Fix Python 3.14 compatibility: defer exception raising from signal handlers

Python 3.14 changed recursion checking to use actual C stack pointers
instead of counters. The old implementation called Python code from within
signal handlers before siglongjmp(), causing _Py_CheckRecursiveCall() to
see inconsistent stack pointers and report 'Unrecoverable stack overflow'.

Solution: Never call Python code from within signal handlers.

Changes:
- Added signal_to_raise field to cysigs_t to store deferred signal
- Modified signal handlers to only set flag and longjmp (no Python calls)
- Added _sig_raise_deferred() to raise exceptions after longjmp
- Updated _sig_on_postjmp() to call _sig_raise_deferred() in safe context

This maintains async-signal-safety and ensures Python's stack tracking
remains consistent. All 65 tests pass on Python 3.14.0rc3.

Also updates:
- pyproject.toml: requires-python = '>=3.9,<3.15' (was <3.14)
- CI/CD: Add Python 3.14 to test matrix

Author: cxzhong

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
       fail-fast: false
       matrix:
         os: ['macos-13', 'macos-latest', 'ubuntu-latest', 'windows-latest']
-        python-version: ['3.10', '3.11', '3.12', '3.13']
+        python-version: ['3.10', '3.11', '3.12', '3.13', '3.14']
     steps:
     - name: Set up the repository
       uses: actions/checkout@v4

pyproject.toml

@@ -30,7 +30,7 @@ classifiers = [
     "Topic :: Software Development :: Debuggers",
 ]
 urls = { Homepage = "https://github.com/sagemath/cysignals" }
-requires-python = ">=3.9,<3.14"
+requires-python = ">=3.9,<3.15"
 
 [project.entry-points.pkg_config]
 cysignals = 'cysignals'

src/cysignals/implementation.c

@@ -392,8 +392,9 @@ static void cysigs_interrupt_handler(int sig)
     {
         if (!cysigs.block_sigint && !custom_signal_is_blocked())
         {
-            /* Raise an exception so Python can see it */
-            do_raise_exception(sig);
+            /* Store signal number to raise exception after longjmp.
+             * Do NOT call Python code from signal handler! */
+            cysigs.signal_to_raise = sig;
 
 #if !_WIN32
             /* Jump back to sig_on() (the first one if there is a stack) */
@@ -449,8 +450,9 @@ static void cysigs_signal_handler(int sig)
         }
 #endif
 
-        /* Raise an exception so Python can see it */
-        do_raise_exception(sig);
+        /* Store signal number to raise exception after longjmp.
+         * Do NOT call Python code from signal handler! */
+        cysigs.signal_to_raise = sig;
     #if !_WIN32
         /* Jump back to sig_on() (the first one if there is a stack) */
         siglongjmp(trampoline, sig);
@@ -616,6 +618,20 @@ static void _sig_on_recover(void)
     cysigs.inside_signal_handler = 0;
 }
 
+/* Raise exception for signal that was deferred from signal handler.
+ * This MUST be called from a safe context (after longjmp), NOT from
+ * inside the signal handler itself. */
+static void _sig_raise_deferred(void)
+{
+    int sig = cysigs.signal_to_raise;
+    if (sig != 0)
+    {
+        cysigs.signal_to_raise = 0;
+        /* Now safe to call Python code - we're back in normal context */
+        do_raise_exception(sig);
+    }
+}
+
 /* Give a warning that sig_off() was called without sig_on() */
 static void _sig_off_warning(const char* file, int line)
 {

src/cysignals/macros.h

@@ -148,7 +148,10 @@ static inline int _sig_on_postjmp(int jmpret)
 {
     if (unlikely(jmpret > 0))
     {
-        /* An exception occurred */
+        /* A signal occurred and we jumped back via longjmp.
+         * Now we're back in a safe context (not in signal handler),
+         * so it's safe to call Python code to raise the exception. */
+        _sig_raise_deferred();
         _sig_on_recover();
         return 0;
     }

src/cysignals/signals.pxd

@@ -91,6 +91,7 @@ cdef nogil:
     cysigs_t cysigs "cysigs"
     void _sig_on_interrupt_received "_sig_on_interrupt_received"() noexcept
     void _sig_on_recover "_sig_on_recover"() noexcept
+    void _sig_raise_deferred "_sig_raise_deferred"() noexcept
     void _sig_off_warning "_sig_off_warning"(const char*, int) noexcept
     void print_backtrace "print_backtrace"() noexcept
 
@@ -99,5 +100,6 @@ cdef inline void __generate_declarations() noexcept:
     cysigs
     _sig_on_interrupt_received
     _sig_on_recover
+    _sig_raise_deferred
     _sig_off_warning
     print_backtrace

src/cysignals/signals.pyx

@@ -56,6 +56,7 @@ cdef extern from "implementation.c":
     void print_backtrace() nogil
     void _sig_on_interrupt_received() nogil
     void _sig_on_recover() nogil
+    void _sig_raise_deferred() nogil
     void _sig_off_warning(const char*, int) nogil
 
     # Python library functions for raising exceptions without "except"

src/cysignals/struct_signals.h

@@ -91,6 +91,12 @@ typedef struct
      * been received. This is set by sig_on(). */
     cyjmp_buf env;
 
+    /* Signal number to be raised as exception after longjmp.
+     * Set by signal handler, checked and cleared by _sig_on_postjmp.
+     * This allows us to defer calling Python code until we're back
+     * in a safe context (not inside signal handler). */
+    cy_atomic_int signal_to_raise;
+
     /* An optional string (in UTF-8 encoding) to be used as text for
      * the exception raised by sig_raise_exception(). If this is NULL,
      * use some default string depending on the type of signal. This can