gh-109853: Fix sys.path[0] For Subinterpreters (gh-109994)

This change makes sure sys.path[0] is set properly for subinterpreters. Before, it wasn't getting set at all. This PR does not address the broader concerns from gh-109853.

2 files changed, 28 insertions, 0 deletions

diff --git a/Python/initconfig.c b/Python/initconfig.c
index 089ede4..6b76b4d 100644
--- a/Python/initconfig.c
+++ b/Python/initconfig.c
@@ -97,6 +97,7 @@ static const PyConfigSpec PYCONFIG_SPEC[] = {
     SPEC(pythonpath_env, WSTR_OPT),
     SPEC(home, WSTR_OPT),
     SPEC(platlibdir, WSTR),
+    SPEC(sys_path_0, WSTR_OPT),
     SPEC(module_search_paths_set, UINT),
     SPEC(module_search_paths, WSTR_LIST),
     SPEC(stdlib_dir, WSTR_OPT),
@@ -770,6 +771,7 @@ PyConfig_Clear(PyConfig *config)
     CLEAR(config->exec_prefix);
     CLEAR(config->base_exec_prefix);
     CLEAR(config->platlibdir);
+    CLEAR(config->sys_path_0);
 
     CLEAR(config->filesystem_encoding);
     CLEAR(config->filesystem_errors);
@@ -3051,6 +3053,7 @@ _Py_DumpPathConfig(PyThreadState *tstate)
     PySys_WriteStderr("  import site = %i\n", config->site_import);
     PySys_WriteStderr("  is in build tree = %i\n", config->_is_python_build);
     DUMP_CONFIG("stdlib dir", stdlib_dir);
+    DUMP_CONFIG("sys.path[0]", sys_path_0);
 #undef DUMP_CONFIG
 
 #define DUMP_SYS(NAME) \
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index f3ed77e..c032376 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -1209,6 +1209,31 @@ init_interp_main(PyThreadState *tstate)
         }
     }
 
+    if (!is_main_interp) {
+        // The main interpreter is handled in Py_Main(), for now.
+        if (config->sys_path_0 != NULL) {
+            PyObject *path0 = PyUnicode_FromWideChar(config->sys_path_0, -1);
+            if (path0 == NULL) {
+                return _PyStatus_ERR("can't initialize sys.path[0]");
+            }
+            PyObject *sysdict = interp->sysdict;
+            if (sysdict == NULL) {
+                Py_DECREF(path0);
+                return _PyStatus_ERR("can't initialize sys.path[0]");
+            }
+            PyObject *sys_path = PyDict_GetItemWithError(sysdict, &_Py_ID(path));
+            if (sys_path == NULL) {
+                Py_DECREF(path0);
+                return _PyStatus_ERR("can't initialize sys.path[0]");
+            }
+            int res = PyList_Insert(sys_path, 0, path0);
+            Py_DECREF(path0);
+            if (res) {
+                return _PyStatus_ERR("can't initialize sys.path[0]");
+            }
+        }
+    }
+
     assert(!_PyErr_Occurred(tstate));
 
     return _PyStatus_OK();