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/*
* Copyright (C) 2011 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include "scoped_flock.h"
#include <sys/file.h>
#include <sys/stat.h>
#include <cstring>
#include <android-base/logging.h>
#include <android-base/stringprintf.h>
#include "file_utils.h"
#include "unix_file/fd_file.h"
namespace art {
using android::base::StringPrintf;
/* static */ ScopedFlock LockedFile::Open(const char* filename, std::string* error_msg) {
return Open(filename, O_CREAT | O_RDWR, true, error_msg);
}
/* static */ ScopedFlock LockedFile::Open(const char* filename, int flags, bool block,
std::string* error_msg) {
#ifdef _WIN32
// TODO: implement file locking for Windows.
UNUSED(filename);
UNUSED(flags);
UNUSED(block);
*error_msg = "flock is unsupported on Windows";
return nullptr;
#else
while (true) {
// NOTE: We don't check usage here because the ScopedFlock should *never* be
// responsible for flushing its underlying FD. Its only purpose should be
// to acquire a lock, and the unlock / close in the corresponding
// destructor. Callers should explicitly flush files they're writing to if
// that is the desired behaviour.
std::unique_ptr<File> file(OS::OpenFileWithFlags(filename, flags, /* auto_flush= */ false));
if (file.get() == nullptr) {
*error_msg = StringPrintf("Failed to open file '%s': %s", filename, strerror(errno));
return nullptr;
}
int operation = block ? LOCK_EX : (LOCK_EX | LOCK_NB);
int flock_result = TEMP_FAILURE_RETRY(flock(file->Fd(), operation));
if (flock_result == EWOULDBLOCK) {
// File is locked by someone else and we are required not to block;
return nullptr;
}
if (flock_result != 0) {
*error_msg = StringPrintf("Failed to lock file '%s': %s", filename, strerror(errno));
return nullptr;
}
struct stat fstat_stat;
int fstat_result = TEMP_FAILURE_RETRY(fstat(file->Fd(), &fstat_stat));
if (fstat_result != 0) {
*error_msg = StringPrintf("Failed to fstat file '%s': %s", filename, strerror(errno));
return nullptr;
}
struct stat stat_stat;
int stat_result = TEMP_FAILURE_RETRY(stat(filename, &stat_stat));
if (stat_result != 0) {
PLOG(WARNING) << "Failed to stat, will retry: " << filename;
// ENOENT can happen if someone racing with us unlinks the file we created so just retry.
if (block) {
continue;
} else {
// Note that in theory we could race with someone here for a long time and end up retrying
// over and over again. This potential behavior does not fit well in the non-blocking
// semantics. Thus, if we are not require to block return failure when racing.
return nullptr;
}
}
if (fstat_stat.st_dev != stat_stat.st_dev || fstat_stat.st_ino != stat_stat.st_ino) {
LOG(WARNING) << "File changed while locking, will retry: " << filename;
if (block) {
continue;
} else {
// See comment above.
return nullptr;
}
}
return ScopedFlock(new LockedFile(std::move((*file.get()))));
}
#endif
}
ScopedFlock LockedFile::DupOf(const int fd, const std::string& path,
const bool read_only_mode, std::string* error_msg) {
#ifdef _WIN32
// TODO: implement file locking for Windows.
UNUSED(fd);
UNUSED(path);
UNUSED(read_only_mode);
*error_msg = "flock is unsupported on Windows.";
return nullptr;
#else
// NOTE: We don't check usage here because the ScopedFlock should *never* be
// responsible for flushing its underlying FD. Its only purpose should be
// to acquire a lock, and the unlock / close in the corresponding
// destructor. Callers should explicitly flush files they're writing to if
// that is the desired behaviour.
ScopedFlock locked_file(
new LockedFile(DupCloexec(fd), path, /* check_usage= */ false, read_only_mode));
if (locked_file->Fd() == -1) {
*error_msg = StringPrintf("Failed to duplicate open file '%s': %s",
locked_file->GetPath().c_str(), strerror(errno));
return nullptr;
}
if (0 != TEMP_FAILURE_RETRY(flock(locked_file->Fd(), LOCK_EX))) {
*error_msg = StringPrintf(
"Failed to lock file '%s': %s", locked_file->GetPath().c_str(), strerror(errno));
return nullptr;
}
return locked_file;
#endif
}
void LockedFile::ReleaseLock() {
#ifndef _WIN32
if (this->Fd() != -1) {
int flock_result = TEMP_FAILURE_RETRY(flock(this->Fd(), LOCK_UN));
if (flock_result != 0) {
// Only printing a warning is okay since this is only used with either:
// 1) a non-blocking Init call, or
// 2) as a part of a seperate binary (eg dex2oat) which has it's own timeout logic to prevent
// deadlocks.
// This means we can be sure that the warning won't cause a deadlock.
PLOG(WARNING) << "Unable to unlock file " << this->GetPath();
}
}
#endif
}
} // namespace art
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