vdm/core/src/io/sparse_file.cpp

// vdm/io/sparse_file.cpp

#include "vdm/io/sparse_file.hpp"

#include <fcntl.h>
#include <unistd.h>

#include <cerrno>
#include <cstring>
#include <utility>

namespace vdm::io {
namespace {

Error errno_to_error(int e) noexcept {
    switch (e) {
        case ENOSPC:
        case EDQUOT:
            return Error::disk_full;
        case EACCES:
        case EPERM:
        case EROFS:
            return Error::permission_denied;
        case ENOENT:
        case ENOTDIR:
        case EISDIR:
        case ENAMETOOLONG:
        case ELOOP:
            return Error::path_rejected;
        default:
            return Error::io_error;
    }
}

ErrorInfo sys_error(std::string_view what, int e) {
    return ErrorInfo(errno_to_error(e), std::string(what) + ": " + std::strerror(e));
}

}  // namespace

SparseFile::~SparseFile() {
    if (fd_ >= 0)
        ::close(fd_);
}

SparseFile::SparseFile(SparseFile &&o) noexcept
    : fd_(std::exchange(o.fd_, -1)),
      preallocated_(std::exchange(o.preallocated_, false)),
      path_(std::move(o.path_)) {}

SparseFile &SparseFile::operator=(SparseFile &&o) noexcept {
    if (this != &o) {
        if (fd_ >= 0)
            ::close(fd_);
        fd_ = std::exchange(o.fd_, -1);
        preallocated_ = std::exchange(o.preallocated_, false);
        path_ = std::move(o.path_);
    }
    return *this;
}

void SparseFile::reset() noexcept {
    fd_ = -1;
    preallocated_ = false;
    path_.clear();
}

Result<void> SparseFile::open(std::string_view path) {
    return open(path, OpenOptions{});
}

Result<void> SparseFile::open(std::string_view path, const OpenOptions &opts) {
    if (fd_ >= 0)
        return ErrorInfo(Error::internal, "SparseFile already open");

    std::string p(path);
    // O_NOFOLLOW: the final component of a download target must never be a symlink, on
    // create or on resume. DAEMON canonicalises the path and checks it against the allowed
    // roots before start(), but a symlink swapped in afterwards would redirect our writes
    // outside those roots (daemon/docs/safepath-adversarial.md leans on this open closing
    // that TOCTOU window). A symlinked leaf fails here with ELOOP -> Error::path_rejected.
    int flags = O_WRONLY | O_CREAT | O_CLOEXEC | O_NOFOLLOW;
    if (opts.truncate_existing)
        flags |= O_TRUNC;

    int fd = ::open(p.c_str(), flags, 0644);
    if (fd < 0)
        return sys_error("open " + p, errno);

    bool prealloc = false;
    if (opts.total_size > 0) {
        if (opts.preallocate) {
            // posix_fallocate returns the error number directly and does not set errno.
            int rc = ::posix_fallocate(fd, 0, static_cast<off_t>(opts.total_size));
            if (rc == 0) {
                prealloc = true;
            } else if (rc == EOPNOTSUPP || rc == ENOSYS || rc == EINVAL) {
                if (::ftruncate(fd, static_cast<off_t>(opts.total_size)) != 0) {
                    int e = errno;
                    ::close(fd);
                    return sys_error("ftruncate " + p, e);
                }
            } else {
                ::close(fd);
                return sys_error("posix_fallocate " + p, rc);
            }
        } else if (!opts.truncate_existing) {
            // Resuming: make sure the file is at least total_size so pwrite offsets land.
            if (::ftruncate(fd, static_cast<off_t>(opts.total_size)) != 0) {
                int e = errno;
                ::close(fd);
                return sys_error("ftruncate " + p, e);
            }
        }
    }

    fd_ = fd;
    preallocated_ = prealloc;
    path_ = std::move(p);
    return ok();
}

Result<void> SparseFile::write_at(std::uint64_t offset, ConstByteSpan data) {
    if (fd_ < 0)
        return ErrorInfo(Error::internal, "write_at on a closed SparseFile");

    const std::byte *p = data.data();
    std::size_t remaining = data.size();
    off_t pos = static_cast<off_t>(offset);

    while (remaining > 0) {
        ssize_t n = ::pwrite(fd_, p, remaining, pos);
        if (n < 0) {
            if (errno == EINTR)
                continue;
            return sys_error("pwrite", errno);
        }
        if (n == 0)
            return ErrorInfo(Error::io_error, "pwrite returned 0");
        p += n;
        pos += n;
        remaining -= static_cast<std::size_t>(n);
    }
    return ok();
}

Result<void> SparseFile::sync() {
    if (fd_ < 0)
        return ErrorInfo(Error::internal, "sync on a closed SparseFile");
    while (::fdatasync(fd_) != 0) {
        if (errno == EINTR)
            continue;
        return sys_error("fdatasync", errno);
    }
    return ok();
}

void SparseFile::advise_dontneed(std::uint64_t offset, std::uint64_t len) noexcept {
    if (fd_ < 0 || len == 0)
        return;
    ::posix_fadvise(fd_, static_cast<off_t>(offset), static_cast<off_t>(len), POSIX_FADV_DONTNEED);
}

Result<void> SparseFile::resize(std::uint64_t size) {
    if (fd_ < 0)
        return ErrorInfo(Error::internal, "resize on a closed SparseFile");
    while (::ftruncate(fd_, static_cast<off_t>(size)) != 0) {
        if (errno == EINTR)
            continue;
        return sys_error("ftruncate", errno);
    }
    return ok();
}

Result<void> SparseFile::close() {
    if (fd_ < 0)
        return ok();
    int fd = std::exchange(fd_, -1);
    int rc = ::close(fd);
    reset();
    if (rc != 0)
        return sys_error("close", errno);
    return ok();
}

}  // namespace vdm::io