vm-ext: gate serving-JIT behind SX_SERVING_JIT + fix continuation-guest regressions

Enabling the epoch serving-mode JIT globally regressed continuation-based guest
interpreters (the epoch mode is the shared command channel every loop's
conformance runner uses). Two-part fix:

1. SAFE DEFAULT GATE. register_jit_hook in the persistent server branch is now
   opt-in via SX_SERVING_JIT=1 (default OFF). Default behaviour is unchanged
   (no JIT in epoch serving) → zero regression for sibling loops. The
   content/Smalltalk page server opts in.

2. GENERAL FIXES + per-guest interpret-only declarations:
   - callable? (sx_server/run_tests/integration_tests/mcp_tree) now accepts
     VmClosure. A JIT-compiled higher-order function returns its inner closure
     as a VmClosure; callable? previously rejected it, so scheme-apply's
     (callable? proc) guard failed with "not a procedure: <vm:anon>".
   - jit-exclude! gains a trailing-"*" namespace-prefix form
     (Sx_types.jit_excluded_prefixes), the robust way to mark a whole guest
     interpreter interpret-only (a name-list misses functions in extra files —
     it left erlang's vm/dispatcher JIT'd and 13 tests short).
   - Per-guest exclusions in each guest's runtime.sx:
       scheme  "scheme-*" "scm-*"   erlang "er-*" "erlang-*"
       prolog  "pl-*"               common-lisp "cl-*" "clos-*"
       js      "js-*"               haskell "hk-*"

Verified under opt-in JIT (== CEK, no hang): smalltalk 847/847, scheme/flow
166/166, erlang 530/530, prolog 590/590, apl 152/152, js 147/148. Residual
(documented, protected by the default gate): common-lisp 6 fails in advanced
suites (parser-recovery/debugger/CLOS/MOP). lua (0/16) and tcl (3/4) fail
identically on CEK — pre-existing, not JIT. run_tests --jit/no-jit unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/integration_tests.ml

@@ -263,7 +263,7 @@ let make_integration_env () =
 
   (* Type predicates — needed by adapter-sx.sx *)
   bind "callable?" (fun args ->
-    match args with [NativeFn _] | [Lambda _] | [Component _] | [Island _] -> Bool true | _ -> Bool false);
+    match args with [NativeFn _] | [Lambda _] | [Component _] | [Island _] | [VmClosure _] -> Bool true | _ -> Bool false);
   bind "lambda?" (fun args -> match args with [Lambda _] -> Bool true | _ -> Bool false);
   bind "macro?" (fun args -> match args with [Macro _] -> Bool true | _ -> Bool false);
   bind "island?" (fun args -> match args with [Island _] -> Bool true | _ -> Bool false);

hosts/ocaml/bin/mcp_tree.ml

@@ -477,7 +477,7 @@ let setup_env () =
   bind "number?" (fun args -> match args with
     | [Number _] -> Bool true | _ -> Bool false);
   bind "callable?" (fun args -> match args with
-    | [NativeFn _ | Lambda _ | Component _ | Island _] -> Bool true | _ -> Bool false);
+    | [NativeFn _ | Lambda _ | Component _ | Island _ | VmClosure _] -> Bool true | _ -> Bool false);
   bind "empty?" (fun args -> match args with
     | [List []] | [ListRef { contents = [] }] -> Bool true
     | [Nil] -> Bool true | _ -> Bool false);

hosts/ocaml/bin/run_tests.ml

@@ -595,7 +595,7 @@ let make_test_env () =
   (* regex-find-all now provided by sx_primitives.ml *)
   bind "callable?" (fun args ->
     match args with
-    | [NativeFn _] | [Lambda _] | [Component _] | [Island _] -> Bool true
+    | [NativeFn _] | [Lambda _] | [Component _] | [Island _] | [VmClosure _] -> Bool true
     | _ -> Bool false);
   bind "make-sx-expr" (fun args -> match args with [String s] -> SxExpr s | _ -> raise (Eval_error "make-sx-expr: expected string"));
   bind "sx-expr-source" (fun args -> match args with [SxExpr s] -> String s | [String s] -> String s | _ -> raise (Eval_error "sx-expr-source: expected sx-expr or string"));

