Stop monkeypatching `linecache`

It turns out this has never been necessary if we just change the cache
entry to forbid expiry in the first place.

IPython/core/compilerop.py

@@ -73,25 +73,6 b' class CachingCompiler(codeop.Compile):'
     def __init__(self):
         codeop.Compile.__init__(self)
 
-        # This is ugly, but it must be done this way to allow multiple
-        # simultaneous ipython instances to coexist.  Since Python itself
-        # directly accesses the data structures in the linecache module, and
-        # the cache therein is global, we must work with that data structure.
-        # We must hold a reference to the original checkcache routine and call
-        # that in our own check_cache() below, but the special IPython cache
-        # must also be shared by all IPython instances.  If we were to hold
-        # separate caches (one in each CachingCompiler instance), any call made
-        # by Python itself to linecache.checkcache() would obliterate the
-        # cached data from the other IPython instances.
-        if not hasattr(linecache, '_ipython_cache'):
-            linecache._ipython_cache = {}
-        if not hasattr(linecache, '_checkcache_ori'):
-            linecache._checkcache_ori = linecache.checkcache
-        # Now, we must monkeypatch the linecache directly so that parts of the
-        # stdlib that call it outside our control go through our codepath
-        # (otherwise we'd lose our tracebacks).
-        linecache.checkcache = check_linecache_ipython
-
         # Caching a dictionary { filename: execution_count } for nicely
         # rendered tracebacks. The filename corresponds to the filename
         # argument used for the builtins.compile function.
@@ -161,14 +142,24 b' class CachingCompiler(codeop.Compile):'
         # Save the execution count
         self._filename_map[name] = number
 
+        # Since Python 2.5, setting mtime to `None` means the lines will
+        # never be removed by `linecache.checkcache`.  This means all the
+        # monkeypatching has *never* been necessary, since this code was
+        # only added in 2010, at which point IPython had already stopped
+        # supporting Python 2.4.
+        #
+        # Note that `linecache.clearcache` and `linecache.updatecache` may
+        # still remove our code from the cache, but those show explicit
+        # intent, and we should not try to interfere.  Normally the former
+        # is never called except when out of memory, and the latter is only
+        # called for lines *not* in the cache.
         entry = (
             len(transformed_code),
-            time.time(),
+            None,
             [line + "\n" for line in transformed_code.splitlines()],
             name,
         )
         linecache.cache[name] = entry
-        linecache._ipython_cache[name] = entry
         return name
 
     @contextmanager
@@ -187,10 +178,20 b' class CachingCompiler(codeop.Compile):'
 
 
 def check_linecache_ipython(*args):
-    """Call linecache.checkcache() safely protecting our cached values.
+    """Deprecated since IPython 8.6.  Call linecache.checkcache() directly.
+
+    It was already not necessary to call this function directly.  If no
+    CachingCompiler had been created, this function would fail badly.  If
+    an instance had been created, this function would've been monkeypatched
+    into place.
+
+    As of IPython 8.6, the monkeypatching has gone away entirely.  But there
+    were still internal callers of this function, so maybe external callers
+    also existed?
     """
-    # First call the original checkcache as intended
-    linecache._checkcache_ori(*args)
-    # Then, update back the cache with our data, so that tracebacks related
-    # to our compiled codes can be produced.
-    linecache.cache.update(linecache._ipython_cache)
+    import warnings
+    warnings.warn(
+        'Just call linecache.checkcache() directly.',
+        DeprecationWarning
+    )
+    linecache.checkcache()

IPython/core/interactiveshell.py

@@ -61,7 +61,7 b' from IPython.core import magic, oinspect, page, prefilter, ultratb'
 from IPython.core.alias import Alias, AliasManager
 from IPython.core.autocall import ExitAutocall
 from IPython.core.builtin_trap import BuiltinTrap
-from IPython.core.compilerop import CachingCompiler, check_linecache_ipython
+from IPython.core.compilerop import CachingCompiler
 from IPython.core.debugger import InterruptiblePdb
 from IPython.core.display_trap import DisplayTrap
 from IPython.core.displayhook import DisplayHook
@@ -1810,7 +1810,6 b' class InteractiveShell(SingletonConfigurable):'
         self.InteractiveTB = ultratb.AutoFormattedTB(mode = 'Plain',
                                                      color_scheme='NoColor',
                                                      tb_offset = 1,
-                                   check_cache=check_linecache_ipython,
                                    debugger_cls=self.debugger_cls, parent=self)
 
         # The instance will store a pointer to the system-wide exception hook,

IPython/core/ultratb.py

@@ -644,10 +644,8 b' class VerboseTB(TBTools):'
         self.long_header = long_header
         self.include_vars = include_vars
         # By default we use linecache.checkcache, but the user can provide a
-        # different check_cache implementation.  This is used by the IPython
-        # kernel to provide tracebacks for interactive code that is cached,
-        # by a compiler instance that flushes the linecache but preserves its
-        # own code cache.
+        # different check_cache implementation.  This was formerly used by the
+        # IPython kernel for interactive code, but is no longer necessary.
         if check_cache is None:
             check_cache = linecache.checkcache
         self.check_cache = check_cache