Dunk on NT and Darwin bc they don't use O_DIRECT which causes serious

problems for this project.

src/cli.rs

@@ -35,6 +35,12 @@ pub struct Args {
     // ArgAction::SetFalse by default evaluates to true,
     // ArgAction::SetTrue by default evaluates to false.
     /// Upon encountering a read error, reopen the source file before continuing.
-    #[arg(short, long, action=ArgAction::SetTrue)]
+    #[arg(short, long, action = ArgAction::SetTrue)]
     pub reopen_on_error: bool,
+
+    /// Use O_DIRECT to bypass kernel buffer when reading.
+    // BSD seems to support O_DIRECT, but MacOS for certain does not.
+    #[cfg(all(unix, not(target_os = "macos")))]
+    #[arg(short, long = "direct", action = ArgAction::SetFalse)]
+    pub direct_io: bool,
 }

src/io.rs

@@ -1,6 +1,5 @@
 use std::fs::{File, OpenOptions};
 use std::io::{self, Seek, SeekFrom};
-use std::os::unix::fs::OpenOptionsExt;
 
 use crate::cli::CONFIG;
 
@@ -20,10 +19,26 @@ pub fn get_stream_length<S: Seek>(stream: &mut S) -> io::Result<u64> {
     len
 }
 
+#[cfg(all(unix, not(target_os = "macos")))]
+pub fn load_input() -> anyhow::Result<File> {
+    use std::os::unix::fs::OpenOptionsExt;
+
+    let mut options = OpenOptions::new();
+    options.read(true);
+
+    if CONFIG.direct_io {
+        options.custom_flags(libc::O_DIRECT);
+    }
+
+    options
+        .open(&CONFIG.input)
+        .with_context(|| format!("Failed to open input file: {}", &CONFIG.input.display()))
+}
+
+#[cfg(any(not(unix), target_os = "macos"))]
 pub fn load_input() -> anyhow::Result<File> {
     OpenOptions::new()
         .read(true)
-        .custom_flags(libc::O_DIRECT)
         .open(&CONFIG.input)
         .with_context(|| format!("Failed to open input file: {}", &CONFIG.input.display()))
 }