[2.2.x] Fixed #30171 -- Fixed DatabaseError in servers tests.

Made DatabaseWrapper thread sharing logic reentrant. Used a reference
counting like scheme to allow nested uses.

The error appeared after 8c775391b78b2a4a2b57c5e89ed4888f36aada4b.
Backport of 76990cbbda5d93fda560c8a5ab019860f7efaab7 from master.

3 files changed, 28 insertions, 20 deletions

diff --git a/django/db/backends/base/base.py b/django/db/backends/base/base.py
index f97d171c96..9fa03cc0ee 100644
--- a/django/db/backends/base/base.py
+++ b/django/db/backends/base/base.py
@@ -1,4 +1,5 @@
 import copy
+import threading
 import time
 import warnings
 from collections import deque
@@ -43,8 +44,7 @@ class BaseDatabaseWrapper:
 
     queries_limit = 9000
 
-    def __init__(self, settings_dict, alias=DEFAULT_DB_ALIAS,
-                 allow_thread_sharing=False):
+    def __init__(self, settings_dict, alias=DEFAULT_DB_ALIAS):
         # Connection related attributes.
         # The underlying database connection.
         self.connection = None
@@ -80,7 +80,8 @@ class BaseDatabaseWrapper:
         self.errors_occurred = False
 
         # Thread-safety related attributes.
-        self.allow_thread_sharing = allow_thread_sharing
+        self._thread_sharing_lock = threading.Lock()
+        self._thread_sharing_count = 0
         self._thread_ident = _thread.get_ident()
 
         # A list of no-argument functions to run when the transaction commits.
@@ -515,12 +516,27 @@ class BaseDatabaseWrapper:
 
     # ##### Thread safety handling #####
 
+    @property
+    def allow_thread_sharing(self):
+        with self._thread_sharing_lock:
+            return self._thread_sharing_count > 0
+
+    def inc_thread_sharing(self):
+        with self._thread_sharing_lock:
+            self._thread_sharing_count += 1
+
+    def dec_thread_sharing(self):
+        with self._thread_sharing_lock:
+            if self._thread_sharing_count <= 0:
+                raise RuntimeError('Cannot decrement the thread sharing count below zero.')
+            self._thread_sharing_count -= 1
+
     def validate_thread_sharing(self):
         """
         Validate that the connection isn't accessed by another thread than the
         one which originally created it, unless the connection was explicitly
-        authorized to be shared between threads (via the `allow_thread_sharing`
-        property). Raise an exception if the validation fails.
+        authorized to be shared between threads (via the `inc_thread_sharing()`
+        method). Raise an exception if the validation fails.
         """
         if not (self.allow_thread_sharing or self._thread_ident == _thread.get_ident()):
             raise DatabaseError(
@@ -589,11 +605,7 @@ class BaseDatabaseWrapper:
         potential child threads while (or after) the test database is destroyed.
         Refs #10868, #17786, #16969.
         """
-        return self.__class__(
-            {**self.settings_dict, 'NAME': None},
-            alias=NO_DB_ALIAS,
-            allow_thread_sharing=False,
-        )
+        return self.__class__({**self.settings_dict, 'NAME': None}, alias=NO_DB_ALIAS)
 
     def schema_editor(self, *args, **kwargs):
         """
@@ -635,7 +647,7 @@ class BaseDatabaseWrapper:
         finally:
             self.execute_wrappers.pop()
 
-    def copy(self, alias=None, allow_thread_sharing=None):
+    def copy(self, alias=None):
         """
         Return a copy of this connection.
 
@@ -644,6 +656,4 @@ class BaseDatabaseWrapper:
         settings_dict = copy.deepcopy(self.settings_dict)
         if alias is None:
             alias = self.alias
-        if allow_thread_sharing is None:
-            allow_thread_sharing = self.allow_thread_sharing
-        return type(self)(settings_dict, alias, allow_thread_sharing)
+        return type(self)(settings_dict, alias)
diff --git a/django/db/backends/postgresql/base.py b/django/db/backends/postgresql/base.py
index e9db668a4d..4376b8609c 100644
--- a/django/db/backends/postgresql/base.py
+++ b/django/db/backends/postgresql/base.py
@@ -277,7 +277,6 @@ class DatabaseWrapper(BaseDatabaseWrapper):
                     return self.__class__(
                         {**self.settings_dict, 'NAME': connection.settings_dict['NAME']},
                         alias=self.alias,
-                        allow_thread_sharing=False,
                     )
         return nodb_connection
 
diff --git a/django/test/testcases.py b/django/test/testcases.py
index 991165c04d..dea7fedbcc 100644
--- a/django/test/testcases.py
+++ b/django/test/testcases.py
@@ -1442,7 +1442,7 @@ class LiveServerTestCase(TransactionTestCase):
             # the server thread.
             if conn.vendor == 'sqlite' and conn.is_in_memory_db():
                 # Explicitly enable thread-shareability for this connection
-                conn.allow_thread_sharing = True
+                conn.inc_thread_sharing()
                 connections_override[conn.alias] = conn
 
         cls._live_server_modified_settings = modify_settings(
@@ -1478,10 +1478,9 @@ class LiveServerTestCase(TransactionTestCase):
             # Terminate the live server's thread
             cls.server_thread.terminate()
 
-        # Restore sqlite in-memory database connections' non-shareability
-        for conn in connections.all():
-            if conn.vendor == 'sqlite' and conn.is_in_memory_db():
-                conn.allow_thread_sharing = False
+            # Restore sqlite in-memory database connections' non-shareability.
+            for conn in cls.server_thread.connections_override.values():
+                conn.dec_thread_sharing()
 
     @classmethod
     def tearDownClass(cls):