hosts/ocaml/bin/sx_server.ml

@@ -789,7 +789,11 @@ let setup_introspection env =
   bind "component?" (fun args ->
     match args with [Component _] | [Island _] -> Bool true | _ -> Bool false);
   bind "callable?" (fun args ->
-    match args with [NativeFn _] | [Lambda _] | [Component _] | [Island _] -> Bool true | _ -> Bool false);
+    (* VmClosure must count as callable: a JIT-compiled higher-order function
+       returns its inner closure as a VmClosure, and downstream code (e.g.
+       scheme-apply's `(callable? proc)` guard) must recognize it — it is
+       invocable via the normal call path. *)
+    match args with [NativeFn _] | [Lambda _] | [Component _] | [Island _] | [VmClosure _] -> Bool true | _ -> Bool false);
   bind "spread?" (fun args -> match args with [Spread _] -> Bool true | _ -> Bool false);
   bind "continuation?" (fun args ->
     match args with [Continuation _] -> Bool true | [_] -> Bool false | _ -> Bool false);
@@ -4556,29 +4560,38 @@ let () =
   else begin
     (* Normal persistent server mode *)
     let env = make_server_env () in
-    (* JIT needs the SX bytecode compiler (lib/compiler.sx) as its `compile`
-       binding — the native Sx_compiler.compile is an incomplete stub that
-       miscompiles parameters (emits arity-0 bytecode with params as
-       GLOBAL_GET). http/cli/site modes already load compiler.sx; the
-       persistent (epoch) serving mode must too before enabling the hook,
-       or every JIT-compiled function fails at runtime with "VM undefined:
-       <param>" and falls back to CEK (with double-executed side effects). *)
-    (_import_env := Some env;
-     let project_dir = try Sys.getenv "SX_PROJECT_DIR" with Not_found ->
-       try Sys.getenv "SX_ROOT" with Not_found ->
-       if Sys.file_exists "/app/spec" then "/app" else Sys.getcwd () in
-     let lib_base = try Sys.getenv "SX_LIB_DIR" with Not_found ->
-       project_dir ^ "/lib" in
-     let compiler_path = lib_base ^ "/compiler.sx" in
-     let compiler_path =
-       if Sys.file_exists compiler_path then compiler_path
-       else if Sys.file_exists "lib/compiler.sx" then "lib/compiler.sx"
-       else compiler_path in
-     try load_library_file compiler_path; rebind_host_extensions env
-     with exn ->
-       Printf.eprintf "[sx-server] WARNING: failed to load compiler.sx for JIT (%s) — JIT disabled\n%!"
-         (Printexc.to_string exn));
-    register_jit_hook env;
+    (* JIT in the epoch serving mode is OPT-IN via SX_SERVING_JIT=1.
+       Default OFF: this mode is the shared command channel used by every
+       loop's conformance runner, and enabling JIT globally regresses
+       continuation-based guest interpreters (Scheme/Erlang/Prolog/CL: their
+       eval/dispatch cores capture call/cc continuations the stack VM can't
+       escape, and deep AST recursion can miscompile into a non-terminating
+       loop). Guests that are safe declare their interpret-only namespace with
+       `(jit-exclude! "<ns>-*")`; until every guest is validated, the safe
+       default is no JIT here. Opt in (SX_SERVING_JIT=1) for validated
+       workloads — e.g. the content/Smalltalk page server. *)
+    (match Sys.getenv_opt "SX_SERVING_JIT" with
+     | Some ("1" | "true" | "yes" | "on") ->
+       (* Load the SX bytecode compiler (lib/compiler.sx) as `compile` — the
+          native Sx_compiler.compile is an incomplete stub (arity-0 bytecode,
+          params as GLOBAL_GET). http/cli/site modes already load it. *)
+       (_import_env := Some env;
+        let project_dir = try Sys.getenv "SX_PROJECT_DIR" with Not_found ->
+          try Sys.getenv "SX_ROOT" with Not_found ->
+          if Sys.file_exists "/app/spec" then "/app" else Sys.getcwd () in
+        let lib_base = try Sys.getenv "SX_LIB_DIR" with Not_found ->
+          project_dir ^ "/lib" in
+        let compiler_path = lib_base ^ "/compiler.sx" in
+        let compiler_path =
+          if Sys.file_exists compiler_path then compiler_path
+          else if Sys.file_exists "lib/compiler.sx" then "lib/compiler.sx"
+          else compiler_path in
+        try load_library_file compiler_path; rebind_host_extensions env
+        with exn ->
+          Printf.eprintf "[sx-server] WARNING: failed to load compiler.sx for JIT (%s) — JIT disabled\n%!"
+            (Printexc.to_string exn));
+       register_jit_hook env
+     | _ -> ());
     send "(ready)";
     (* Main command loop *)
     try

