Avoid segfault when ptls->safepoint is NULL

There are cases (eg: when __gmpz_init is called from python via PyCall in a
secondary thread) where ptls->safepoint is NULL when maybe_collect is called,
and *ptls->safepoint causes a NULL pointer dereference and a segfault.  This
change fixes that case.

Included test in test/threads.jl that will reproduce this issue.

See #27020 for backtrace and more details

src/gc.c

@@ -3034,23 +3034,29 @@ JL_DLLEXPORT void jl_throw_out_of_memory_error(void)
 JL_DLLEXPORT void *jl_gc_counted_malloc(size_t sz)
 {
     jl_ptls_t ptls = jl_get_ptls_states();
-    maybe_collect(ptls);
-    ptls->gc_num.allocd += sz;
-    ptls->gc_num.malloc++;
+    if (ptls && ptls->safepoint) {
+        maybe_collect(ptls);
+        ptls->gc_num.allocd += sz;
+        ptls->gc_num.malloc++;
+    }
     void *b = malloc(sz);
-    if (b == NULL)
+    // If ptls->safepoint is NULL, then let the caller deal with the failed malloc
+    if (b == NULL && ptls && ptls->safepoint)
         jl_throw(jl_memory_exception);
     return b;
 }
 
 JL_DLLEXPORT void *jl_gc_counted_calloc(size_t nm, size_t sz)
 {
     jl_ptls_t ptls = jl_get_ptls_states();
-    maybe_collect(ptls);
-    ptls->gc_num.allocd += nm*sz;
-    ptls->gc_num.malloc++;
+    if (ptls && ptls->safepoint) {
+        maybe_collect(ptls);
+        ptls->gc_num.allocd += nm*sz;
+        ptls->gc_num.malloc++;
+    }
     void *b = calloc(nm, sz);
-    if (b == NULL)
+    // If ptls->safepoint is NULL, then let the caller deal with the failed calloc
+    if (b == NULL && ptls && ptls->safepoint)
         jl_throw(jl_memory_exception);
     return b;
 }
@@ -3059,8 +3065,10 @@ JL_DLLEXPORT void jl_gc_counted_free_with_size(void *p, size_t sz)
 {
     jl_ptls_t ptls = jl_get_ptls_states();
     free(p);
-    ptls->gc_num.freed += sz;
-    ptls->gc_num.freecall++;
+    if (ptls && ptls->safepoint) {
+        ptls->gc_num.freed += sz;
+        ptls->gc_num.freecall++;
+    }
 }
 
 // older name for jl_gc_counted_free_with_size
@@ -3072,14 +3080,17 @@ JL_DLLEXPORT void jl_gc_counted_free(void *p, size_t sz)
 JL_DLLEXPORT void *jl_gc_counted_realloc_with_old_size(void *p, size_t old, size_t sz)
 {
     jl_ptls_t ptls = jl_get_ptls_states();
-    maybe_collect(ptls);
-    if (sz < old)
-        ptls->gc_num.freed += (old - sz);
-    else
-        ptls->gc_num.allocd += (sz - old);
-    ptls->gc_num.realloc++;
+    if (ptls && ptls->safepoint) {
+        maybe_collect(ptls);
+        if (sz < old)
+            ptls->gc_num.freed += (old - sz);
+        else
+            ptls->gc_num.allocd += (sz - old);
+        ptls->gc_num.realloc++;
+    }
     void *b = realloc(p, sz);
-    if (b == NULL)
+    // If ptls->safepoint is NULL, then let the caller deal with the failed realloc
+    if (b == NULL && ptls && ptls->safepoint)
         jl_throw(jl_memory_exception);
     return b;
 }

src/julia_threads.h

@@ -240,7 +240,7 @@ JL_DLLEXPORT void (jl_cpu_wake)(void);
 
 // gc safepoint and gc states
 // This triggers a SegFault when we are in GC
-// Assign it to a variable to make sure the compiler emit the load
+// Assign it to a variable to make sure the compiler emits the load
 // and to avoid Clang warning for -Wunused-volatile-lvalue
 #define jl_gc_safepoint_(ptls) do {                     \
         jl_signal_fence();                              \

test/threads_exec.jl

@@ -652,6 +652,12 @@ function pfib(n::Int)
 end
 @test pfib(20) == 6765
 
+function test_27020_SIGSEGV()
+    x = BigInt()
+    # We only care that this call does not throw a SIGSEGV, so no need to check return value
+    @threadcall((:__gmpz_init, :libgmp), Cvoid, (Ptr{BigInt}, ), Ref(x))
+end
+test_27020_SIGSEGV()
 
 # scheduling wake/sleep test (#32511)
 let timeout = 300 # this test should take about 1-10 seconds