Remove unused JL_HAVE_SIGALTSTACK code (#54568)

Nothing is ever activating this, and it probably hasn't been tested in a
while.

As far as I can tell, this code path was added by @vtjnash in
082d7d2

I have no particular beef with this code other than "it complicates the
thread / task logic, so dealing with this requires understanding one
more moving part, removing it makes it simpler". If it is decided that
this code should rather be kept, "just in case" (which, though), that's
equally fine by me. Thought perhaps a comment could be added explaining
the motivation for keeping it even thought it is currently always
disabled?

Author: fingolfin

src/julia_threads.h

@@ -31,7 +31,6 @@ JL_DLLEXPORT int8_t jl_threadpoolid(int16_t tid) JL_NOTSAFEPOINT;
 // JL_HAVE_ASM && JL_HAVE_UNW_CONTEXT -- libunwind-based
 // JL_HAVE_UNW_CONTEXT -- libunwind-based
 // JL_HAVE_UCONTEXT -- posix standard API, requires syscall for resume
-// JL_HAVE_SIGALTSTACK -- requires several syscall for start, setjmp for resume
 
 #ifdef _OS_WINDOWS_
 #define JL_HAVE_UCONTEXT
@@ -53,8 +52,7 @@ typedef struct {
 } jl_stack_context_t;
 #if !defined(JL_HAVE_UCONTEXT) && \
     !defined(JL_HAVE_ASM) && \
-    !defined(JL_HAVE_UNW_CONTEXT) && \
-    !defined(JL_HAVE_SIGALTSTACK)
+    !defined(JL_HAVE_UNW_CONTEXT)
 #if (defined(_CPU_X86_64_) || defined(_CPU_X86_) || defined(_CPU_AARCH64_) ||  \
      defined(_CPU_ARM_) || defined(_CPU_PPC64_))
 #define JL_HAVE_ASM
@@ -70,7 +68,7 @@ typedef struct {
 #endif
 #endif
 
-#if (!defined(JL_HAVE_UNW_CONTEXT) && defined(JL_HAVE_ASM)) || defined(JL_HAVE_SIGALTSTACK)
+#if !defined(JL_HAVE_UNW_CONTEXT) && defined(JL_HAVE_ASM)
 typedef jl_stack_context_t _jl_ucontext_t;
 #endif
 #pragma GCC visibility push(default)

src/stackwalk.c

@@ -1151,8 +1151,6 @@ static void jl_rec_backtrace(jl_task_t *t) JL_NOTSAFEPOINT
       #pragma message("jl_rec_backtrace not defined for ASM/SETJMP on unknown system")
       (void)c;
      #endif
-#elif defined(JL_HAVE_SIGALTSTACK)
-     #pragma message("jl_rec_backtrace not defined for SIGALTSTACK")
 #else
      #pragma message("jl_rec_backtrace not defined for unknown task system")
 #endif

src/task.c

@@ -272,7 +272,7 @@ JL_NO_ASAN static void restore_stack2(jl_task_t *t, jl_ptls_t ptls, jl_task_t *l
         return;
     if (r != 0 || returns != 1)
         abort();
-#elif defined(JL_HAVE_ASM) || defined(JL_HAVE_SIGALTSTACK) || defined(_OS_WINDOWS_)
+#elif defined(JL_HAVE_ASM) || defined(_OS_WINDOWS_)
     if (jl_setjmp(lastt->ctx.copy_ctx.uc_mcontext, 0))
         return;
 #else
@@ -1510,98 +1510,6 @@ JL_NO_ASAN static void jl_start_fiber_set(jl_ucontext_t *t)
 }
 #endif
 
