Send an explicit interrupt to cancel an attach waitfor. (#72565)

Currently when you interrupt a:

(lldb) process attach -w -n some_process

lldb just closes the connection to the stub and kills the
lldb_private::Process it made for the attach. The stub at the other end
notices the connection go down and exits because of that. But when
communication to a device is handled through some kind of proxy server
which isn't as well behaved as one would wish, that signal might not be
reliable, causing debugserver to persist on the machine, waiting to
steal the next instance of that process.

We can work around those failures by sending an explicit interrupt
before closing down the connection. The stub will also have to be
waiting for the interrupt for this to make any difference. I changed
debugserver to do that.

I didn't make the equivalent change in lldb-server. So long as you
aren't faced with a flakey connection, this should not be necessary.

lldb/test/API/python_api/process/cancel_attach/TestCancelAttach.py

@@ -0,0 +1,57 @@
+"""
+Test using SendAsyncInterrupt to interrupt an "attach wait"
+"""
+
+import lldb
+import sys
+import time
+import threading
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+import lldbsuite.test.lldbutil
+
+
+class AttachCancelTestCase(TestBase):
+    NO_DEBUG_INFO_TESTCASE = True
+
+    def test_scripted_implementation(self):
+        """Test that cancelling a stuck "attach waitfor" works."""
+        # First make an empty target for the attach:
+        target = self.dbg.CreateTarget(None)
+
+        # We need to cancel this, so we need to do the attach
+        # on a separate thread:
+        class AttachThread(threading.Thread):
+            def __init__(self, target, error):
+                # Make this a daemon thread so if we don't manage to interrupt,
+                # Python will keep this thread from hanging the test.
+                threading.Thread.__init__(self, daemon=True)
+                self.target = target
+                self.error = error
+                
+            def run(self):
+                self.target.AttachToProcessWithName(lldb.SBListener(), "LLDB-No-Such-Process", True, self.error)
+                
+        error = lldb.SBError()
+        thread = AttachThread(target, error)
+        thread.start()
+
+        # Now wait till the attach on the child thread has made a process
+        # for the attach attempt:
+        while not target.process.IsValid():
+            time.sleep(1)
+        # I don't have a positive signal for "we started the attach attempt"
+        # so the best I can do is sleep a bit more here to give that a chance
+        # to start:
+        time.sleep(1)
+
+        # Now send the attach interrupt:
+        target.process.SendAsyncInterrupt()
+        # We don't want to stall if we can't interrupt, so join with a timeout:
+        thread.join(60)
+        if thread.is_alive():
+          self.fail("The attach thread is alive after timeout interval")
+
+        # Now check the error, should say the attach was interrupted:
+        self.assertTrue(error.Fail(), "We succeeded in not attaching")
+