Add CreateEmptyDevice and WaitForDevice APIs

These APIs support a flow in which dm devices can be created before they
are actually needed, hence minimizing the time a process will wait for
ueventd to create user space paths.

Bug: 190618831
Test: atest libdm_test
Change-Id: I4dfa14e5271a6a13de6da73ec3c7efb1ebc0f8b8

fs_mgr/libdm/dm.cpp

@@ -170,19 +170,18 @@ static bool IsRecovery() {
     return access("/system/bin/recovery", F_OK) == 0;
 }
 
-bool DeviceMapper::CreateDevice(const std::string& name, const DmTable& table, std::string* path,
-                                const std::chrono::milliseconds& timeout_ms) {
+bool DeviceMapper::CreateEmptyDevice(const std::string& name) {
     std::string uuid = GenerateUuid();
-    if (!CreateDevice(name, uuid)) {
-        return false;
-    }
+    return CreateDevice(name, uuid);
+}
 
+bool DeviceMapper::WaitForDevice(const std::string& name,
+                                 const std::chrono::milliseconds& timeout_ms, std::string* path) {
     // We use the unique path for testing whether the device is ready. After
     // that, it's safe to use the dm-N path which is compatible with callers
     // that expect it to be formatted as such.
     std::string unique_path;
-    if (!LoadTableAndActivate(name, table) || !GetDeviceUniquePath(name, &unique_path) ||
-        !GetDmDevicePathByName(name, path)) {
+    if (!GetDeviceUniquePath(name, &unique_path) || !GetDmDevicePathByName(name, path)) {
         DeleteDevice(name);
         return false;
     }
@@ -208,6 +207,25 @@ bool DeviceMapper::CreateDevice(const std::string& name, const DmTable& table, s
     return true;
 }
 
+bool DeviceMapper::CreateDevice(const std::string& name, const DmTable& table, std::string* path,
+                                const std::chrono::milliseconds& timeout_ms) {
+    if (!CreateEmptyDevice(name)) {
+        return false;
+    }
+
+    if (!LoadTableAndActivate(name, table)) {
+        DeleteDevice(name);
+        return false;
+    }
+
+    if (!WaitForDevice(name, timeout_ms, path)) {
+        DeleteDevice(name);
+        return false;
+    }
+
+    return true;
+}
+
 bool DeviceMapper::GetDeviceUniquePath(const std::string& name, std::string* path) {
     struct dm_ioctl io;
     InitIo(&io, name);

fs_mgr/libdm/dm_test.cpp

@@ -29,6 +29,7 @@
 #include <thread>
 
 #include <android-base/file.h>
+#include <android-base/scopeguard.h>
 #include <android-base/strings.h>
 #include <android-base/unique_fd.h>
 #include <gtest/gtest.h>
@@ -679,3 +680,17 @@ TEST(libdm, DeleteDeviceDeferredWaitsForLastReference) {
     ASSERT_NE(0, access(path.c_str(), F_OK));
     ASSERT_EQ(ENOENT, errno);
 }
+
+TEST(libdm, CreateEmptyDevice) {
+    DeviceMapper& dm = DeviceMapper::Instance();
+    ASSERT_TRUE(dm.CreateEmptyDevice("empty-device"));
+    auto guard = android::base::make_scope_guard([&]() { dm.DeleteDevice("empty-device", 5s); });
+
+    // Empty device should be in suspended state.
+    ASSERT_EQ(DmDeviceState::SUSPENDED, dm.GetState("empty-device"));
+
+    std::string path;
+    ASSERT_TRUE(dm.WaitForDevice("empty-device", 5s, &path));
+    // Path should exist.
+    ASSERT_EQ(0, access(path.c_str(), F_OK));
+}

fs_mgr/libdm/include/libdm/dm.h

@@ -115,6 +115,19 @@ class DeviceMapper final {
     // - ACTIVE: resumes the device.
     bool ChangeState(const std::string& name, DmDeviceState state);
 
+    // Creates empty device.
+    // This supports a use case when a caller doesn't need a device straight away, but instead
+    // asks kernel to create it beforehand, thus avoiding blocking itself from waiting for ueventd
+    // to create user space paths.
+    // Callers are expected to then activate their device by calling LoadTableAndActivate function.
+    // To avoid race conditions, callers must still synchronize with ueventd by calling
+    // WaitForDevice function.
+    bool CreateEmptyDevice(const std::string& name);
+
+    // Waits for device paths to be created in the user space.
+    bool WaitForDevice(const std::string& name, const std::chrono::milliseconds& timeout_ms,
+                       std::string* path);
+
     // Creates a device, loads the given table, and activates it. If the device
     // is not able to be activated, it is destroyed, and false is returned.
     // After creation, |path| contains the result of calling