OCaml 5.2 support (#7)

* OCaml 5.2 support

This patch adds OCaml 5.2 support. The main change is to account for
the new continuation representation in OCaml 5.2, which represents a
continuation as a pair. Legacy support is preserved via the use of the
compile-time `MULTICONT52` variable. When toggled code for the new
continuation representation is included, otherwise this code is
excluded.

Resolves #6.

* Use ocaml-5.2.0+trunk in CI until the proper release is available.

* Follow the OCaml manual's FFI rules even if they may not be strictly required.

.github/workflows/default.yml

@@ -18,6 +18,7 @@ jobs:
           - ubuntu-22.04
         ocaml-compiler:
           - 5.1.0
+          - ocaml-variants.5.2.0+trunk
 
     runs-on: ${{ matrix.os }}
 

lib/multicont_stubs.c

@@ -16,30 +16,49 @@
 #include "fiber_primitives.h" // provides [MULTICONT_NEXT_FIBER_ID]
                               // generator.
 
+// NOTE(dhil): The representation of continuations was changed in
+// OCaml 5.2. In OCaml 5.2+ a continuation is a pair of stack segments
+// (first_segment, last_segment) which together forms the complete
+// stack chain from effect invocation site to handle site. Here,
+// first_segment is the segment where the effect was initially
+// performed and last_segment is segment that had the appropriate
+// handler installed.
+//
+// Prior to OCaml 5.2 the continuation was simply a pointer to
+// previous stack segment that performed or reperformed the effect.
 #if OCAML_VERSION_MAJOR >= 5 && OCAML_VERSION_MINOR > 1
-#error "multicont is not yet supported on OCaml 5.2+"
+#define MULTICONT52 1
+#else
+#define MULTICONT52 0
 #endif
 
-value multicont_promote(value k) {
+CAMLprim value multicont_promote(value k) {
   CAMLparam1(k);
   CAMLlocal1(r);
 
   value null_stk = Val_ptr(NULL);
 
+#if MULTICONT52
+  r = caml_alloc_2(Cont_tag, null_stk, null_stk);
+#else
   r = caml_alloc_1(Cont_tag, null_stk);
+#endif
 
   // Move the stack from [k] to [r]
   {
     // Prevent the GC from running between [caml_continuation_use] and
     // [caml_continuation_replace]
     CAMLnoalloc;
     caml_continuation_replace(r, Ptr_val(caml_continuation_use(k)));
+#if MULTICONT52
+    caml_modify(&Field(r, 1), Field(k, 1));
+#endif
   }
 
   CAMLreturn(r);
 }
 
-value multicont_clone_continuation(value k) {
+CAMLprim value multicont_clone_continuation(value k) {
   CAMLparam1(k);      // input continuation object
   CAMLlocal1(kclone); // resulting continuation object clone
 
@@ -51,9 +70,16 @@ value multicont_clone_continuation(value k) {
                     *clone,     // clone of [current]
                     *result;    // clone of [source]
   struct stack_info **link = &result;
+#if MULTICONT52
+  struct stack_info *last_segment; // the last segment of the stack chain
+#endif
 
   // Allocate an OCaml object with the continuation tag
+#if MULTICONT52
+  kclone = caml_alloc_2(Cont_tag, null_stk, null_stk);
+#else
   kclone = caml_alloc_1(Cont_tag, null_stk);
+#endif
   {
     // Prevent the GC from running between the use of
     // [caml_continuation_use] and [caml_continuation_replace]
@@ -97,11 +123,18 @@ value multicont_clone_continuation(value k) {
       clone->sp = Stack_high(clone) - space_used;
 
       // Prepare to handle the next stack segment
+#if MULTICONT52
+      last_segment = clone;
+#endif
       *link = clone;
       link = &Stack_parent(clone);
       current = Stack_parent(current);
     }
 
+#if MULTICONT52
+    caml_modify(&Field(kclone, 1), Val_ptr(last_segment));
+#endif
+
     // Reattach the [source] stack to [k] (necessary as
     // [caml_continuation_use] deattaches it) and attach [result] to
     // [kclone]
@@ -112,13 +145,14 @@ value multicont_clone_continuation(value k) {
   CAMLreturn(kclone);
 }
 
-value multicont_drop_continuation(value k) {
+CAMLprim value multicont_drop_continuation(value k) {
+  CAMLparam1(k);
   struct stack_info *current,
                     *next = Ptr_val(caml_continuation_use(k));
   while (next != NULL) {
     current = next;
     next = Stack_parent(current);
     caml_free_stack(current);
   }
-  return Val_unit;
+  CAMLreturn(Val_unit);
 }