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Commit 55974f14 authored by Yifan Hong's avatar Yifan Hong
Browse files

libsnapshot: write files atomically 

... by writing to a temporary file then rename()'ing it
back.

Test: libsnapshot_test
Bug: 144549076
Change-Id: Ide400aff8d67d56d422d0adea3a4f1673ebc9994
parent b6663cfb

fs_mgr/libsnapshot/snapshot.cpp

+7 −7
Original line number Diff line number Diff line
@@ -217,7 +217,7 @@ bool SnapshotManager::FinishedSnapshotWrites() { 
    // we can tell whether or not we performed a rollback.

    auto contents = device_->GetSlotSuffix();

    auto boot_file = GetSnapshotBootIndicatorPath();

    if (!android::base::WriteStringToFile(contents, boot_file)) {

    if (!WriteStringToFileAtomic(contents, boot_file)) {

        PLOG(ERROR) << "write failed: " << boot_file;

        return false;

    }
@@ -1731,7 +1731,7 @@ bool SnapshotManager::WriteUpdateState(LockedFile* lock, UpdateState state) { 
    }

#endif



    if (!android::base::WriteStringToFile(contents, GetStateFilePath())) {

    if (!WriteStringToFileAtomic(contents, GetStateFilePath())) {

        PLOG(ERROR) << "Could not write to state file";

        return false;

    }
@@ -1780,14 +1780,14 @@ bool SnapshotManager::WriteSnapshotStatus(LockedFile* lock, const SnapshotStatus 
    CHECK(!status.name().empty());



    auto path = GetSnapshotStatusFilePath(status.name());

    unique_fd fd(

            open(path.c_str(), O_RDWR | O_CLOEXEC | O_NOFOLLOW | O_CREAT | O_SYNC | O_TRUNC, 0660));

    if (fd < 0) {

        PLOG(ERROR) << "Open failed: " << path;



    std::string content;

    if (!status.SerializeToString(&content)) {

        LOG(ERROR) << "Unable to serialize SnapshotStatus for " << status.name();

        return false;

    }



    if (!status.SerializeToFileDescriptor(fd.get())) {

    if (!WriteStringToFileAtomic(content, path)) {

        PLOG(ERROR) << "Unable to write SnapshotStatus to " << path;

        return false;

    }

fs_mgr/libsnapshot/utility.cpp

+12 −0
Original line number Diff line number Diff line
@@ -140,5 +140,17 @@ AutoUnmountDevice::~AutoUnmountDevice() { 
    }

}



bool WriteStringToFileAtomic(const std::string& content, const std::string& path) {

    std::string tmp_path = path + ".tmp";

    if (!android::base::WriteStringToFile(content, tmp_path)) {

        return false;

    }

    if (rename(tmp_path.c_str(), path.c_str()) == -1) {

        PLOG(ERROR) << "rename failed from " << tmp_path << " to " << path;

        return false;

    }

    return true;

}



}  // namespace snapshot

}  // namespace android

fs_mgr/libsnapshot/utility.h

+7 −0
Original line number Diff line number Diff line
@@ -112,5 +112,12 @@ std::vector<android::fs_mgr::Partition*> ListPartitionsWithSuffix( 
// Initialize a device before using it as the COW device for a dm-snapshot device.

bool InitializeCow(const std::string& device);



// "Atomically" write string to file. This is done by a series of actions:

// 1. Write to path + ".tmp"

// 2. Move temporary file to path using rename()

// Note that rename() is an atomic operation. This function may not work properly if there

// is an open fd to |path|, because that fd has an old view of the file.

bool WriteStringToFileAtomic(const std::string& content, const std::string& path);



}  // namespace snapshot

}  // namespace android