bpo-45613: Set `sqlite3.threadsafety` dynamically (GH-29227)

Use the compile-time selected default SQLite threaded mode to set the
DB-API 2.0 attribute 'threadsafety'

Mappings:
  - SQLITE_THREADSAFE=0 => threadsafety=0
  - SQLITE_THREADSAFE=1 => threadsafety=3
  - SQLITE_THREADSAFE=2 => threadsafety=1

1 files changed, 30 insertions, 0 deletions

diff --git a/Modules/_sqlite/module.c b/Modules/_sqlite/module.c
index 3bca33b..3b9f797 100644
--- a/Modules/_sqlite/module.c
+++ b/Modules/_sqlite/module.c
@@ -540,6 +540,28 @@ add_integer_constants(PyObject *module) {
     return 0;
 }
 
+/* Convert SQLite default threading mode (as set by the compile-time constant
+ * SQLITE_THREADSAFE) to the corresponding DB-API 2.0 (PEP 249) threadsafety
+ * level. */
+static int
+get_threadsafety(pysqlite_state *state)
+{
+    int mode = sqlite3_threadsafe();
+    switch (mode) {
+    case 0:        // Single-thread mode; threads may not share the module.
+        return 0;
+    case 1:        // Serialized mode; threads may share the module,
+        return 3;  // connections, and cursors.
+    case 2:        // Multi-thread mode; threads may share the module, but not
+        return 1;  // connections.
+    default:
+        PyErr_Format(state->InterfaceError,
+                     "Unable to interpret SQLite threadsafety mode. Got %d, "
+                     "expected 0, 1, or 2", mode);
+        return -1;
+    }
+}
+
 static int
 module_traverse(PyObject *module, visitproc visit, void *arg)
 {
@@ -689,6 +711,14 @@ module_exec(PyObject *module)
         goto error;
     }
 
+    int threadsafety = get_threadsafety(state);
+    if (threadsafety < 0) {
+        goto error;
+    }
+    if (PyModule_AddIntConstant(module, "threadsafety", threadsafety) < 0) {
+        goto error;
+    }
+
     /* initialize microprotocols layer */
     if (pysqlite_microprotocols_init(module) < 0) {
         goto error;