[6/6] Implement DAP cancellation

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index e4c00143fd1..c7c56f2e967 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -39541,6 +39541,22 @@  to return the bytes of each instruction in an implementation-defined
 format.  @value{GDBN} implements this by sending a string with the
 bytes encoded in hex, like @code{"55a2b900"}.
 
+When the @code{repl} context is used for the @code{evaluate} request,
+@value{GDBN} evaluates the provided expression as a CLI command.
+
+Evaluation in general can cause the inferior to continue execution.
+For example, evaluating the @code{continue} command could do this, as
+could evaluating an expression that involves an inferior function
+call.
+
+@code{repl} evaluation can also cause @value{GDBN} to appear to stop
+responding to requests, for example if a CLI script does a lengthy
+computation.
+
+Evaluations like this can be interrupted using the DAP @code{cancel}
+request.  (In fact, @code{cancel} should work for any request, but it
+is unlikely to be useful for most of them.)
+
 @node JIT Interface
 @chapter JIT Compilation Interface
 @cindex just-in-time compilation
diff --git a/gdb/python/lib/gdb/dap/server.py b/gdb/python/lib/gdb/dap/server.py
index 53a0ca7f448..44dffb1b809 100644
--- a/gdb/python/lib/gdb/dap/server.py
+++ b/gdb/python/lib/gdb/dap/server.py
@@ -14,8 +14,11 @@ 
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import functools
+import gdb
+import heapq
 import inspect
 import json
+import threading
 
 from .io import start_json_writer, read_json
 from .startup import (
@@ -48,6 +51,52 @@  class NotStoppedException(Exception):
     pass
 
 
+# This is used to handle cancellation requests.  It tracks all the
+# needed state, so that we can cancel both requests that are in flight
+# as well as queued requests.
+class CancellationHandler:
+    def __init__(self):
+        # Methods on this class acquire this lock before proceeding.
+        self.lock = threading.Lock()
+        # The request currently being handled, or None.
+        self.in_flight = None
+        self.reqs = []
+
+    def starting(self, req):
+        """Call at the start of the given request.
+
+        Throws the appropriate exception if the request should be
+        immediately cancelled."""
+        with self.lock:
+            self.in_flight = req
+            while len(self.reqs) > 0 and self.reqs[0] <= req:
+                if heapq.heappop(self.reqs) == req:
+                    raise KeyboardInterrupt()
+
+    def done(self, req):
+        """Indicate that the request is done."""
+        with self.lock:
+            self.in_flight = None
+
+    def cancel(self, req):
+        """Call to cancel a request.
+
+        If the request has already finished, this is ignored.
+        If the request is in flight, it is interrupted.
+        If the request has not yet been seen, the cancellation is queued."""
+        with self.lock:
+            if req == self.in_flight:
+                gdb.interrupt()
+            else:
+                # We don't actually ignore the request here, but in
+                # the 'starting' method.  This way we don't have to
+                # track as much state.  Also, this implementation has
+                # the weird property that a request can be cancelled
+                # before it is even sent.  It didn't seem worthwhile
+                # to try to check for this.
+                heapq.heappush(self.reqs, req)
+
+
 class Server:
     """The DAP server class."""
 
@@ -64,6 +113,7 @@  class Server:
         # requests is kept.
         self.read_queue = DAPQueue()
         self.done = False
+        self.canceller = CancellationHandler()
         global _server
         _server = self
 
@@ -71,13 +121,14 @@  class Server:
     # PARAMS is just a dictionary from the JSON.
     @in_dap_thread
     def _handle_command(self, params):
-        # We don't handle 'cancel' for now.
+        req = params["seq"]
         result = {
-            "request_seq": params["seq"],
+            "request_seq": req,
             "type": "response",
             "command": params["command"],
         }
         try:
+            self.canceller.starting(req)
             if "arguments" in params:
                 args = params["arguments"]
             else:
@@ -90,10 +141,15 @@  class Server:
         except NotStoppedException:
             result["success"] = False
             result["message"] = "notStopped"
+        except KeyboardInterrupt:
+            # This can only happen when a request has been canceled.
+            result["success"] = False
+            result["message"] = "cancelled"
         except BaseException as e:
             log_stack()
             result["success"] = False
             result["message"] = str(e)
+        self.canceller.done(req)
         return result
 
     # Read inferior output and sends OutputEvents to the client.  It
@@ -115,11 +171,25 @@  class Server:
         self.write_queue.put(obj)
 
