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, 13 insertions, 2 deletions

diff --git a/Programs/_testembed.c b/Programs/_testembed.c
index d635c5a..ba59959 100644
--- a/Programs/_testembed.c
+++ b/Programs/_testembed.c
@@ -681,8 +681,6 @@ static int test_init_from_config(void)
 
     config.safe_path = 1;
 
-    config._isolated_interpreter = 1;
-
     putenv("PYTHONINTMAXSTRDIGITS=6666");
     config.int_max_str_digits = 31337;
 
@@ -1901,6 +1899,18 @@ static int test_unicode_id_init(void)
 }
 
 
+static int test_init_main_interpreter_settings(void)
+{
+    _testembed_Py_Initialize();
+    (void) PyRun_SimpleStringFlags(
+        "import _testinternalcapi, json; "
+        "print(json.dumps(_testinternalcapi.get_interp_settings(0)))",
+        0);
+    Py_Finalize();
+    return 0;
+}
+
+
 #ifndef MS_WINDOWS
 #include "test_frozenmain.h"      // M_test_frozenmain
 
@@ -2087,6 +2097,7 @@ static struct TestCase TestCases[] = {
     {"test_run_main_loop", test_run_main_loop},
     {"test_get_argc_argv", test_get_argc_argv},
     {"test_init_use_frozen_modules", test_init_use_frozen_modules},
+    {"test_init_main_interpreter_settings", test_init_main_interpreter_settings},
 
     // Audit
     {"test_open_code_hook", test_open_code_hook},