gh-98608: Change _Py_NewInterpreter() to _Py_NewInterpreterFromConfig() (gh-98609)

(see https://github.com/python/cpython/issues/98608)

This change does the following:

1. change the argument to a new `_PyInterpreterConfig` struct
2. rename the function to `_Py_NewInterpreterFromConfig()`, inspired by `Py_InitializeFromConfig()` (takes a `_PyInterpreterConfig`  instead of `isolated_subinterpreter`)
3. split up the boolean `isolated_subinterpreter` into the corresponding multiple granular settings
   * allow_fork
   * allow_subprocess
   * allow_threads
4. add `PyInterpreterState.feature_flags` to store those settings
5. add a function for checking if a feature is enabled on an opaque `PyInterpreterState *`
6. drop `PyConfig._isolated_interpreter`

The existing default (see `Py_NewInterpeter()` and `Py_Initialize*()`) allows fork, subprocess, and threads and the optional "isolated" interpreter (see the `_xxsubinterpreters` module) disables all three.  None of that changes here; the defaults are preserved.

Note that the given `_PyInterpreterConfig` will not be used outside `_Py_NewInterpreterFromConfig()`, nor preserved.  This contrasts with how `PyConfig` is currently preserved, used, and even modified outside `Py_InitializeFromConfig()`.  I'd rather just avoid that mess from the start for `_PyInterpreterConfig`.  We can preserve it later if we find an actual need.

This change allows us to follow up with a number of improvements (e.g. stop disallowing subprocess and support disallowing exec instead).

(Note that this PR adds "private" symbols.  We'll probably make them public, and add docs, in a separate change.)

1 files changed, 15 insertions, 4 deletions

diff --git a/Include/cpython/initconfig.h b/Include/cpython/initconfig.h
index c22c8d5..64748cf 100644
--- a/Include/cpython/initconfig.h
+++ b/Include/cpython/initconfig.h
@@ -213,10 +213,6 @@ typedef struct PyConfig {
     // If equal to 0, stop Python initialization before the "main" phase.
     int _init_main;
 
-    // If non-zero, disallow threads, subprocesses, and fork.
-    // Default: 0.
-    int _isolated_interpreter;
-
     // If non-zero, we believe we're running from a source tree.
     int _is_python_build;
 } PyConfig;
@@ -245,6 +241,21 @@ PyAPI_FUNC(PyStatus) PyConfig_SetWideStringList(PyConfig *config,
     Py_ssize_t length, wchar_t **items);
 
 
+/* --- PyInterpreterConfig ------------------------------------ */
+
+typedef struct {
+    int allow_fork;
+    int allow_subprocess;
+    int allow_threads;
+} _PyInterpreterConfig;
+
+#define _PyInterpreterConfig_LEGACY_INIT \
+    { \
+        .allow_fork = 1, \
+        .allow_subprocess = 1, \
+        .allow_threads = 1, \
+    }
+
 /* --- Helper functions --------------------------------------- */
 
 /* Get the original command line arguments, before Python modified them.