handle unprivileged retry logic inside library

docs/guide.md

@@ -231,7 +231,7 @@ ex::sync_wait(ex::starts_on(sched, s));
 
 ### Unprivileged Mode
 
-The control commands provided by ublk-cpp support unprivileged mode natively, and this is transparent to upper-layer users. You only need one set of code to deal with both modes. (Of course, you may still need udev configuration logic similar to what [ublksrv describes](https://github.com/ublk-org/ublksrv#unprivileged-mode), and use sleep and retry to handle the race conditions caused by permission changes. But there is only so much ublk-cpp can do.)
+The control commands provided by ublk-cpp support unprivileged mode natively, and this is transparent to upper-layer users. You only need one set of code to deal with both modes. (Of course, you may still need udev configuration logic similar to what [ublksrv describes](https://github.com/ublk-org/ublksrv#unprivileged-mode).)
 
 ### Raw APIs
 

include/ublk/detail/control.hpp

@@ -6,9 +6,12 @@
 #pragma once
 
 #include "ublk/detail/path.hpp"
+#include "ublk/detail/retry.hpp"
 #include "ublk/detail/task.hpp"
 #include "ublk/raw.hpp"
+#include <cerrno>
 #include <condy.hpp>
+#include <system_error>
 
 namespace ublk {
 namespace detail {
@@ -20,19 +23,33 @@ struct control_get_dev_info_t {
     ex::task<void, TaskEnv<Sched, Alloc>> invoke(int fd, uint32_t dev_id,
                                                  ublksrv_ctrl_dev_info *info) {
         DevPathBuf<ublksrv_ctrl_dev_info> buf(dev_id);
-        auto s =
-            raw::get_dev_info2(fd, dev_id, &buf, sizeof(buf), DEV_PATH_LEN) |
-            ex::then([&](int32_t r) noexcept {
-                *info = buf.payload;
-                return r;
-            }) |
-            ex::let_error([&](std::error_code ec) {
-                if (ec.value() != EOPNOTSUPP) {
-                    throw std::system_error(ec, "get_dev_info2");
+        bool ok = co_await retry<Sched, Alloc>(
+            [&]() -> ex::task<bool, TaskEnv<Sched, Alloc>> {
+                int32_t r =
+                    co_await (raw::get_dev_info2(fd, dev_id, &buf, sizeof(buf),
+                                                 DEV_PATH_LEN) |
+                              ex::upon_error([](std::error_code ec) noexcept {
+                                  return -ec.value();
+                              }));
+                if (r >= 0) {
+                    *info = buf.payload;
+                    co_return true;
                 }
-                return raw::get_dev_info(fd, dev_id, info);
-            });
-        co_await std::move(s);
+                if (r == -EACCES) {
+                    co_return false;
+                }
+                if (r == -EOPNOTSUPP) {
+                    co_await raw::get_dev_info(fd, dev_id, info);
+                    co_return true;
+                }
+                throw std::system_error(-r, std::generic_category(),
+                                        "get_dev_info2");
+            },
+            20, 100);
+        if (!ok) {
+            throw std::system_error(EACCES, std::generic_category(),
+                                    "get_dev_info2");
+        }
     }
 };
 

include/ublk/detail/retry.hpp

@@ -0,0 +1,37 @@
+#pragma once
+
+#include "ublk/detail/task.hpp"
+#include <condy.hpp>
+#include <cstddef>
+#include <cstdint>
+#include <system_error>
+
+namespace ublk {
+namespace detail {
+
+namespace ex = condy::detail::ex;
+
+template <typename Sched, typename Alloc, typename Fn>
+inline ex::task<bool, TaskEnv<Sched, Alloc>> retry(Fn fn, size_t max_retry,
+                                                   int64_t sleep_ms) {
+    if (co_await fn()) {
+        co_return true;
+    }
+    __kernel_timespec ts = {};
+    ts.tv_sec = sleep_ms / 1000;
+    ts.tv_nsec = (sleep_ms % 1000) * 1'000'000;
+    bool ok = false;
+    for (size_t i = 0; i < max_retry; i++) {
+        co_await (condy::async_timeout(&ts, 0, 0) |
+                  ex::upon_error(
+                      [](std::error_code ec) noexcept { return -ec.value(); }));
+        ok = co_await fn();
+        if (ok) {
+            break;
+        }
+    }
+    co_return ok;
+}
+
+} // namespace detail
+} // namespace ublk

tests/test_control.cpp

@@ -33,9 +33,6 @@ TEST_CASE("test control") {
                       unprivileged ? UBLK_F_UNPRIVILEGED_DEV : 0);
         ex::sync_wait(ex::starts_on(sched, ublk::add_dev(control_fd, &info)));
         REQUIRE(info.dev_id == dev_id);
-        if (unprivileged) {
-            std::this_thread::sleep_for(std::chrono::milliseconds(100));
-        }
         ex::sync_wait(
             ex::starts_on(sched, ublk::del_dev(control_fd, info.dev_id)));
     }
@@ -62,10 +59,6 @@ TEST_CASE("test control") {
                                [](const std::exception_ptr &) noexcept {})));
         });
 
-        if (unprivileged) {
-            std::this_thread::sleep_for(std::chrono::milliseconds(100));
-        }
-
         SUBCASE("del_dev_async") {
             // TODO: Kernel bug
             // https://lore.kernel.org/linux-block/20260901142438.237586-1-yi1tang.yang@gmail.com/

tests/test_daemon.cpp

@@ -47,9 +47,6 @@ TEST_CASE("test daemon") {
     auto d2 = ublk::detail::defer([&]() noexcept {
         ex::sync_wait(ex::starts_on(sched, ublk::del_dev(control_fd, dev_id)));
     });
-    if (unprivileged) {
-        std::this_thread::sleep_for(std::chrono::milliseconds(100));
-    }
 
     std::tie(applyed) =
         // NOLINTNEXTLINE(bugprone-unchecked-optional-access)

tests/test_io.cpp

@@ -80,10 +80,6 @@ TEST_CASE("test io") {
                 ex::sync_wait(
                     ex::starts_on(sched, ublk::del_dev(control_fd, dev_id)));
             });
-            if (unprivileged) {
-                // Wait for udev to chown /dev/ublkcN.
-                std::this_thread::sleep_for(std::chrono::milliseconds(100));
-            }
 
             ublk_params params = {};
             fill_params(params);
@@ -236,9 +232,6 @@ TEST_CASE("test io") {
                         ex::upon_error(
                             [](const std::exception_ptr &) noexcept {})));
             });
-            if (unprivileged) {
-                std::this_thread::sleep_for(std::chrono::milliseconds(100));
-            }
 
             ublk_params params = {};
             fill_params(params);

tests/test_query.cpp

@@ -74,9 +74,6 @@ TEST_CASE("test query - cached_get_dev_info cache miss") {
     auto d2 = ublk::detail::defer([&]() noexcept {
         ex::sync_wait(ex::starts_on(sched, ublk::del_dev(control_fd, dev_id)));
     });
-    if (unprivileged) {
-        std::this_thread::sleep_for(std::chrono::milliseconds(100));
-    }
 
     // No cached info in the env -> miss -> fetch via syscall.
     ublksrv_ctrl_dev_info out{};