Python: flush stdout and stderr after running Python script

When debugging with gdb in vscode, the stuff I print when executing a script in
the text editor does not show up in the terminal. It does work when I flush
explicitly though using `print(..., flush=True)`. This is quite annoying.

The solution is to always flush `stdout` and `stderr` automatically when running
a script. This is done using the CPython API, as just using
`fflush(stdout/stderr)` did not solve the issue.

Pull Request: https://projects.blender.org/blender/blender/pulls/136632

source/blender/python/generic/py_capi_utils.cc

@@ -1687,6 +1687,46 @@ bool PyC_RunString_AsStringOrNone(const char *imports[],
 
 #endif /* #ifndef MATH_STANDALONE */
 
+/* -------------------------------------------------------------------- */
+/** \name Std Files Flush
+ *
+ * \{ */
+
+void PyC_StdFilesFlush()
+{
+  /* This is ported from CPython's internal #flush_std_files (2025-03-31). The code is a bit
+   * different because the original code uses some internal APIs.
+   *
+   * This is approximately equivalent to:
+   * ```
+   * try:
+   *     sys.stdout.flush()
+   *     sys.stderr.flush()
+   * except Exception:
+   *     pass
+   * ```
+   */
+  PyObject *py_flush = PyUnicode_FromString("flush");
+  BLI_assert(py_flush);
+  for (const char *name : {"stdout", "stderr"}) {
+    PyObject *py_file = PySys_GetObject(name);
+    if (!py_file) {
+      PyErr_Clear();
+      continue;
+    }
+    PyObject *py_flush_retval = PyObject_CallMethodNoArgs(py_file, py_flush);
+    if (py_flush_retval) {
+      Py_DECREF(py_flush_retval);
+    }
+    else {
+      PyErr_Clear();
+    }
+  }
+  Py_DECREF(py_flush);
+}
+
+/** \} */
+
 /* -------------------------------------------------------------------- */
 /** \name Int Conversion
  *

source/blender/python/generic/py_capi_utils.hh

@@ -259,6 +259,11 @@ bool PyC_RunString_AsStringOrNone(const char **imports,
                                   const char *filename,
                                   char **r_value) ATTR_NONNULL(2, 3, 4) ATTR_WARN_UNUSED_RESULT;
 
+/**
+ * Flush stdout and stderr. Errors are ignored.
+ */
+void PyC_StdFilesFlush();
+
 /**
  * Use with PyArg_ParseTuple's "O&" formatting.
  *

source/blender/python/intern/bpy_interface_run.cc

@@ -217,6 +217,10 @@ static bool python_script_exec(
 
   PyC_MainModule_Restore(main_mod);
 
+  /* Flush stdout/stderr to ensure the script output is visible.
+   * Using fflush(stdout) does not solve it. */
+  PyC_StdFilesFlush();
+
   bpy_context_clear(C, &gilstate);
 
   return (py_result != nullptr);