faux-folly/folly/FileUtil.h

/*
 * Copyright 2015 Facebook, Inc.
 *
 * 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.
 */

#ifndef FOLLY_FILEUTIL_H_
#define FOLLY_FILEUTIL_H_

#include <folly/Conv.h>
#include <folly/Portability.h>
#include <folly/ScopeGuard.h>

#include <cassert>
#include <limits>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <fcntl.h>
#include <unistd.h>

namespace folly {

/**
 * Convenience wrappers around some commonly used system calls.  The *NoInt
 * wrappers retry on EINTR.  The *Full wrappers retry on EINTR and also loop
 * until all data is written.  Note that *Full wrappers weaken the thread
 * semantics of underlying system calls.
 */
int openNoInt(const char* name, int flags, mode_t mode = 0666);
int closeNoInt(int fd);
int dupNoInt(int fd);
int dup2NoInt(int oldfd, int newfd);
int fsyncNoInt(int fd);
int fdatasyncNoInt(int fd);
int ftruncateNoInt(int fd, off_t len);
int truncateNoInt(const char* path, off_t len);
int flockNoInt(int fd, int operation);
int shutdownNoInt(int fd, int how);

ssize_t readNoInt(int fd, void* buf, size_t n);
ssize_t preadNoInt(int fd, void* buf, size_t n, off_t offset);
ssize_t readvNoInt(int fd, const iovec* iov, int count);

ssize_t writeNoInt(int fd, const void* buf, size_t n);
ssize_t pwriteNoInt(int fd, const void* buf, size_t n, off_t offset);
ssize_t writevNoInt(int fd, const iovec* iov, int count);

/**
 * Wrapper around read() (and pread()) that, in addition to retrying on
 * EINTR, will loop until all data is read.
 *
 * This wrapper is only useful for blocking file descriptors (for non-blocking
 * file descriptors, you have to be prepared to deal with incomplete reads
 * anyway), and only exists because POSIX allows read() to return an incomplete
 * read if interrupted by a signal (instead of returning -1 and setting errno
 * to EINTR).
 *
 * Note that this wrapper weakens the thread safety of read(): the file pointer
 * is shared between threads, but the system call is atomic.  If multiple
 * threads are reading from a file at the same time, you don't know where your
 * data came from in the file, but you do know that the returned bytes were
 * contiguous.  You can no longer make this assumption if using readFull().
 * You should probably use pread() when reading from the same file descriptor
 * from multiple threads simultaneously, anyway.
 *
 * Note that readvFull and preadvFull require iov to be non-const, unlike
 * readv and preadv.  The contents of iov after these functions return
 * is unspecified.
 */
ssize_t readFull(int fd, void* buf, size_t n);
ssize_t preadFull(int fd, void* buf, size_t n, off_t offset);
ssize_t readvFull(int fd, iovec* iov, int count);
#if FOLLY_HAVE_PREADV
ssize_t preadvFull(int fd, iovec* iov, int count, off_t offset);
#endif

/**
 * Similar to readFull and preadFull above, wrappers around write() and
 * pwrite() that loop until all data is written.
 *
 * Generally, the write() / pwrite() system call may always write fewer bytes
 * than requested, just like read().  In certain cases (such as when writing to
 * a pipe), POSIX provides stronger guarantees, but not in the general case.
 * For example, Linux (even on a 64-bit platform) won't write more than 2GB in
 * one write() system call.
 *
 * Note that writevFull and pwritevFull require iov to be non-const, unlike
 * writev and pwritev.  The contents of iov after these functions return
 * is unspecified.
 */
ssize_t writeFull(int fd, const void* buf, size_t n);
ssize_t pwriteFull(int fd, const void* buf, size_t n, off_t offset);
ssize_t writevFull(int fd, iovec* iov, int count);
#if FOLLY_HAVE_PWRITEV
ssize_t pwritevFull(int fd, iovec* iov, int count, off_t offset);
#endif

/**
 * Read entire file (if num_bytes is defaulted) or no more than
 * num_bytes (otherwise) into container *out. The container is assumed
 * to be contiguous, with element size equal to 1, and offer size(),
 * reserve(), and random access (e.g. std::vector<char>, std::string,
 * fbstring).
 *
 * Returns: true on success or false on failure. In the latter case
 * errno will be set appropriately by the failing system primitive.
 */
template <class Container>
bool readFile(const char* file_name, Container& out,
              size_t num_bytes = std::numeric_limits<size_t>::max()) {
  static_assert(sizeof(out[0]) == 1,
                "readFile: only containers with byte-sized elements accepted");
  assert(file_name);

  const auto fd = openNoInt(file_name, O_RDONLY);
  if (fd == -1) return false;

  size_t soFar = 0; // amount of bytes successfully read
  SCOPE_EXIT {
    assert(out.size() >= soFar); // resize better doesn't throw
    out.resize(soFar);
    // Ignore errors when closing the file
    closeNoInt(fd);
  };

  // Obtain file size:
  struct stat buf;
  if (fstat(fd, &buf) == -1) return false;
  // Some files (notably under /proc and /sys on Linux) lie about
  // their size, so treat the size advertised by fstat under advise
  // but don't rely on it. In particular, if the size is zero, we
  // should attempt to read stuff. If not zero, we'll attempt to read
  // one extra byte.
  constexpr size_t initialAlloc = 1024 * 4;
  out.resize(
    std::min(
      buf.st_size > 0 ? folly::to<size_t>(buf.st_size + 1) : initialAlloc,
      num_bytes));

  while (soFar < out.size()) {
    const auto actual = readFull(fd, &out[soFar], out.size() - soFar);
    if (actual == -1) {
      return false;
    }
    soFar += actual;
    if (soFar < out.size()) {
      // File exhausted
      break;
    }
    // Ew, allocate more memory. Use exponential growth to avoid
    // quadratic behavior. Cap size to num_bytes.
    out.resize(std::min(out.size() * 3 / 2, num_bytes));
  }

  return true;
}

/**
 * Writes container to file. The container is assumed to be
 * contiguous, with element size equal to 1, and offering STL-like
 * methods empty(), size(), and indexed access
 * (e.g. std::vector<char>, std::string, fbstring, StringPiece).
 *
 * "flags" dictates the open flags to use. Default is to create file
 * if it doesn't exist and truncate it.
 *
 * Returns: true on success or false on failure. In the latter case
 * errno will be set appropriately by the failing system primitive.
 */
template <class Container>
bool writeFile(const Container& data, const char* filename,
              int flags = O_WRONLY | O_CREAT | O_TRUNC) {
  static_assert(sizeof(data[0]) == 1,
                "writeFile works with element size equal to 1");
  int fd = open(filename, flags, 0666);
  if (fd == -1) {
    return false;
  }
  bool ok = data.empty() ||
    writeFull(fd, &data[0], data.size()) == static_cast<ssize_t>(data.size());
  return closeNoInt(fd) == 0 && ok;
}

}  // namespaces

#endif /* FOLLY_FILEUTIL_H_ */