pythongh-105699: Use a _Py_hashtable_t for the PyModuleDef Cache (pythongh-106974)

This fixes a crasher due to a race condition, triggered infrequently when two isolated (own GIL) subinterpreters simultaneously initialize their sys or builtins modules.  The crash happened due the combination of the "detached" thread state we were using and the "last holder" logic we use for the GIL.  It turns out it's tricky to use the same thread state for different threads.  Who could have guessed?

We solve the problem by eliminating the one object we were still sharing between interpreters.  We replace it with a low-level hashtable, using the "raw" allocator to avoid tying it to the main interpreter.

We also remove the accommodations for "detached" thread states, which were a dubious idea to start with.

Author: ericsnowcurrently

Include/internal/pycore_import.h

@@ -5,6 +5,7 @@
 extern "C" {
 #endif
 
+#include "pycore_hashtable.h"     // _Py_hashtable_t
 #include "pycore_time.h"          // _PyTime_t
 
 extern int _PyImport_IsInitialized(PyInterpreterState *);
@@ -36,19 +37,15 @@ struct _import_runtime_state {
        See PyInterpreterState.modules_by_index for more info. */
     Py_ssize_t last_module_index;
     struct {
-        /* A thread state tied to the main interpreter,
-           used exclusively for when the extensions dict is access/modified
-           from an arbitrary thread. */
-        PyThreadState main_tstate;
-        /* A lock to guard the dict. */
+        /* A lock to guard the cache. */
         PyThread_type_lock mutex;
-        /* A dict mapping (filename, name) to PyModuleDef for modules.
+        /* The actual cache of (filename, name, PyModuleDef) for modules.
            Only legacy (single-phase init) extension modules are added
            and only if they support multiple initialization (m_size >- 0)
            or are imported in the main interpreter.
            This is initialized lazily in _PyImport_FixupExtensionObject().
            Modules are added there and looked up in _imp.find_extension(). */
-        PyObject *dict;
+        _Py_hashtable_t *hashtable;
     } extensions;
     /* Package context -- the full module name for package imports */
     const char * pkgcontext;

Include/internal/pycore_pystate.h

@@ -125,11 +125,6 @@ extern PyThreadState * _PyThreadState_New(PyInterpreterState *interp);
 extern void _PyThreadState_Bind(PyThreadState *tstate);
 extern void _PyThreadState_DeleteExcept(PyThreadState *tstate);
 
-extern void _PyThreadState_InitDetached(PyThreadState *, PyInterpreterState *);
-extern void _PyThreadState_ClearDetached(PyThreadState *);
-extern void _PyThreadState_BindDetached(PyThreadState *);
-extern void _PyThreadState_UnbindDetached(PyThreadState *);
-
 // Export for '_testinternalcapi' shared extension
 PyAPI_FUNC(PyObject*) _PyThreadState_GetDict(PyThreadState *tstate);
 

Include/internal/pycore_runtime_init.h

@@ -97,11 +97,6 @@ extern PyTypeObject _PyExc_MemoryError;
            in accordance with the specification. */ \
         .autoTSSkey = Py_tss_NEEDS_INIT, \
         .parser = _parser_runtime_state_INIT, \
-        .imports = { \
-            .extensions = { \
-                .main_tstate = _PyThreadState_INIT, \
-            }, \
-        }, \
         .ceval = { \
             .perf = _PyEval_RUNTIME_PERF_INIT, \
         }, \

Python/import.c

@@ -2,10 +2,12 @@
 
 #include "Python.h"
 
+#include "pycore_hashtable.h"     // _Py_hashtable_new_full()
 #include "pycore_import.h"        // _PyImport_BootstrapImp()
 #include "pycore_initconfig.h"    // _PyStatus_OK()
 #include "pycore_interp.h"        // struct _import_runtime_state
 #include "pycore_namespace.h"     // _PyNamespace_Type
+#include "pycore_object.h"        // _Py_SetImmortal()
 #include "pycore_pyerrors.h"      // _PyErr_SetString()
 #include "pycore_pyhash.h"        // _Py_KeyedHash()
 #include "pycore_pylifecycle.h"
@@ -905,172 +907,192 @@ extensions_lock_release(void)
    dictionary, to avoid loading shared libraries twice.
 */
 
