fix mistakes in C and throw an error from Koka

kklib/src/thread.c

@@ -530,19 +530,24 @@ kk_lvar_t kk_lvar_alloc(kk_box_t lattice_bottom, kk_context_t* ctx) {
 // add value to lvar
 void kk_lvar_put(kk_lvar_t lvar, kk_function_t update, kk_context_t *ctx) {
   lvar_t *lv = (lvar_t *)kk_cptr_raw_unbox(lvar);
+  kk_box_t result = null;
 
   if (lv->is_frozen != 0) goto err;
 
   pthread_mutex_lock(&lv->lock);
-  lv->result =
+  result =
     kk_function_call(kk_box_t, (kk_function_t, kk_box_t, kk_context_t *),
                      update, (update, lv->result, ctx));
 
   kk_box_mark_shared(lv->result, ctx); // TODO: can we mark outside the mutex?
 
   // null means that update went to top
-  if (kk_box_is_null(lv->result)) goto err;
+  if (kk_box_is_null(result)) {
+    pthread_mutex_unlock(&lv->lock);
+    goto err;
+  }
 
+  lv->result = result;
   pthread_mutex_unlock(&lv->lock);
 
   pthread_cond_broadcast(&lv->available);
@@ -594,12 +599,12 @@ kk_box_t kk_lvar_get( kk_lvar_t lvar, kk_function_t in_threshold_set, kk_context
     kk_lvar_wait(lvar, ctx);
     pthread_mutex_unlock(&lv->lock);
   }
+
   kk_function_drop(in_threshold_set, ctx);
   kk_box_drop(lvar,ctx);
   return result;
+
  err:
-  // get should block forever in this situation
-  // but maybe we can do something better?
   pthread_mutex_lock(&lv->lock);
   pthread_cond_wait(&lv->available, &lv->lock);
   return kk_box_null;

lib/std/os/lvar.kk

@@ -57,8 +57,11 @@ public fun put( lvar : lvar<a>, d : a ) : pure () {
   lvar.put( fn (d') { lvar.join(d, d') } )
 }
 
-public fun get( lvar : lvar<a>, in-threshold-set : (a, bool) -> maybe<a> ) : pure a {
-  lvar.lv.unsafe-get( fn (d, frz) { in-threshold-set(d, frz.bool).null } )
+public fun get( lvar : lvar<a>, in-threshold-set : (a, bool) -> maybe<a> ) : exn a {
+  match (lvar.lv.unsafe-get( fn (d, frz) { in-threshold-set(d, frz.bool).null } )) {
+    Nothing -> throw("Threshold set could not be reached")
+    Just a -> a
+  }
 }
 
 // Variant of `get` which assumes every value in the threshold set is unfrozen.