hosts/ocaml/lib/sx_primitives.ml

@@ -4154,17 +4154,26 @@ let () =
     Queue.clear Sx_types.jit_cache_queue;
     Nil);
   register "jit-exclude!" (fun args ->
-    (* Mark one or more function names as interpret-only (never JIT-compiled).
-       A guest interpreter calls this for its continuation-using dispatch core.
-       Accepts any number of string/symbol names. *)
+    (* Mark function names as interpret-only (never JIT-compiled). A guest
+       interpreter calls this for its continuation-using dispatch core.
+       Accepts string/symbol names; a trailing "*" makes it a namespace prefix
+       (e.g. "er-*" excludes every function whose name starts with "er-") —
+       the robust way to declare a whole guest interpreter core. *)
     List.iter (fun a ->
       match a with
-      | String n | Symbol n -> Hashtbl.replace Sx_types.jit_excluded n ()
+      | String n | Symbol n ->
+        let len = String.length n in
+        if len > 0 && n.[len - 1] = '*' then begin
+          let prefix = String.sub n 0 (len - 1) in
+          if not (List.mem prefix !Sx_types.jit_excluded_prefixes) then
+            Sx_types.jit_excluded_prefixes := prefix :: !Sx_types.jit_excluded_prefixes
+        end else
+          Hashtbl.replace Sx_types.jit_excluded n ()
       | _ -> ()) args;
     Nil);
   register "jit-excluded?" (fun args ->
     match args with
-    | [String n] | [Symbol n] -> Bool (Hashtbl.mem Sx_types.jit_excluded n)
+    | [String n] | [Symbol n] -> Bool (Sx_types.jit_name_excluded n)
     | _ -> Bool false);
   register "jit-reset-counters!" (fun _args ->
     Sx_types.jit_compiled_count := 0;

hosts/ocaml/lib/sx_types.ml

@@ -487,6 +487,21 @@ let jit_threshold_skipped_count = ref 0
     points: the CEK call hook and the in-VM tiered-compilation path. *)
 let jit_excluded : (string, unit) Hashtbl.t = Hashtbl.create 64
 
+(** Namespace-prefix exclusions. A guest interpreter declares its whole
+    function namespace interpret-only with one entry (e.g. ["er-"], ["scm-"]),
+    which is far more robust than enumerating every function — a name-list
+    misses functions in extra files (the erlang VM dispatcher, etc.) and
+    silently regresses. Set via [jit-exclude!] with a trailing ["*"]
+    (e.g. [(jit-exclude! "er-*")]). Checked via [jit_name_excluded]. *)
+let jit_excluded_prefixes : string list ref = ref []
+
+(** True if [name] is excluded from JIT — by exact name or by namespace prefix. *)
+let jit_name_excluded name =
+  Hashtbl.mem jit_excluded name
+  || List.exists (fun p ->
+       String.length name >= String.length p
+       && String.sub name 0 (String.length p) = p) !jit_excluded_prefixes
+
 (** {2 JIT cache LRU eviction — Phase 2}
 
     Once a lambda crosses the threshold, its [l_compiled] slot is filled.

hosts/ocaml/lib/sx_vm.ml

@@ -1144,10 +1144,12 @@ let jit_compile_lambda (l : lambda) globals =
     None
   ) else if _jit_is_broken_name fn_name then (
     None
-  ) else if Hashtbl.mem Sx_types.jit_excluded fn_name then (
+  ) else if Sx_types.jit_name_excluded fn_name then (
     (* Guest-declared interpret-only function (continuation-using dispatch
-       core). Run on the CEK; the stack VM can't escape through a CEK
-       continuation. See Sx_types.jit_excluded. *)
+       core, or a whole namespace via prefix). Run on the CEK; the stack VM
+       can't escape through a CEK continuation and may miscompile deep AST
+       recursion into a non-terminating loop. See Sx_types.jit_excluded /
+       jit_excluded_prefixes. *)
     None
   ) else
   try