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use std::fs::File;
use std::path::Path;
use memmap::Mmap;
/// Controls the strategy used for determining when to use memory maps.
///
/// If a searcher is called in circumstances where it is possible to use memory
/// maps, and memory maps are enabled, then it will attempt to do so if it
/// believes it will make the search faster.
///
/// By default, memory maps are disabled.
#[derive(Clone, Debug)]
pub struct MmapChoice(MmapChoiceImpl);
#[derive(Clone, Debug)]
enum MmapChoiceImpl {
Auto,
Never,
}
impl Default for MmapChoice {
fn default() -> MmapChoice {
MmapChoice(MmapChoiceImpl::Never)
}
}
impl MmapChoice {
/// Use memory maps when they are believed to be advantageous.
///
/// The heuristics used to determine whether to use a memory map or not
/// may depend on many things, including but not limited to, file size
/// and platform.
///
/// If memory maps are unavailable or cannot be used for a specific input,
/// then normal OS read calls are used instead.
///
/// # Safety
///
/// This constructor is not safe because there is no obvious way to
/// encapsulate the safety of file backed memory maps on all platforms
/// without simultaneously negating some or all of their benefits.
///
/// The specific contract the caller is required to uphold isn't precise,
/// but it basically amounts to something like, "the caller guarantees that
/// the underlying file won't be mutated." This, of course, isn't feasible
/// in many environments. However, command line tools may still decide to
/// take the risk of, say, a `SIGBUS` occurring while attempting to read a
/// memory map.
pub unsafe fn auto() -> MmapChoice {
MmapChoice(MmapChoiceImpl::Auto)
}
/// Never use memory maps, no matter what. This is the default.
pub fn never() -> MmapChoice {
MmapChoice(MmapChoiceImpl::Never)
}
/// Return a memory map if memory maps are enabled and if creating a
/// memory from the given file succeeded and if memory maps are believed
/// to be advantageous for performance.
///
/// If this does attempt to open a memory map and it fails, then `None`
/// is returned and the corresponding error (along with the file path, if
/// present) is logged at the debug level.
pub(crate) fn open(
&self,
file: &File,
path: Option<&Path>,
) -> Option<Mmap> {
if !self.is_enabled() {
return None;
}
if cfg!(target_os = "macos") {
// I guess memory maps on macOS aren't great. Should re-evaluate.
return None;
}
// SAFETY: This is acceptable because the only way `MmapChoiceImpl` can
// be `Auto` is if the caller invoked the `auto` constructor, which
// is itself not safe. Thus, this is a propagation of the caller's
// assertion that using memory maps is safe.
match unsafe { Mmap::map(file) } {
Ok(mmap) => Some(mmap),
Err(err) => {
if let Some(path) = path {
log::debug!(
"{}: failed to open memory map: {}",
path.display(),
err
);
} else {
log::debug!("failed to open memory map: {}", err);
}
None
}
}
}
/// Whether this strategy may employ memory maps or not.
pub(crate) fn is_enabled(&self) -> bool {
match self.0 {
MmapChoiceImpl::Auto => true,
MmapChoiceImpl::Never => false,
}
}
}
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