Add max_threads kwarg to faulthandler.register() too

Per @vstinner's follow-up: also wire the kwarg through register() /
faulthandler_user. Stored in the per-signal user_signal_t struct; the
user-signal handler now passes user->max_threads to
faulthandler_dump_traceback instead of 0.

Doc, whatsnew, and NEWS entries updated to list all four functions.

Author: efroemling

Doc/library/faulthandler.rst

@@ -189,11 +189,12 @@ Dumping the tracebacks after a timeout
 Dumping the traceback on a user signal
 --------------------------------------
 
-.. function:: register(signum, file=sys.stderr, all_threads=True, chain=False)
+.. function:: register(signum, file=sys.stderr, all_threads=True, chain=False, *, max_threads=100)
 
    Register a user signal: install a handler for the *signum* signal to dump
    the traceback of all threads, or of the current thread if *all_threads* is
    ``False``, into *file*. Call the previous handler if chain is ``True``.
+   *max_threads* caps the number of threads dumped.
 
    The *file* must be kept open until the signal is unregistered by
    :func:`unregister`: see :ref:`issue with file descriptors <faulthandler-fd>`.
@@ -203,6 +204,9 @@ Dumping the traceback on a user signal
    .. versionchanged:: 3.5
       Added support for passing file descriptor to this function.
 
+   .. versionchanged:: next
+      Added the *max_threads* keyword argument.
+
 .. function:: unregister(signum)
 
    Unregister a user signal: uninstall the handler of the *signum* signal

Doc/whatsnew/3.15.rst

@@ -898,7 +898,8 @@ faulthandler
 ------------
 
 * Added the *max_threads* parameter in :func:`faulthandler.enable`,
-  :func:`faulthandler.dump_traceback`, and :func:`faulthandler.dump_traceback_later`.
+  :func:`faulthandler.dump_traceback`, :func:`faulthandler.dump_traceback_later`,
+  and :func:`faulthandler.register`.
   (Contributed by Eric Froemling in :gh:`149085`.)
 
 

Include/internal/pycore_faulthandler.h

@@ -42,6 +42,7 @@ struct faulthandler_user_signal {
     int chain;
     _Py_sighandler_t previous;
     PyInterpreterState *interp;
+    Py_ssize_t max_threads;
 };
 #endif /* FAULTHANDLER_USER */
 

Misc/NEWS.d/next/Library/2026-04-28-16-30-48.gh-issue-149085.5aNgBD.rst

@@ -1,2 +1,3 @@
 Add a *max_threads* keyword argument to :func:`faulthandler.dump_traceback`,
-:func:`faulthandler.dump_traceback_later`, and :func:`faulthandler.enable`.
+:func:`faulthandler.dump_traceback_later`, :func:`faulthandler.enable`, and
+:func:`faulthandler.register`.

Modules/clinic/faulthandler.c.h

@@ -961,7 +961,8 @@ faulthandler_user(int signum)
     if (!user->enabled)
         return;
 
-    faulthandler_dump_traceback(user->fd, user->all_threads, user->interp, 0);
+    faulthandler_dump_traceback(user->fd, user->all_threads, user->interp,
+                                user->max_threads);
 
 #ifdef HAVE_SIGACTION
     if (user->chain) {
@@ -1011,17 +1012,21 @@ faulthandler.register as faulthandler_register_py
     file: object(py_default="sys.stderr") = NULL
     all_threads: bool = True
     chain: bool = False
+    *
+    max_threads: Py_ssize_t = 100
 
 Register a handler for the signal 'signum'.
 
 Dump the traceback of the current thread, or of all threads if
-all_threads is True, into file.
+all_threads is True, into file. max_threads caps the number of threads
+dumped.
 [clinic start generated code]*/
 
 static PyObject *
 faulthandler_register_py_impl(PyObject *module, int signum, PyObject *file,
-                              int all_threads, int chain)
-/*[clinic end generated code: output=1f770cee150a56cd input=ae9de829e850907b]*/
+                              int all_threads, int chain,
+                              Py_ssize_t max_threads)
+/*[clinic end generated code: output=d63a5b4f388dee5f input=c75096a20de502fe]*/
 {
     int fd;
     user_signal_t *user;
@@ -1072,6 +1077,7 @@ faulthandler_register_py_impl(PyObject *module, int signum, PyObject *file,
     user->all_threads = all_threads;
     user->chain = chain;
     user->interp = PyThreadState_GetInterpreter(tstate);
+    user->max_threads = max_threads;
     user->enabled = 1;
 
     Py_RETURN_NONE;