Refactor register_readline()

* make readline stuff more symmetrical wrt reading/writing
* prevent truncation of the history file
* add regression test

Should finally address python#121245.

Lib/site.py

@@ -509,35 +509,35 @@ def register_readline():
         pass
 
     if readline.get_current_history_length() == 0:
+        try:
+            from _pyrepl.main import CAN_USE_PYREPL
+        except ImportError:
+            CAN_USE_PYREPL = False
         # If no history was loaded, default to .python_history,
         # or PYTHON_HISTORY.
         # The guard is necessary to avoid doubling history size at
         # each interpreter exit when readline was already configured
         # through a PYTHONSTARTUP hook, see:
         # http://bugs.python.org/issue5845#msg198636
         history = gethistoryfile()
+        if os.getenv("PYTHON_BASIC_REPL") or not CAN_USE_PYREPL:
+            my_readline = readline
+        else:
+            my_readline = _pyrepl.readline
         try:
-            if os.getenv("PYTHON_BASIC_REPL"):
-                readline.read_history_file(history)
-            else:
-                _pyrepl.readline.read_history_file(history)
+            my_readline.read_history_file(history)
         except (OSError,* _pyrepl.unix_console._error):
             pass
 
         def write_history():
             try:
-                from _pyrepl.main import CAN_USE_PYREPL
-            except ImportError:
-                CAN_USE_PYREPL = False
-
-            try:
-                if os.getenv("PYTHON_BASIC_REPL") or not CAN_USE_PYREPL:
-                    readline.write_history_file(history)
-                else:
-                    _pyrepl.readline.write_history_file(history)
-            except (FileNotFoundError, PermissionError):
+                if my_readline.get_current_history_length() > 0:
+                    my_readline.write_history_file(history)
+            except (FileNotFoundError, PermissionError,
+                    _pyrepl.unix_console.InvalidTerminal):
                 # home directory does not exist or is not writable
                 # https://bugs.python.org/issue19891
+                # or terminal doesn't have the required capability
                 pass
 
         atexit.register(write_history)

Lib/test/test_pyrepl/test_pyrepl.py

@@ -1,10 +1,12 @@
 import io
 import itertools
 import os
+import pathlib
 import rlcompleter
 import select
 import subprocess
 import sys
+import tempfile
 from unittest import TestCase, skipUnless
 from unittest.mock import patch
 from test.support import force_not_colorized
@@ -898,6 +900,21 @@ def test_python_basic_repl(self):
         self.assertNotIn("Exception", output)
         self.assertNotIn("Traceback", output)
 
+    def setUp(self):
+        self.hfile = tempfile.NamedTemporaryFile(delete=False)
+
+    def tearDown(self):
+        self.hfile.close()
+
+    def test_not_wiping_history_file(self):
+        env = os.environ.copy()
+        env.update({"PYTHON_HISTORY": self.hfile.name})
+        commands = "123\nspam\nexit()\n"
+        output, exit_code = self.run_repl(commands, env=env)
+        self.assertIn("123", output)
+        self.assertIn("spam", output)
+        self.assertNotEqual(pathlib.Path(self.hfile.name).stat().st_size, 0)
+
     def run_repl(self, repl_input: str | list[str], env: dict | None = None) -> tuple[str, int]:
         master_fd, slave_fd = pty.openpty()
         process = subprocess.Popen(