Fix hang on empty {} blocks in generators and async functions

NEWS.md

@@ -1,5 +1,7 @@
 # coro (development version)
 
+* Generators and async functions no longer hang when their body contains an empty `{}` block, such as `generator(function() {})`. Empty blocks now behave like `NULL` (#75).
+
 * Fixed performance issue related to JIT compilation of generator/async instances (#71). The JIT compiler is now able to properly cache the instances, making repeated instantiations fast, and preventing the JIT from kicking in on every call.
 
 * Async functions and generators can now be R6 methods (#63).

R/parser.R

@@ -314,6 +314,11 @@ block_states <- function(block, counter, continue, last, return, info) {
   saved_last <- last
   last <- FALSE
 
+  # An empty block has no states, which would loop forever (#75)
+  if (is_null(node_cdr(block))) {
+    block <- quote({ NULL })
+  }
+
   block <- duplicate(block, shallow = TRUE)
   refs <- block_refs(block)
   states <- NULL
@@ -412,6 +417,12 @@ block_states <- function(block, counter, continue, last, return, info) {
       },
       `{` = {
         nested_node <- node_cdr(expr)
+        # Splicing an empty block adds nothing and can drop the final
+        # state, so treat it as `NULL` (#75)
+        if (is_null(nested_node)) {
+          node_poke_car(node, NULL)
+          next
+        }
         nested_refs <- block_refs(expr)
         splice(nested_node, nested_refs)
         next

tests/testthat/helper-flowery.R

@@ -10,3 +10,10 @@ expect_exhausted <- function(x) {
 local_methods <- function(..., .frame = caller_env()) {
   local_bindings(..., .env = global_env(), .frame = .frame)
 }
+
+# Turns an infinite loop into an error so a hang fails the test
+with_time_limit <- function(expr, seconds = 5) {
+  setTimeLimit(elapsed = seconds, transient = TRUE)
+  on.exit(setTimeLimit(elapsed = Inf))
+  expr
+}

tests/testthat/test-async.R

@@ -405,3 +405,10 @@ test_that("async methods in R6 classes", {
     expect_promise(out, status = "pending")
   })
 })
+
+test_that("empty `{}` blocks behave like `NULL` (#75)", {
+  with_time_limit({
+    expect_null(wait_for(async(function() {})()))
+    expect_null(wait_for(async_generator(function() {})()()))
+  })
+})

tests/testthat/test-generator.R

@@ -609,3 +609,12 @@ test_that("compiled instances do not pin their environment (#36)", {
 
   expect_null(wref_key(ref))
 })
+
+test_that("empty `{}` blocks behave like `NULL` (#75)", {
+  with_time_limit({
+    expect_exhausted(generator(function() {})()())
+    expect_equal(collect(generator(function() { yield(1); {} })()), list(1))
+    expect_equal(collect(generator(function() { {}; yield(1) })()), list(1))
+    expect_equal(collect(generator(function() { {}; yield(1); {} })()), list(1))
+  })
+})