Add no-caml-startup feature flag that avoids the caml_startup sym…

…bol.

Will also be enabled if the `OCAML_INTEROP_NO_CAML_STARTUP` environment variable is set.

This will make `OCamlRuntime::init_persistent()` a noop.

The main motivation for this feature flag is to make code that uses
ocaml-rs and ocaml-interop loadable from `dune utop`:

zshipko/ocaml-rs#70

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `no-caml-startup` feature flag that disables calls to `caml_startup` when initializing the runtime. This makes `OCamlRuntime::init_persistent()` a noop. It is useful for being able to load code that uses `ocaml-rs` in an utop toplevel (for example, when running `dune utop`). Will be enabled if the environment variable `OCAML_INTEROP_NO_CAML_STARTUP` is set. This is a temporary option that is likely to be removed in the future once a better solution is implemented.
+
 ## [0.8.4] - 2021-05-18
 
 ### Added

Cargo.toml

@@ -23,3 +23,4 @@ static_assertions = "1.1.0"
 [features]
 without-ocamlopt = ["ocaml-sys/without-ocamlopt", "ocaml-boxroot-sys/without-ocamlopt"]
 caml-state = ["ocaml-sys/caml-state"]
+no-caml-startup = []

build.rs

@@ -0,0 +1,14 @@
+// Copyright (c) Viable Systems and TezEdge Contributors
+// SPDX-License-Identifier: MIT
+
+const OCAML_INTEROP_NO_CAML_STARTUP: &str = "OCAML_INTEROP_NO_CAML_STARTUP";
+
+fn main() {
+    println!(
+        "cargo:rerun-if-env-changed={}",
+        OCAML_INTEROP_NO_CAML_STARTUP
+    );
+    if std::env::var(OCAML_INTEROP_NO_CAML_STARTUP).is_ok() {
+        println!("cargo:rustc-cfg=feature=\"no-caml-startup\"");
+    }
+}

src/runtime.rs

@@ -2,8 +2,7 @@
 // SPDX-License-Identifier: MIT
 
 use ocaml_boxroot_sys::{boxroot_setup, boxroot_teardown};
-use ocaml_sys::{caml_shutdown, caml_startup};
-use std::{marker::PhantomData, sync::Once};
+use std::marker::PhantomData;
 
 use crate::{memory::OCamlRef, value::OCaml};
 
@@ -30,15 +29,20 @@ impl OCamlRuntime {
     ///
     /// After the first invocation, this method does nothing.
     pub fn init_persistent() {
-        static INIT: Once = Once::new();
-
-        INIT.call_once(|| {
-            let arg0 = "ocaml\0".as_ptr() as *const ocaml_sys::Char;
-            let c_args = vec![arg0, core::ptr::null()];
-            unsafe {
-                caml_startup(c_args.as_ptr());
-            }
-        })
+        #[cfg(not(feature = "no-caml-startup"))]
+        {
+            static INIT: std::sync::Once = std::sync::Once::new();
+
+            INIT.call_once(|| {
+                let arg0 = "ocaml\0".as_ptr() as *const ocaml_sys::Char;
+                let c_args = vec![arg0, core::ptr::null()];
+                unsafe {
+                    ocaml_sys::caml_startup(c_args.as_ptr());
+                }
+            })
+        }
+        #[cfg(feature = "no-caml-startup")]
+        panic!("Rust code that is called from an OCaml program should not try to initialize the runtime.");
     }
 
     /// Recover the runtime handle.
@@ -77,7 +81,7 @@ impl Drop for OCamlRuntime {
     fn drop(&mut self) {
         unsafe {
             boxroot_teardown();
-            caml_shutdown();
+            ocaml_sys::caml_shutdown();
         }
     }
 }

testing/ocaml-caller/rust/Cargo.toml

@@ -9,5 +9,5 @@ crate-type = ["staticlib", "cdylib"]
 
 [dependencies]
 ocaml-interop = { path = "../../../../.." }
-# Above is for building with dune, bellow is for biulding from this directory
+# Above is for building with dune, bellow is for building from this directory
 # ocaml-interop = { path = "../../.." }