scripts/qmp-shell: Use context manager instead of atexit

We can invoke the shell history writing when we leave the QMPShell scope
instead of relying on atexit. Doing so may be preferable to avoid global
state being registered from within a class instead of from the
application logic directly.

Use QMP's context manager to hook this history saving at close time,
which gets invoked when we leave the context block.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 20210607200649.1840382-32-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

scripts/qmp/qmp-shell

@@ -66,7 +66,6 @@
 # sent to QEMU, which is useful for debugging and documentation generation.
 import argparse
 import ast
-import atexit
 import json
 import os
 import re
@@ -142,6 +141,11 @@ class QMPShell(qmp.QEMUMonitorProtocol):
         self.pretty = pretty
         self.verbose = verbose
 
+    def close(self) -> None:
+        # Hook into context manager of parent to save shell history.
+        self._save_history()
+        super().close()
+
     def _fill_completion(self) -> None:
         cmds = self.cmd('query-commands')
         if 'error' in cmds:
@@ -164,9 +168,8 @@ class QMPShell(qmp.QEMUMonitorProtocol):
             pass
         except IOError as err:
             print(f"Failed to read history '{self._histfile}': {err!s}")
-        atexit.register(self.__save_history)
 
-    def __save_history(self) -> None:
+    def _save_history(self) -> None:
         try:
             readline.write_history_file(self._histfile)
         except IOError as err:
@@ -448,25 +451,25 @@ def main() -> None:
         parser.error("QMP socket or TCP address must be specified")
 
     shell_class = HMPShell if args.hmp else QMPShell
+
     try:
         address = shell_class.parse_address(args.qmp_server)
     except qmp.QMPBadPortError:
         parser.error(f"Bad port number: {args.qmp_server}")
         return  # pycharm doesn't know error() is noreturn
 
-    qemu = shell_class(address, args.pretty, args.verbose)
-
-    try:
-        qemu.connect(negotiate=not args.skip_negotiation)
-    except qmp.QMPConnectError:
-        die("Didn't get QMP greeting message")
-    except qmp.QMPCapabilitiesError:
-        die("Couldn't negotiate capabilities")
-    except OSError as err:
-        die(f"Couldn't connect to {args.qmp_server}: {err!s}")
-
-    for _ in qemu.repl():
-        pass
+    with shell_class(address, args.pretty, args.verbose) as qemu:
+        try:
+            qemu.connect(negotiate=not args.skip_negotiation)
+        except qmp.QMPConnectError:
+            die("Didn't get QMP greeting message")
+        except qmp.QMPCapabilitiesError:
+            die("Couldn't negotiate capabilities")
+        except OSError as err:
+            die(f"Couldn't connect to {args.qmp_server}: {err!s}")
+
+        for _ in qemu.repl():
+            pass
 
 
 if __name__ == '__main__':