+static void *
+hashtable_key_from_2_strings(PyObject *str1, PyObject *str2, const char sep)
+{
+    Py_ssize_t str1_len, str2_len;
+    const char *str1_data = PyUnicode_AsUTF8AndSize(str1, &str1_len);
+    const char *str2_data = PyUnicode_AsUTF8AndSize(str2, &str2_len);
+    if (str1_data == NULL || str2_data == NULL) {
+        return NULL;
+    }
+    /* Make sure sep and the NULL byte won't cause an overflow. */
+    assert(SIZE_MAX - str1_len - str2_len > 2);
+    size_t size = str1_len + 1 + str2_len + 1;
+
+    char *key = PyMem_RawMalloc(size);
+    if (key == NULL) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+
+    strncpy(key, str1_data, str1_len);
+    key[str1_len] = sep;
+    strncpy(key + str1_len + 1, str2_data, str2_len + 1);
+    assert(strlen(key) == size - 1);
+    return key;
+}
+
+static Py_uhash_t
+hashtable_hash_str(const void *key)
+{
+    return _Py_HashBytes(key, strlen((const char *)key));
+}
+
+static int
+hashtable_compare_str(const void *key1, const void *key2)
+{
+    return strcmp((const char *)key1, (const char *)key2) == 0;
+}
+
 static void
-_extensions_cache_init(void)
+hashtable_destroy_str(void *ptr)
 {
-    /* The runtime (i.e. main interpreter) must be initializing,
-       so we don't need to worry about the lock. */
-    _PyThreadState_InitDetached(&EXTENSIONS.main_tstate,
-                                _PyInterpreterState_Main());
+    PyMem_RawFree(ptr);
 }
 
+#define HTSEP ':'
+
 static PyModuleDef *
 _extensions_cache_get(PyObject *filename, PyObject *name)
 {
     PyModuleDef *def = NULL;
+    void *key = NULL;
     extensions_lock_acquire();
 
-    PyObject *key = PyTuple_Pack(2, filename, name);
-    if (key == NULL) {
+    if (EXTENSIONS.hashtable == NULL) {
         goto finally;
     }
 
-    PyObject *extensions = EXTENSIONS.dict;
-    if (extensions == NULL) {
+    key = hashtable_key_from_2_strings(filename, name, HTSEP);
+    if (key == NULL) {
+        goto finally;
+    }
+    _Py_hashtable_entry_t *entry = _Py_hashtable_get_entry(
+            EXTENSIONS.hashtable, key);
+    if (entry == NULL) {
         goto finally;
     }
-    def = (PyModuleDef *)PyDict_GetItemWithError(extensions, key);
+    def = (PyModuleDef *)entry->value;
 
 finally:
-    Py_XDECREF(key);
     extensions_lock_release();
+    if (key != NULL) {
+        PyMem_RawFree(key);
+    }
     return def;
 }
 
 static int
 _extensions_cache_set(PyObject *filename, PyObject *name, PyModuleDef *def)
 {
     int res = -1;
-    PyThreadState *oldts = NULL;
     extensions_lock_acquire();
 
