temporal: add test for tgis.init() across a session change

pyproject.toml

@@ -522,6 +522,7 @@ docstring-code-format = true
 
 [tool.pytest.ini_options]
 addopts = """
+    --import-mode=importlib
     --ignore-glob='dist.*'
     --ignore-glob='bin.*'
     --ignore-glob='*/tests/data/*'

python/grass/temporal/core.py

@@ -596,6 +596,7 @@ def init(raise_fatal_error: bool = False, skip_db_version_check: bool = False):
     global raise_on_error  # noqa: FURB154
     global enable_mapset_check, enable_timestamp_write  # noqa: FURB154
     global current_mapset, current_location, current_gisdbase  # noqa: FURB154
+    global message_interface, c_library_interface  # noqa: FURB154
 
     raise_on_error = raise_fatal_error
 
@@ -604,10 +605,30 @@ def init(raise_fatal_error: bool = False, skip_db_version_check: bool = False):
     gs.run_command("t.connect", flags="c")
     grassenv = gs.gisenv()
 
+    new_mapset = grassenv["MAPSET"]
+    new_location = grassenv["LOCATION_NAME"]
+    new_gisdbase = grassenv["GISDBASE"]
+
+    # The message and C-library subprocesses inherit GISRC at spawn time,
+    # so a session change leaves them bound to the old (possibly deleted)
+    # session. Stop them so the _init_* helpers below respawn them in the
+    # current environment.
+    if current_mapset is not None and (
+        current_mapset != new_mapset
+        or current_location != new_location
+        or current_gisdbase != new_gisdbase
+    ):
+        if message_interface is not None:
+            message_interface.stop()
+            message_interface = None
+        if c_library_interface is not None:
+            c_library_interface.stop()
+            c_library_interface = None
+
     # Set the global variable for faster access
-    current_mapset = grassenv["MAPSET"]
-    current_location = grassenv["LOCATION_NAME"]
-    current_gisdbase = grassenv["GISDBASE"]
+    current_mapset = new_mapset
+    current_location = new_location
+    current_gisdbase = new_gisdbase
 
     # Check environment variable GRASS_TGIS_RAISE_ON_ERROR
     if (

python/grass/temporal/tests/grass_temporal_core_init_session_change_test.py

@@ -0,0 +1,47 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+"""Regression test for tgis.init() across a GRASS session change.
+
+tgis.init() spawns message and C-library subprocesses that capture
+GISRC at spawn time. When the parent process switches to a different
+project (different LOCATION_NAME or GISDBASE) and calls tgis.init()
+again, those subprocesses must be restarted; otherwise they remain
+bound to the previous session's GISRC, which may have been deleted.
+"""
+
+import pytest
+
+from grass.grassdb.create import create_mapset
+import grass.script as gs
+import grass.temporal as tgis
+
+
+@pytest.fixture
+def two_projects(tmp_path):
+    """Create two empty projects and yield their paths."""
+    project_a = tmp_path / "a"
+    project_b = tmp_path / "b"
+    gs.create_project(project_a)
+    create_mapset(project_a / "a")
+    gs.create_project(project_b)
+    create_mapset(project_b / "b")
+    return project_a, project_b
+
+
+def test_init_restarts_subprocesses_on_session_change(two_projects):
+    """tgis.init() must respawn its subprocesses after a session change."""
+    project_a, project_b = two_projects
+    with gs.setup.init(project_a / "a"):
+        tgis.init()
+        first_ciface = tgis.get_tgis_c_library_interface()
+        assert first_ciface.available_mapsets() == ["a", "PERMANENT"]
+
+    with gs.setup.init(project_b / "b"):
+        tgis.init()
+        second_ciface = tgis.get_tgis_c_library_interface()
+
+        # The C-library interface must have been replaced. Without this,
+        # the subprocess is still bound to session A's deleted GISRC and
+        # the call below fails.
+        assert second_ciface is not first_ciface
+        assert second_ciface.available_mapsets() == ["b", "PERMANENT"]