rust: Call _coz_add_delays() after counter increments

The Rust crate was not calling `_coz_add_delays()` after incrementing
progress counters. The C `COZ_INCREMENT_COUNTER` macro in `coz.h` calls
`_call_coz_add_delays()` after every increment — this is required for the
profiler to inject virtual delays for causal profiling experiments.
Without this call, the profiler detects 0 experiments despite counters
being incremented correctly.

Also changes the atomic ordering from SeqCst to Relaxed to match the C
implementation (`__ATOMIC_RELAXED`), which is sufficient since libcoz
uses its own synchronization.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: camelid

rust/src/lib.rs

@@ -14,7 +14,7 @@
 use once_cell::sync::OnceCell;
 use std::ffi::{CStr, CString};
 use std::mem;
-use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
+use std::sync::atomic::{AtomicUsize, Ordering::Relaxed};
 
 /// Equivalent of the `COZ_PROGRESS` and `COZ_PROGRESS_NAMED` macros
 ///
@@ -158,7 +158,8 @@ impl Counter {
                 mem::size_of_val(&counter.count),
                 mem::size_of::<libc::size_t>()
             );
-            counter.count.fetch_add(1, SeqCst);
+            counter.count.fetch_add(1, Relaxed);
+            coz_add_delays();
         }
     }
 
@@ -203,6 +204,11 @@ struct coz_counter_t {
 /// `typedef coz_counter_t* (*coz_get_counter_t)(int, const char*);`
 type GetCounterFn = unsafe extern "C" fn(libc::c_int, *const libc::c_char) -> *mut coz_counter_t;
 
+/// The type of `_coz_add_delays` as defined in `include/coz.h`
+///
+/// `typedef void (*coz_add_delays_t)(void);`
+type AddDelaysFn = unsafe extern "C" fn();
+
 #[cfg(target_os = "linux")]
 fn coz_get_counter(ty: libc::c_int, name: &CStr) -> Option<*mut coz_counter_t> {
     static GET_COUNTER: OnceCell<Option<GetCounterFn>> = OnceCell::new();
@@ -226,7 +232,34 @@ fn coz_get_counter(ty: libc::c_int, name: &CStr) -> Option<*mut coz_counter_t> {
     func.map(|f| unsafe { f(ty, name.as_ptr()) })
 }
 
+/// Calls `_coz_add_delays()` from libcoz.
+///
+/// This must be called after every counter increment to allow the profiler to
+/// inject virtual delays for causal profiling experiments. Without this call,
+/// the profiler cannot detect progress points and will report 0 experiments.
+#[cfg(target_os = "linux")]
+fn coz_add_delays() {
+    static ADD_DELAYS: OnceCell<Option<AddDelaysFn>> = OnceCell::new();
+    let func = ADD_DELAYS.get_or_init(|| {
+        let name = CStr::from_bytes_with_nul(b"_coz_add_delays\0").unwrap();
+        let func = unsafe { libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr()) };
+        if func.is_null() {
+            None
+        } else {
+            Some(unsafe { mem::transmute(func) })
+        }
+    });
+
+    if let Some(f) = func {
+        // SAFETY: _coz_add_delays is a void->void function with no invariants.
+        unsafe { f() };
+    }
+}
+
 #[cfg(not(target_os = "linux"))]
 fn coz_get_counter(_ty: libc::c_int, _name: &CStr) -> Option<*mut coz_counter_t> {
     None
 }
+
+#[cfg(not(target_os = "linux"))]
+fn coz_add_delays() {}