-    /* Swap to the main interpreter, if necessary.  This matters if
-       the dict hasn't been created yet or if the item isn't in the
-       dict yet.  In both cases we must ensure the relevant objects
-       are created using the main interpreter. */
-    PyThreadState *main_tstate = &EXTENSIONS.main_tstate;
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (!_Py_IsMainInterpreter(interp)) {
-        _PyThreadState_BindDetached(main_tstate);
-        oldts = _PyThreadState_Swap(interp->runtime, main_tstate);
-        assert(!_Py_IsMainInterpreter(oldts->interp));
-
-        /* Make sure the name and filename objects are owned
-           by the main interpreter. */
-        name = PyUnicode_InternFromString(PyUnicode_AsUTF8(name));
-        assert(name != NULL);
-        filename = PyUnicode_InternFromString(PyUnicode_AsUTF8(filename));
-        assert(filename != NULL);
+    if (EXTENSIONS.hashtable == NULL) {
+        _Py_hashtable_allocator_t alloc = {PyMem_RawMalloc, PyMem_RawFree};
+        EXTENSIONS.hashtable = _Py_hashtable_new_full(
+            hashtable_hash_str,
+            hashtable_compare_str,
+            hashtable_destroy_str,  // key
+            /* There's no need to decref the def since it's immortal. */
+            NULL,  // value
+            &alloc
+        );
+        if (EXTENSIONS.hashtable == NULL) {
+            PyErr_NoMemory();
+            goto finally;
+        }
     }
 
-    PyObject *key = PyTuple_Pack(2, filename, name);
+    void *key = hashtable_key_from_2_strings(filename, name, HTSEP);
     if (key == NULL) {
         goto finally;
     }
 
-    PyObject *extensions = EXTENSIONS.dict;
-    if (extensions == NULL) {
-        extensions = PyDict_New();
-        if (extensions == NULL) {
+    int already_set = 0;
+    _Py_hashtable_entry_t *entry = _Py_hashtable_get_entry(
+            EXTENSIONS.hashtable, key);
+    if (entry == NULL) {
+        if (_Py_hashtable_set(EXTENSIONS.hashtable, key, def) < 0) {
+            PyMem_RawFree(key);
+            PyErr_NoMemory();
             goto finally;
         }
-        EXTENSIONS.dict = extensions;
-    }
-
-    PyModuleDef *actual = (PyModuleDef *)PyDict_GetItemWithError(extensions, key);
-    if (PyErr_Occurred()) {
-        goto finally;
     }
-    else if (actual != NULL) {
-        /* We expect it to be static, so it must be the same pointer. */
-        assert(def == actual);
-        res = 0;
-        goto finally;
+    else {
+        if (entry->value == NULL) {
+            entry->value = def;
+        }
+        else {
+            /* We expect it to be static, so it must be the same pointer. */
+            assert((PyModuleDef *)entry->value == def);
+            already_set = 1;
+        }
+        PyMem_RawFree(key);
     }
-
-    /* This might trigger a resize, which is why we must switch
-       to the main interpreter. */
-    res = PyDict_SetItem(extensions, key, (PyObject *)def);
-    if (res < 0) {
-        res = -1;
-        goto finally;
+    if (!already_set) {
+        /* We assume that all module defs are statically allocated
+           and will never be freed.  Otherwise, we would incref here. */
+        _Py_SetImmortal(def);
     }
     res = 0;
 
 finally:
-    Py_XDECREF(key);
-    if (oldts != NULL) {
-        _PyThreadState_Swap(interp->runtime, oldts);
-        _PyThreadState_UnbindDetached(main_tstate);
-        Py_DECREF(name);
-        Py_DECREF(filename);
-    }
     extensions_lock_release();
     return res;
 }
 
-static int
+static void
 _extensions_cache_delete(PyObject *filename, PyObject *name)
 {
-    int res = -1;
-    PyThreadState *oldts = NULL;
+    void *key = NULL;
     extensions_lock_acquire();
 
-    PyObject *key = PyTuple_Pack(2, filename, name);
-    if (key == NULL) {
+    if (EXTENSIONS.hashtable == NULL) {
+        /* It was never added. */
         goto finally;
     }
 
-    PyObject *extensions = EXTENSIONS.dict;
-    if (extensions == NULL) {
-        res = 0;
+    key = hashtable_key_from_2_strings(filename, name, HTSEP);
+    if (key == NULL) {
         goto finally;
     }
 