-#if defined(JL_HAVE_SIGALTSTACK)
-#if defined(_COMPILER_TSAN_ENABLED_)
-#error TSAN support not currently implemented for this tasking model
-#endif
-
-static void start_basefiber(int sig)
-{
-    jl_ptls_t ptls = jl_current_task->ptls;
-    if (jl_setjmp(ptls->base_ctx.uc_mcontext, 0))
-        start_task(); // sanitizer_finish_switch_fiber is part of start_task
-}
-static char *jl_alloc_fiber(_jl_ucontext_t *t, size_t *ssize, jl_task_t *owner)
-{
-    stack_t uc_stack, osigstk;
-    struct sigaction sa, osa;
-    sigset_t set, oset;
-    void *stk = jl_malloc_stack(ssize, owner);
-    if (stk == NULL)
-        return NULL;
-    // setup
-    jl_ptls_t ptls = jl_current_task->ptls;
-    _jl_ucontext_t base_ctx;
-    memcpy(&base_ctx, &ptls->base_ctx, sizeof(base_ctx));
-    sigfillset(&set);
-    if (pthread_sigmask(SIG_BLOCK, &set, &oset) != 0) {
-       jl_free_stack(stk, *ssize);
-       jl_error("pthread_sigmask failed");
-    }
-    uc_stack.ss_sp = stk;
-    uc_stack.ss_size = *ssize;
-    uc_stack.ss_flags = 0;
-    if (sigaltstack(&uc_stack, &osigstk) != 0) {
-       jl_free_stack(stk, *ssize);
-       jl_error("sigaltstack failed");
-    }
-    memset(&sa, 0, sizeof(sa));
-    sigemptyset(&sa.sa_mask);
-    sa.sa_handler = start_basefiber;
-    sa.sa_flags = SA_ONSTACK;
-    if (sigaction(SIGUSR2, &sa, &osa) != 0) {
-       jl_free_stack(stk, *ssize);
-       jl_error("sigaction failed");
-    }
-    // emit signal
-    pthread_kill(pthread_self(), SIGUSR2); // initializes jl_basectx
-    sigdelset(&set, SIGUSR2);
-    sigsuspend(&set);
-    // cleanup
-    if (sigaction(SIGUSR2, &osa, NULL) != 0) {
-       jl_free_stack(stk, *ssize);
-       jl_error("sigaction failed");
-    }
-    if (osigstk.ss_size < MINSTKSZ && (osigstk.ss_flags | SS_DISABLE))
-       osigstk.ss_size = MINSTKSZ;
-    if (sigaltstack(&osigstk, NULL) != 0) {
-       jl_free_stack(stk, *ssize);
-       jl_error("sigaltstack failed");
-    }
-    if (pthread_sigmask(SIG_SETMASK, &oset, NULL) != 0) {
-       jl_free_stack(stk, *ssize);
-       jl_error("pthread_sigmask failed");
-    }
-    if (&ptls->base_ctx != t) {
-        memcpy(&t, &ptls->base_ctx, sizeof(base_ctx));
-        memcpy(&ptls->base_ctx, &base_ctx, sizeof(base_ctx)); // restore COPY_STACKS context
-    }
-    return (char*)stk;
-}
-static void jl_start_fiber_set(jl_ucontext_t *t) {
-    jl_longjmp(t->ctx.uc_mcontext, 1); // (doesn't return)
-}
-static void jl_start_fiber_swap(jl_ucontext_t *lastt, jl_ucontext_t *t)
-{
-    assert(lastt);
-    if (lastt && jl_setjmp(lastt->ctx.uc_mcontext, 0))
-        return;
-    tsan_switch_to_ctx(t);
-    jl_start_fiber_set(t);
-}
-static void jl_swap_fiber(jl_ucontext_t *lastt, jl_ucontext_t *t)
-{
-    if (jl_setjmp(lastt->ctx.uc_mcontext, 0))
-        return;
-    tsan_switch_to_ctx(t);
-    jl_start_fiber_set(t); // doesn't return
-}
-static void jl_set_fiber(jl_ucontext_t *t)
-{
-    jl_longjmp(t->ctx.uc_mcontext, 1);
-}
-#endif
-
 // Initialize a root task using the given stack.
 jl_task_t *jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi)
 {