io_utils.cc

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/* Copyright (C) 2004-2025 Olly Betts
 * Copyright (C) 2010 Richard Boulton
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 */
 
#include <config.h>
 
#include "io_utils.h"
#include "posixy_wrapper.h"
 
#include "safeunistd.h"
 
#include <cerrno>
#include <cstring>
#include <string>
 
#include <xapian/error.h>
 
#include "noreturn.h"
#include "omassert.h"
#include "str.h"
 
// Trying to include the correct headers with the correct defines set to
// get pread() and pwrite() prototyped on every platform without breaking any
// other platform is a real can of worms.  So instead we probe for what
// prototypes (if any) are required in configure and put them into
// PREAD_PROTOTYPE and PWRITE_PROTOTYPE.
#if defined HAVE_PREAD && defined PREAD_PROTOTYPE
PREAD_PROTOTYPE
#endif
#if defined HAVE_PWRITE && defined PWRITE_PROTOTYPE
PWRITE_PROTOTYPE
#endif
 
bool
io_unlink(const std::string & filename)
{
    if (posixy_unlink(filename.c_str()) == 0) {
        return true;
    }
    if (errno != ENOENT) {
        throw Xapian::DatabaseError(filename + ": delete failed", errno);
    }
    return false;
}
 
// The smallest fd we want to use for a writable handle.
//
// We want to avoid using fd < MIN_WRITE_FD, in case some other code in the
// same process tries to write to stdout or stderr, which would end up
// corrupting our database.
const int MIN_WRITE_FD = 3;
 
static int
move_to_higher_fd_(int fd)
{
    int badfd = fd;
#ifdef F_DUPFD_CLOEXEC
    // dup to the first unused fd >= MIN_WRITE_FD.
    fd = fcntl(badfd, F_DUPFD_CLOEXEC, MIN_WRITE_FD);
    // F_DUPFD_CLOEXEC may not be supported.
    if (fd < 0 && errno == EINVAL)
#endif
#ifdef F_DUPFD
    {
        fd = fcntl(badfd, F_DUPFD, MIN_WRITE_FD);
# ifdef FD_CLOEXEC
        if (fd >= 0)
            (void)fcntl(fd, F_SETFD, FD_CLOEXEC);
# endif
    }
    int save_errno = errno;
    (void)close(badfd);
    errno = save_errno;
#else
    {
        char toclose[MIN_WRITE_FD];
        memset(toclose, 0, sizeof(toclose));
        fd = badfd;
        do {
            toclose[fd] = 1;
            fd = dup(fd);
        } while (fd >= 0 && fd < MIN_WRITE_FD);
        int save_errno = errno;
        for (badfd = 0; badfd != MIN_WRITE_FD; ++badfd)
            if (toclose[badfd])
                close(badfd);
        if (fd < 0) {
            errno = save_errno;
        } else {
# ifdef FD_CLOEXEC
            (void)fcntl(fd, F_SETFD, FD_CLOEXEC);
# endif
        }
    }
#endif
    Assert(fd >= MIN_WRITE_FD || fd < 0);
    return fd;
}
 
static inline int
move_to_higher_fd(int fd)
{
    if (usual(fd >= MIN_WRITE_FD || fd < 0)) return fd;
    return move_to_higher_fd_(fd);
}
 
int
io_open_block_wr(const char* filename, bool anew)
{
    // Use auto because on AIX O_CLOEXEC may be a 64-bit integer constant.
    auto flags = O_RDWR | O_BINARY | O_CLOEXEC;
    if (anew) flags |= O_CREAT | O_TRUNC;
    int fd = ::open(filename, flags, 0666);
    return move_to_higher_fd(fd);
}
 
size_t
io_read(int fd, char * p, size_t n, size_t min)
{
    size_t total = 0;
    while (n) {
        ssize_t c = read(fd, p, n);
        if (c <= 0) {
            if (c == 0) {
                if (total >= min) break;
                throw Xapian::DatabaseCorruptError("Couldn't read enough (EOF)");
            }
            if (errno == EINTR) continue;
            throw Xapian::DatabaseError("Error reading from file", errno);
        }
        p += c;
        total += c;
        n -= c;
    }
    return total;
}
 
void
io_write(int fd, const char * p, size_t n)
{
    while (n) {
        ssize_t c = write(fd, p, n);
        if (c < 0) {
            if (errno == EINTR) continue;
            throw Xapian::DatabaseError("Error writing to file", errno);
        }
        p += c;
        n -= c;
    }
}
 