-    PyModuleDef *actual = (PyModuleDef *)PyDict_GetItemWithError(extensions, key);
-    if (PyErr_Occurred()) {
+    _Py_hashtable_entry_t *entry = _Py_hashtable_get_entry(
+            EXTENSIONS.hashtable, key);
+    if (entry == NULL) {
+        /* It was never added. */
         goto finally;
     }
-    else if (actual == NULL) {
-        /* It was already removed or never added. */
-        res = 0;
+    if (entry->value == NULL) {
+        /* It was already removed. */
         goto finally;
     }
-
-    /* Swap to the main interpreter, if necessary. */
-    PyThreadState *main_tstate = &EXTENSIONS.main_tstate;
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (!_Py_IsMainInterpreter(interp)) {
-        _PyThreadState_BindDetached(main_tstate);
-        oldts = _PyThreadState_Swap(interp->runtime, main_tstate);
-        assert(!_Py_IsMainInterpreter(oldts->interp));
-    }
-
-    if (PyDict_DelItem(extensions, key) < 0) {
-        goto finally;
-    }
-    res = 0;
+    /* If we hadn't made the stored defs immortal, we would decref here.
+       However, this decref would be problematic if the module def were
+       dynamically allocated, it were the last ref, and this function
+       were called with an interpreter other than the def's owner. */
+    entry->value = NULL;
 
 finally:
-    if (oldts != NULL) {
-        _PyThreadState_Swap(interp->runtime, oldts);
-        _PyThreadState_UnbindDetached(main_tstate);
-    }
-    Py_XDECREF(key);
     extensions_lock_release();
-    return res;
+    if (key != NULL) {
+        PyMem_RawFree(key);
+    }
 }
 
 static void
 _extensions_cache_clear_all(void)
 {
     /* The runtime (i.e. main interpreter) must be finalizing,
        so we don't need to worry about the lock. */
-    // XXX assert(_Py_IsMainInterpreter(_PyInterpreterState_GET()));
-    Py_CLEAR(EXTENSIONS.dict);
-    _PyThreadState_ClearDetached(&EXTENSIONS.main_tstate);
+    _Py_hashtable_destroy(EXTENSIONS.hashtable);
+    EXTENSIONS.hashtable = NULL;
 }
 
+#undef HTSEP
+
 
 static bool
 check_multi_interp_extensions(PyInterpreterState *interp)
@@ -1231,6 +1253,8 @@ import_find_extension(PyThreadState *tstate, PyObject *name,
         PyObject *m_copy = def->m_base.m_copy;
         /* Module does not support repeated initialization */
         if (m_copy == NULL) {
+            /* It might be a core module (e.g. sys & builtins),
+               for which we don't set m_copy. */
             m_copy = get_core_module_dict(tstate->interp, name, filename);
             if (m_copy == NULL) {
                 return NULL;
@@ -1300,9 +1324,7 @@ clear_singlephase_extension(PyInterpreterState *interp,
     }
 
     /* Clear the cached module def. */
-    if (_extensions_cache_delete(filename, name) < 0) {
-        return -1;
-    }
+    _extensions_cache_delete(filename, name);
 
     return 0;
 }
@@ -3051,6 +3073,8 @@ void
 _PyImport_Fini(void)
 {
     /* Destroy the database used by _PyImport_{Fixup,Find}Extension */
+    // XXX Should we actually leave them (mostly) intact, since we don't
+    // ever dlclose() the module files?
     _extensions_cache_clear_all();
 
     /* Use the same memory allocator as _PyImport_Init(). */
@@ -3088,10 +3112,6 @@ _PyImport_Fini2(void)
 PyStatus
 _PyImport_InitCore(PyThreadState *tstate, PyObject *sysmod, int importlib)
 {
-    if (_Py_IsMainInterpreter(tstate->interp)) {
-        _extensions_cache_init();
-    }
-
     // XXX Initialize here: interp->modules and interp->import_func.
     // XXX Initialize here: sys.modules and sys.meta_path.