Initial implementation of Debugger Adapter Protocol

The Debugger Adapter Protocol is a JSON-RPC protocol that IDEs can use
to communicate with debuggers.  You can find more information here:

    https://microsoft.github.io/debug-adapter-protocol/

Frequently this is implemented as a shim, but it seemed to me that GDB
could implement it directly, via the Python API.  This patch is the
initial implementation.

DAP is implemented as a new "interp".  This is slightly weird, because
it doesn't act like an ordinary interpreter -- for example it doesn't
implement a command syntax, and doesn't use GDB's ordinary event loop.
However, this seemed like the best approach overall.

To run GDB in this mode, use:

    gdb -i=dap

The DAP code will accept JSON-RPC messages on stdin and print
responses to stdout.  GDB redirects the inferior's stdout to a new
pipe so that output can be encapsulated by the protocol.

The Python code uses multiple threads to do its work.  Separate
threads are used for reading JSON from the client and for writing JSON
to the client.  All GDB work is done in the main thread.  (The first
implementation used asyncio, but this had some limitations, and so I
rewrote it to use threads instead.)

This is not a complete implementation of the protocol, but it does
implement enough to demonstrate that the overall approach works.

There is a rudimentary test suite.  It uses a JSON parser written in
pure Tcl.  This parser is under the same license as Tcl itself, so I
felt it was acceptable to simply import it into the tree.

There is also a bit of documentation -- just documenting the new
interpreter name.

1 files changed, 65 insertions, 0 deletions

diff --git a/gdb/python/lib/gdb/dap/scopes.py b/gdb/python/lib/gdb/dap/scopes.py
new file mode 100644
index 00000000000..0c887db91ad
--- /dev/null
+++ b/gdb/python/lib/gdb/dap/scopes.py
@@ -0,0 +1,65 @@
+# Copyright 2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import gdb
+
+from .frames import frame_for_id
+from .startup import send_gdb_with_response, in_gdb_thread
+from .server import request
+
+
+# Helper function to return a frame's block without error.
+@in_gdb_thread
+def _safe_block(frame):
+    try:
+        return frame.block()
+    except gdb.error:
+        return None
+
+
+# Helper function to return a list of variables of block, up to the
+# enclosing function.
+@in_gdb_thread
+def _block_vars(block):
+    result = []
+    while True:
+        result += list(block)
+        if block.function is not None:
+            break
+        block = block.superblock
+    return result
+
+
+# Helper function to create a DAP scopes for a given frame ID.
+@in_gdb_thread
+def _get_scope(id):
+    frame = frame_for_id(id)
+    block = _safe_block(frame)
+    scopes = []
+    if block is not None:
+        new_scope = {
+            # FIXME
+            "name": "Locals",
+            "expensive": False,
+            "namedVariables": len(_block_vars(block)),
+        }
+        scopes.append(new_scope)
+    return scopes
+
+
+@request("scopes")
+def scopes(*, frameId, **extra):
+    scopes = send_gdb_with_response(lambda: _get_scope(frameId))
+    return {"scopes": scopes}