Fix data loss on cancelled io

src/fibers/fiber.cpp

@@ -1438,7 +1438,6 @@ void FiberScheduler::poll(int fd, uint32_t events, uint64_t * triggeredEvents, I
 
 void FiberScheduler::cancelIo(IoFuture * future) noexcept
 {
-    future->result = nullptr;
     enqueueIo(
         nullptr,
         [=](io_uring_sqe * sqe) noexcept

src/fibers/tests/io-cancel-test.cpp

@@ -0,0 +1,81 @@
+#include <silk/fibers/fiber.h>
+
+#include <gtest/gtest.h>
+
+#include <string>
+
+#include <unistd.h>
+
+namespace silk
+{
+
+// Repeatedly read then immediately cancel.
+// Make sure no data is lost.
+TEST(IoCancel, cancelMustNotDropDeliveredBytes)
+{
+    static constexpr uint64_t TOTAL = 4096;
+
+    struct Params
+    {
+        int readFd;
+        int writeFd;
+
+        static int fiberMain(Params * p) noexcept
+        {
+            std::string expected(TOTAL, '\0');
+            for (uint64_t i = 0; i < TOTAL; ++i)
+            {
+                expected[i] = static_cast<char>(i & 0xFF);
+            }
+
+            EXPECT_EQ(::write(p->writeFd, expected.data(), TOTAL), static_cast<ssize_t>(TOTAL));
+            ::close(p->writeFd);
+
+            std::string got;
+            for (;;)
+            {
+                char buf[64] = {};
+                uint64_t bytes_read = 0;
+                FiberScheduler::IoFuture future;
+                iovec iov{buf, sizeof(buf)};
+                FiberScheduler::read(p->readFd, &iov, 1, 0, &bytes_read, &future);
+                future.cancel();
+                if (future.wait() == 0)
+                {
+                    // Read won (likely).
+                    if (bytes_read == 0)
+                    {
+                        break;
+                    }
+                    got.append(buf, bytes_read);
+                }
+                else
+                {
+                    // Cancel won.
+                    // It's unlikely to happen consistently,
+                    // but to keep the test independent of kernel internals, read to make progress.
+                    if (!FiberScheduler::read(p->readFd, buf, sizeof(buf), 0, &bytes_read))
+                    {
+                        if (bytes_read == 0)
+                        {
+                            break;
+                        }
+                        got.append(buf, bytes_read);
+                    }
+                }
+            }
+
+            EXPECT_EQ(got, expected);
+            return 0;
+        }
+    };
+
+    int fds[2];
+    ASSERT_EQ(::pipe(fds), 0);
+
+    EXPECT_EQ(FiberScheduler::run(Params::fiberMain, Params{fds[0], fds[1]}), 0);
+
+    ::close(fds[0]);
+}
+
+} // namespace silk