fix(savedata): leave the mount result untouched when sceSaveDataMount3 fails

Animal Well (PPSA22520) checks for its save with a CREATE2 mount it never
unmounts, then saves with a second mount of the same directory. That
mount returns BUSY, and the title keeps using the first mount point for
sceSaveDataPrepare, its save file and sceSaveDataUmount2.
sceSaveDataMount3 cleared the result before its checks, so the save was
written to "/save" in the game folder instead of the save directory, and
the first mount was never released. The result is now written only when
the mount succeeds, as shadPS4 does.

core/libs/prx/libSceSaveData.native/Export.cpp

@@ -463,7 +463,6 @@ static int mount3(const SaveDataMount3* mount, SaveDataMountResult* mount_result
     if (mount == nullptr || mount_result == nullptr || mount->dir_name == nullptr) {
         throw std::runtime_error("sceSaveDataMount3: null argument");
     }
-    std::memset(mount_result, 0, sizeof(*mount_result));
     const bool create = (mount->mount_mode & SAVE_DATA_MOUNT_MODE_CREATE) != 0;
     const bool create2 = (mount->mount_mode & SAVE_DATA_MOUNT_MODE_CREATE2) != 0;
     const bool rdonly = (mount->mount_mode & SAVE_DATA_MOUNT_MODE_RDONLY) != 0;
@@ -498,6 +497,7 @@ static int mount3(const SaveDataMount3* mount, SaveDataMountResult* mount_result
     if (slot == -1) {
         return SAVE_DATA_ERROR_MOUNT_FULL;
     }
+    std::memset(mount_result, 0, sizeof(*mount_result));
     if (create || create2) {
         std::filesystem::create_directories(real_path);
     }

core/libs/tests/GuestSaveDataMountStatus.cpp

@@ -18,6 +18,8 @@ namespace {
 constexpr std::uint32_t MountReadWrite = 2;
 constexpr std::uint32_t MountCreate = 4;
 constexpr std::uint32_t MountCreate2 = 32;
+constexpr int SaveDataErrorBusy = -2137063421;
+constexpr int SaveDataErrorNotFound = -2137063416;
 
 int failures = 0;
 
@@ -41,6 +43,26 @@ void MountAndCheck(const char* name, std::uint32_t mode, std::uint32_t expectedS
     Check(sceSaveDataUmount2(0, &result.mount_point) == 0, what + ": unmounts");
 }
 
+void CheckFailedMountKeepsResult() {
+    SceSaveDataDirName held{};
+    std::memcpy(held.data, "held", sizeof("held"));
+    SaveDataMount3 mount{};
+    mount.dir_name = &held;
+    mount.mount_mode = MountReadWrite | MountCreate2;
+    SaveDataMountResult result{};
+    Check(sceSaveDataMount3(&mount, &result) == 0, "first mount of a save");
+    const SaveDataMountResult mounted = result;
+    Check(sceSaveDataMount3(&mount, &result) == SaveDataErrorBusy, "second mount of a mounted save is BUSY");
+    Check(std::memcmp(&result, &mounted, sizeof(result)) == 0, "a BUSY mount leaves the result as it was");
+    SceSaveDataDirName absent{};
+    std::memcpy(absent.data, "absent", sizeof("absent"));
+    mount.dir_name = &absent;
+    mount.mount_mode = MountReadWrite;
+    Check(sceSaveDataMount3(&mount, &result) == SaveDataErrorNotFound, "open of a missing save is NOT_FOUND");
+    Check(std::memcmp(&result, &mounted, sizeof(result)) == 0, "a NOT_FOUND mount leaves the result as it was");
+    Check(sceSaveDataUmount2(0, &result.mount_point) == 0, "the kept mount point unmounts the save");
+}
+
 }
 
 int main() {
@@ -53,6 +75,7 @@ int main() {
     MountAndCheck("fresh", MountReadWrite | MountCreate2, 0u, "CREATE2 of an existing save");
     MountAndCheck("made", MountReadWrite | MountCreate, 1u, "CREATE of a missing save");
     MountAndCheck("made", MountReadWrite, 0u, "open of an existing save");
+    CheckFailedMountKeepsResult();
     std::filesystem::current_path(previous);
     std::error_code error;
     std::filesystem::remove_all(root, error);

docs/dev/debt/libSceSaveData.native.md

@@ -12,4 +12,5 @@
 ### Functional
 
 - [sceSaveDataDirNameSearch2](../../../core/libs/prx/libSceSaveData.native/Export.cpp) (libSceSaveData.native) runs the same search as `sceSaveDataDirNameSearch` with the same condition and result structures, as [SharpEmu](https://github.com/sharpemu/sharpemu/blob/main/src/SharpEmu.Libs/SaveData/SaveDataExports.cs) does, its only source; what distinguishes it on the console is unknown.
+- [sceSaveDataMount3](../../../core/libs/prx/libSceSaveData.native/Export.cpp) (libSceSaveData.native) - a failed mount (BUSY, EXISTS, NOT_FOUND, MOUNT_FULL) leaves the result as it was, as shadPS4 does; not checked on hardware. Animal Well (PPSA22520) depends on it: it checks for its save with a mount it never unmounts, then saves with a second mount of the same directory, ignores its BUSY and keeps using the first mount point for `sceSaveDataPrepare`, its file and `sceSaveDataUmount2`