Default max_spare_txns on cpython impl to 0.

Also, don't create spare transactions if the above parameter is true.

This prevents hanging open transactions across forks. This is exhibited
in a MDB_BAD_RSLOT error.

Fixes #346.

ChangeLog

@@ -31,7 +31,7 @@
 * Add Python 3.10 support.
 
 * Fix crash relating to caching of transactions.  The 'max_spare_txns'
-  parameter to Environment/open is currently ignored.
+  parameter to Environment/open is currently ignored in cpython.
 
 2021-04-19 v1.2.1
 * Resolve CI bug where non-Linux wheels were not being published to PyPI.

lmdb/__init__.py

@@ -50,4 +50,4 @@ def _reading_docs():
     from lmdb.cffi import __all__
     from lmdb.cffi import __doc__
 
-__version__ = '1.6.2'
+__version__ = '1.7.0.dev0'

lmdb/cpython.c

@@ -168,6 +168,9 @@ struct EnvObject {
     int readonly;
     /** 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;
 };
 
 /** TransObject.flags bitfield values. */
@@ -1162,7 +1165,7 @@ env_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         int max_dbs;
         int max_spare_txns;
         int lock;
-    } arg = {NULL, 10485760, 1, 0, 1, 1, 0, 0755, 1, 1, 0, 1, 126, 0, 1, 1};
+    } arg = {NULL, 10485760, 1, 0, 1, 1, 0, 0755, 1, 1, 0, 1, 126, 0, 0, 1};
 
     static const struct argspec argspec[] = {
         {"path", ARG_OBJ, OFFSET(env_new, path)},
@@ -1180,7 +1183,7 @@ env_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         {"max_readers", ARG_INT, OFFSET(env_new, max_readers)},
         {"max_dbs", ARG_INT, OFFSET(env_new, max_dbs)},
         {"max_spare_txns", ARG_INT, OFFSET(env_new, max_spare_txns)},
-        {"lock", ARG_BOOL, OFFSET(env_new, lock)}
+        {"lock", ARG_BOOL, OFFSET(env_new, lock)},
     };
 
     PyObject *fspath_obj = NULL;
@@ -1208,6 +1211,7 @@ env_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     self->main_db = NULL;
     self->env = NULL;
     self->spare_txn = NULL;
+    self->max_spare_txns = arg.max_spare_txns;
 
     if((rc = mdb_env_create(&self->env))) {
         err_set("mdb_env_create", rc);
@@ -3213,8 +3217,8 @@ trans_dealloc(TransObject *self)
         PyObject_ClearWeakRefs((PyObject *) self);
     }
 
-    if(txn && self->env && !self->env->spare_txn &&
-      (self->flags & TRANS_RDONLY)) {
+    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;

tests/fork_test.py

@@ -0,0 +1,28 @@
+import lmdb
+import os
+import time
+
+print("      python: Opening environment")
+env = lmdb.Environment('db.lmdb')
+print("      python: Opened environment %r" % env)
+print()
+
+print("      python: Starting transaction")
+txn = env.begin(write=False)
+print("      python: Started transaction %r" % txn)
+print()
+
+print("      python: Deleting transaction")
+del txn
+print("      python: Deleted transaction")
+print()
+
+if (os.fork() != 0):
+    os.wait()
+    print("      python: Exited forked process")
+    print()
+    print("      python: Starting transaction (no. 2)")
+    txn = env.begin(write=False)
+    print("      python: Started transaction (no. 2) %r" % txn)
+else:
+    print("      python: Inside forked process")