Grab most of the changes suggested in #363.

Capture the PID at environment creation time and don't delete
transactions when we're not on that PID.  This will catch some common
accidental misuses.

lmdb/cpython.c

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2024 The py-lmdb authors, all rights reserved.
+ * Copyright 2013-2025 The py-lmdb authors, all rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted only as authorized by the OpenLDAP
@@ -166,11 +166,13 @@ struct EnvObject {
     DbObject *main_db;
     /**  1 if env opened read-only; transactions must always be read-only. */
     int readonly;
-    /** Spare read-only transaction . */
+    /** Spare read-only transaction. */
     struct MDB_txn *spare_txn;
     /** Maximum number of spare transactions.  In cpython only 0 and 1 are supported.
      *  If process will be forked, this must be set to 0. */
     int max_spare_txns;
+    /** Process ID of the process this Environment was opened in. */
+    pid_t pid;
 };
 
 /** TransObject.flags bitfield values. */
@@ -1212,6 +1214,7 @@ env_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     self->env = NULL;
     self->spare_txn = NULL;
     self->max_spare_txns = arg.max_spare_txns;
+    self->pid = getpid();
 
     if((rc = mdb_env_create(&self->env))) {
         err_set("mdb_env_create", rc);
@@ -3217,16 +3220,21 @@ trans_dealloc(TransObject *self)
         PyObject_ClearWeakRefs((PyObject *) self);
     }
 
-    if(txn && self->env && !self->env->spare_txn && 
-            self->env->max_spare_txns && (self->flags & TRANS_RDONLY)) {
-        MDEBUG("caching trans")
-        mdb_txn_reset(txn);
-        self->env->spare_txn = txn;
-        self->txn = NULL;
+    if(self->env && self->env->pid == getpid()) {
+        if(txn && self->env && !self->env->spare_txn && 
+                self->env->max_spare_txns && (self->flags & TRANS_RDONLY)) {
+            MDEBUG("caching trans")
+            mdb_txn_reset(txn);
+            self->env->spare_txn = txn;
+            self->txn = NULL;
+        }
+        MDEBUG("deleting trans")
+        trans_clear(self);
+    }
+    else {
+       MDEBUG("In forked process, not deleting trans");
     }
 
-    MDEBUG("deleting trans")
-    trans_clear(self);
     PyObject_Del(self);
 }
 

tests/env_test.py

@@ -826,7 +826,6 @@ class SpareTxnTest(unittest.TestCase):
     def tearDown(self):
         testlib.cleanup()
 
-    @unittest.skip('Temporarily removed this functionality')
     def test_none(self):
         _, env = testlib.temp_env(max_spare_txns=0)
         assert 0 == reader_count(env)

tests/txn_test.py

@@ -40,7 +40,7 @@
 ULONG_0002 = struct.pack('L', 2)  # L != size_t
 
 
-class InitTest(unittest.TestCase):
+class InitTest(testlib.LmdbTest):
     def tearDown(self):
         testlib.cleanup()
 
@@ -141,14 +141,40 @@ def test_buffers(self):
         txn.abort()
 
     @unittest.skipIf(sys.platform.startswith('win'), "No fork on Windows")
-    def test_fork(self):
+    def test_cached_txn_across_fork(self):
         _, env = testlib.temp_env()
         txn = env.begin(write=False)
         del txn
 
-        if (os.fork() != 0):
-            os.wait()
-            txn = env.begin(write=False)
+        if os.fork() != 0:
+            # I am the parent
+            os.wait()  # Wait for child to finish
+            txn = env.begin(write=False)  # Used to raise MDB_BAD_RSLOT (#346)
+
+    @unittest.skipIf(sys.platform.startswith('win'), "No fork on Windows")
+    def test_child_deleting_transaction(self):
+        _, env = testlib.temp_env()
+        with env.begin(write=True) as txn:
+            txn.put(b'a', b'foo')
+        txn = env.begin()
+
+        r = env.readers()
+        assert r != '(no active readers)\n'
+
+        pid = os.fork()
+        if pid == 0:
+            del txn   # I am the child
+            os._exit(0)
+
+        os.waitpid(pid, 0)
+        # Make sure my child didn't mess with my environment
+        r2 = env.readers()
+        assert r2 == r, '%r != %r' % (r2, r)
+        assert txn.get(b'a') == b'foo'
+        print 
+        del txn
+        r3 = env.readers()
+        print(r, r2, r3)
 
 
 class ContextManagerTest(unittest.TestCase):