     # This is run in a separate thread and simply reads requests from
-    # the client and puts them into a queue.
+    # the client and puts them into a queue.  A separate thread is
+    # used so that 'cancel' requests can be handled -- the DAP thread
+    # will normally block, waiting for each request to complete.
     def _reader_thread(self):
         while True:
             cmd = read_json(self.in_stream)
             log("READ: <<<" + json.dumps(cmd) + ">>>")
+            # Be extra paranoid about the form here.  If anything is
+            # missing, it will be put in the queue and then an error
+            # issued by ordinary request processing.
+            if (
+                "command" in cmd
+                and cmd["command"] == "cancel"
+                and "arguments" in cmd
+                # gdb does not implement progress, so there's no need
+                # to check for progressId.
+                and "requestId" in cmd["arguments"]
+            ):
+                self.canceller.cancel(cmd["arguments"]["requestId"])
             self.read_queue.put(cmd)
 
     @in_dap_thread
@@ -316,3 +386,18 @@  def disconnect(*, terminateDebuggee: bool = False, **args):
     if terminateDebuggee:
         send_gdb_with_response("kill")
     _server.shutdown()
+
+
+@request("cancel", on_dap_thread=True, expect_stopped=False)
+@capability("supportsCancelRequest")
+def cancel(**args):
+    # If a 'cancel' request can actually be satisfied, it will be
+    # handled specially in the reader thread.  However, in order to
+    # construct a proper response, the request is also added to the
+    # command queue and so ends up here.  Additionally, the spec says:
+    #    The cancel request may return an error if it could not cancel
+    #    an operation but a client should refrain from presenting this
+    #    error to end users.
+    # ... which gdb takes to mean that it is fine for all cancel
+    # requests to report success.
+    return None
diff --git a/gdb/testsuite/gdb.dap/pause.exp b/gdb/testsuite/gdb.dap/pause.exp
index 1b65dcac836..0f2fcbc6b2d 100644
--- a/gdb/testsuite/gdb.dap/pause.exp
+++ b/gdb/testsuite/gdb.dap/pause.exp
@@ -75,4 +75,75 @@  foreach event [lindex $result 1] {
 }
 gdb_assert {$seen == "pass"} "continue event from inferior call"
 
+#
+# Test that a repl evaluation that causes a continue can be canceled.
+#
+
+set cont_id [dap_send_request evaluate \
+		 {o expression [s continue] context [s repl]}]
+dap_wait_for_event_and_check "continued" continued
+
+set cancel_id [dap_send_request cancel \
+		   [format {o requestId [i %d]} $cont_id]]
+
+# The stop event will come before any responses to the requests.
+dap_wait_for_event_and_check "stopped by cancel" stopped
+
+# Now we can wait for the 'continue' request to complete, and then the
+# 'cancel' request.
+dap_read_response evaluate $cont_id
+dap_read_response cancel $cancel_id
+
+#
+# Test that a repl evaluation of a long-running gdb command (that does
+# not continue the inferior) can be canceled.
+#
+
+proc write_file {suffix contents} {
+    global testfile
+
+    set gdbfile [standard_output_file ${testfile}.$suffix]
+    set ofd [open $gdbfile w]
+    puts $ofd $contents
+    close $ofd
+    return $gdbfile
+}
+
+set gdbfile [write_file gdb "set \$x = 0\nwhile 1\nset \$x = \$x + 1\nend"]
+set cont_id [dap_send_request evaluate \
+		 [format {o expression [s "source %s"] context [s repl]} \
+		      $gdbfile]]
+
+# Wait a little to try to ensure the command is running.
+sleep 0.2
+set cancel_id [dap_send_request cancel \
+		   [format {o requestId [i %d]} $cont_id]]
+
+set info [lindex [dap_read_response evaluate $cont_id] 0]
+gdb_assert {[dict get $info success] == "false"} "gdb command failed"
+gdb_assert {[dict get $info message] == "cancelled"} "gdb command canceled"
+
+dap_read_response cancel $cancel_id
+
+#
+# Test that a repl evaluation of a long-running Python command (that
+# does not continue the inferior) can be canceled.
+#
+
+write_file py "while True:\n  pass"
+set cont_id [dap_send_request evaluate \
+		 [format {o expression [s "source %s"] context [s repl]} \
+		      $gdbfile]]
+
+# Wait a little to try to ensure the command is running.
+sleep 0.2
+set cancel_id [dap_send_request cancel \
+		   [format {o requestId [i %d]} $cont_id]]
+
+set info [lindex [dap_read_response evaluate $cont_id] 0]
+gdb_assert {[dict get $info success] == "false"} "python command failed"
+gdb_assert {[dict get $info message] == "cancelled"} "python command canceled"
+
+dap_read_response cancel $cancel_id
+
 dap_shutdown