XAPIAN_NORETURN(
        static void throw_block_error(const char * s, off_t b, int e = 0));
static void
throw_block_error(const char * s, off_t b, int e)
{
    std::string m = s;
    m += str(b);
    throw Xapian::DatabaseError(m, e);
}
 
#ifdef HAVE_POSIX_FADVISE
bool
io_readahead_block(int fd, size_t n, off_t b, off_t o)
{
    o += b * n;
    // Assume that any failure is likely to also happen for another call with
    // the same fd.
    return posix_fadvise(fd, o, n, POSIX_FADV_WILLNEED) == 0;
}
#endif
 
void
io_read_block(int fd, char * p, size_t n, off_t b, off_t o)
{
    o += b * n;
    // Prefer pread if available since it's typically implemented as a
    // separate syscall, and that eliminates the overhead of an extra syscall
    // per block read.
#ifdef HAVE_PREAD
    while (true) {
        ssize_t c = pread(fd, p, n, o);
        // We should get a full read most of the time, so streamline that case.
        if (usual(c == ssize_t(n)))
            return;
        // -1 is error, 0 is EOF
        if (c <= 0) {
            if (c == 0)
                throw_block_error("EOF reading block ", b);
            // We get EINTR if the syscall was interrupted by a signal.
            // In this case we should retry the read.
            if (errno == EINTR) continue;
            throw_block_error("Error reading block ", b, errno);
        }
        p += c;
        n -= c;
        o += c;
    }
#else
    if (rare(lseek(fd, o, SEEK_SET) < 0))
        throw_block_error("Error seeking to block ", b, errno);
    while (true) {
        ssize_t c = read(fd, p, n);
        // We should get a full read most of the time, so streamline that case.
        if (usual(c == ssize_t(n)))
            return;
        if (c <= 0) {
            if (c == 0)
                throw_block_error("EOF reading block ", b);
            // We get EINTR if the syscall was interrupted by a signal.
            // In this case we should retry the read.
            if (errno == EINTR) continue;
            throw_block_error("Error reading block ", b, errno);
        }
        p += c;
        n -= c;
    }
#endif
}
 
void
io_write_block(int fd, const char * p, size_t n, off_t b, off_t o)
{
    o += b * n;
    // Prefer pwrite if available since it's typically implemented as a
    // separate syscall, and that eliminates the overhead of an extra syscall
    // per block write.
#ifdef HAVE_PWRITE
    while (true) {
        ssize_t c = pwrite(fd, p, n, o);
        // We should get a full write most of the time, so streamline that case.
        if (usual(c == ssize_t(n)))
            return;
        if (c < 0) {
            // We get EINTR if the syscall was interrupted by a signal.
            // In this case we should retry the write.
            if (errno == EINTR) continue;
            throw_block_error("Error writing block ", b, errno);
        }
        p += c;
        n -= c;
        o += c;
    }
#else
    if (rare(lseek(fd, o, SEEK_SET) < 0))
        throw_block_error("Error seeking to block ", b, errno);
    while (true) {
        ssize_t c = write(fd, p, n);
        // We should get a full write most of the time, so streamline that case.
        if (usual(c == ssize_t(n)))
            return;
        if (c < 0) {
            // We get EINTR if the syscall was interrupted by a signal.
            // In this case we should retry the write.
            if (errno == EINTR) continue;
            throw_block_error("Error writing block ", b, errno);
        }
        p += c;
        n -= c;
    }
#endif
}
 
bool
io_tmp_rename(const std::string & tmp_file, const std::string & real_file)
{
#ifdef EXDEV
    // We retry on EXDEV a few times as some older Linux kernels are buggy and
    // fail with EXDEV when the two files are on the same device (as they
    // always ought to be when this function is used).  Don't retry forever in
    // case someone calls this with files on different devices.
    //
    // We're not sure exactly which kernels are buggy in this way, but there's
    // discussion here: https://www.spinics.net/lists/linux-nfs/msg17306.html
    //
    // Reported at: https://trac.xapian.org/ticket/698
    int retries = 5;
retry:
#endif
    if (posixy_rename(tmp_file.c_str(), real_file.c_str()) < 0) {
#ifdef EXDEV
        if (errno == EXDEV && --retries > 0) goto retry;
#endif
        // With NFS, rename() failing may just mean that the server crashed
        // after successfully renaming, but before reporting this, and then
        // the retried operation fails.  So we need to check if the source
        // file still exists, which we do by calling unlink(), since we want
        // to remove the temporary file anyway.
        int saved_errno = errno;
        if (unlink(tmp_file.c_str()) == 0 || errno != ENOENT) {
            errno = saved_errno;
            return false;
        }
    }
    return true;
}