends an hex-encoded string representing the working directory
that the remote inferior will use.  There is a slight difference from
the packet proposed by LLDB: GDB's version will accept empty
arguments, meaning that the user wants to clear the previously set
working directory for the inferior (i.e., "set cwd" without arguments
on GDB).

For UNIX-like targets this feature is already implemented on
nat/fork-inferior.c, and all gdbserver has to do is to basically
implement "set_inferior_cwd" and call it whenever such packet arrives.
For other targets, like Windows, it is possible to use the existing
"get_inferior_cwd" function and do the necessary steps to make sure
that the inferior will use the specified working directory.

Aside from that, the patch consists basically of updates to the
testcase (making it available on remote targets) and the
documentation.

No regressions found.

gdb/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* NEWS (Changes since GDB 8.0): Add entry about new
	'set-cwd-on-gdbserver' feature.
	(New remote packets): Add entry for QSetWorkingDir.
	* common/common-inferior.h (set_inferior_cwd): New prototype.
	* infcmd.c (set_inferior_cwd): Remove "static".
	(show_cwd_command): Expand text to include remote debugging.
	* remote.c: Add PACKET_QSetWorkingDir.
	(remote_protocol_features) <QSetWorkingDir>: New entry for
	PACKET_QSetWorkingDir.
	(extended_remote_set_inferior_cwd): New function.
	(extended_remote_create_inferior): Call
	"extended_remote_set_inferior_cwd".
	(_initialize_remote): Call "add_packet_config_cmd" for
	QSetWorkingDir.

gdb/gdbserver/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* inferiors.c (set_inferior_cwd): New function.
	* server.c (handle_general_set): Handle QSetWorkingDir packet.
	(handle_query): Inform that QSetWorkingDir is supported.
	* win32-low.c (create_process): Pass the inferior's cwd to
	CreateProcess.

gdb/testsuite/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/set-cwd.exp: Make it available on
	native-extended-gdbserver.

gdb/doc/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (Starting your Program) <The working directory.>:
	Mention remote debugging.
	(Working Directory) <Your Program's Working Directory>:
	Likewise.
	(Connecting) <Remote Packet>: Add "set-working-dir"
	and "QSetWorkingDir" to the table.
	(Remote Protocol) <QSetWorkingDir>: New item, explaining the
	packet.

1 files changed, 10 insertions, 0 deletions

diff --git a/gdb/NEWS b/gdb/NEWS
index fcf4548..2bad096 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -26,6 +26,12 @@
 
 * New features in the GDB remote stub, GDBserver
 
+  ** GDBserver is now able to start inferior processes with a
+     specified initial working directory.
+
+     The user can set the desired working directory to be used from
+     GDB using the new "set cwd" command.
+
   ** New "--selftest" command line option runs some GDBserver self
      tests.  These self tests are disabled in releases.
 
@@ -59,6 +65,10 @@ QEnvironmentReset
 QStartupWithShell
   Indicates whether the inferior must be started with a shell or not.
 
+QSetWorkingDir
+  Tell GDBserver that the inferior to be started should use a specific
+  working directory.
+
 * The "maintenance print c-tdesc" command now takes an optional
   argument which is the file name of XML target description.