Reference safety for Worker after callbacks

Joining a WorkRequest now ensures that either the after callback has
already been executed, or will never be executed.

Fixes #1438

test/miscellaneous/worker.cpp

@@ -0,0 +1,70 @@
+#include "../fixtures/util.hpp"
+
+#include <mbgl/util/worker.hpp>
+#include <mbgl/util/run_loop.hpp>
+
+using namespace mbgl;
+using namespace mbgl::util;
+
+TEST(Worker, ExecutesWorkAndAfter) {
+    RunLoop loop(uv_default_loop());
+
+    Worker worker(1);
+    WorkRequest request;
+
+    bool didWork = false;
+    bool didAfter = false;
+
+    loop.invoke([&] {
+        request = worker.send([&] {
+            didWork = true;
+        }, [&] {
+            didAfter = true;
+            loop.stop();
+        });
+    });
+
+    uv_run(uv_default_loop(), UV_RUN_DEFAULT);
+    EXPECT_TRUE(didWork);
+    EXPECT_TRUE(didAfter);
+}
+
+TEST(Worker, WorkRequestJoinWaitsForWorkToComplete) {
+    RunLoop loop(uv_default_loop());
+
+    Worker worker(1);
+    bool didWork = false;
+
+    loop.invoke([&] {
+        WorkRequest request = worker.send([&] {
+            usleep(10000);
+            didWork = true;
+        }, [&] {});
+
+        request.join();
+        EXPECT_TRUE(didWork);
+        loop.stop();
+    });
+
+    uv_run(uv_default_loop(), UV_RUN_DEFAULT);
+}
+
+TEST(Worker, WorkRequestJoinCancelsAfter) {
+    RunLoop loop(uv_default_loop());
+
+    Worker worker(1);
+    bool didAfter = false;
+
+    loop.invoke([&] {
+        WorkRequest request = worker.send([&] {
+        }, [&] {
+            didAfter = true;
+        });
+
+        request.join();
+        loop.stop();
+    });
+
+    uv_run(uv_default_loop(), UV_RUN_DEFAULT);
+    EXPECT_FALSE(didAfter);
+}