tac: opportunistically use memory maps

Since tac must read its input files completely to start processing them
from the end, it is particularly suited to use memory maps to benefit
from the page cache maintained by the operating systems to bring the
necessary data into memory as required.

This does also include situations where the input is stdin, but not via
a pipe but for example a file descriptor set up by the user's shell
through an input redirection.

Cargo.lock

@@ -1064,6 +1064,15 @@ version = "2.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b16bd47d9e329435e309c58469fe0791c2d0d1ba96ec0954152a5ae2b04387dc"
 
+[[package]]
+name = "memmap2"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4647a11b578fead29cdbb34d4adef8dd3dc35b876c9c6d5240d83f205abfe96e"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "memoffset"
 version = "0.6.4"
@@ -3025,6 +3034,7 @@ version = "0.0.7"
 dependencies = [
  "clap",
  "memchr 2.4.0",
+ "memmap2",
  "regex",
  "uucore",
  "uucore_procs",

src/uu/tac/Cargo.toml

@@ -1,3 +1,5 @@
+# spell-checker:ignore memmap
+
 [package]
 name = "uu_tac"
 version = "0.0.7"
@@ -16,6 +18,7 @@ path = "src/tac.rs"
 
 [dependencies]
 memchr = "2"
+memmap2 = "0.5"
 regex = "1"
 clap = { version = "2.33", features = ["wrap_help"] }
 uucore = { version=">=0.0.9", package="uucore", path="../../uucore" }

src/uu/tac/src/tac.rs

@@ -5,15 +5,19 @@
 //  * For the full copyright and license information, please view the LICENSE
 //  * file that was distributed with this source code.
 
-// spell-checker:ignore (ToDO) sbytes slen dlen memmem
+// spell-checker:ignore (ToDO) sbytes slen dlen memmem memmap Mmap mmap SIGBUS
 
 #[macro_use]
 extern crate uucore;
 
 use clap::{crate_version, App, Arg};
 use memchr::memmem;
+use memmap2::Mmap;
 use std::io::{stdin, stdout, BufWriter, Read, Write};
-use std::{fs::read, path::Path};
+use std::{
+    fs::{read, File},
+    path::Path,
+};
 use uucore::display::Quotable;
 use uucore::InvalidEncodingHandling;
 
@@ -220,14 +224,23 @@ fn tac(filenames: Vec<&str>, before: bool, regex: bool, separator: &str) -> i32
     };
 
     for &filename in &filenames {
-        let data = if filename == "-" {
-            let mut data = Vec::new();
-            if let Err(e) = stdin().read_to_end(&mut data) {
-                show_error!("failed to read from stdin: {}", e);
-                exit_code = 1;
-                continue;
+        let mmap;
+        let buf;
+
+        let data: &[u8] = if filename == "-" {
+            if let Some(mmap1) = try_mmap_stdin() {
+                mmap = mmap1;
+                &mmap
+            } else {
+                let mut buf1 = Vec::new();
+                if let Err(e) = stdin().read_to_end(&mut buf1) {
+                    show_error!("failed to read from stdin: {}", e);
+                    exit_code = 1;
+                    continue;
+                }
+                buf = buf1;
+                &buf
             }
-            data
         } else {
             let path = Path::new(filename);
             if path.is_dir() || path.metadata().is_err() {
@@ -242,22 +255,47 @@ fn tac(filenames: Vec<&str>, before: bool, regex: bool, separator: &str) -> i32
                 exit_code = 1;
                 continue;
             }
-            match read(path) {
-                Ok(data) => data,
-                Err(e) => {
-                    show_error!("failed to read {}: {}", filename.quote(), e);
-                    exit_code = 1;
-                    continue;
+
+            if let Some(mmap1) = try_mmap_path(path) {
+                mmap = mmap1;
+                &mmap
+            } else {
+                match read(path) {
+                    Ok(buf1) => {
+                        buf = buf1;
+                        &buf
+                    }
+                    Err(e) => {
+                        show_error!("failed to read {}: {}", filename.quote(), e);
+                        exit_code = 1;
+                        continue;
+                    }
                 }
             }
         };
 
         if let Some(pattern) = &pattern {
-            buffer_tac_regex(&data, pattern, before)
+            buffer_tac_regex(data, pattern, before)
         } else {
-            buffer_tac(&data, before, separator)
+            buffer_tac(data, before, separator)
         }
         .unwrap_or_else(|e| crash!(1, "failed to write to stdout: {}", e));
     }
     exit_code
 }
+
+fn try_mmap_stdin() -> Option<Mmap> {
+    // SAFETY: If the file is truncated while we map it, SIGBUS will be raised
+    // and our process will be terminated, thus preventing access of invalid memory.
+    unsafe { Mmap::map(&stdin()).ok() }
+}
+
+fn try_mmap_path(path: &Path) -> Option<Mmap> {
+    let file = File::open(path).ok()?;
+
+    // SAFETY: If the file is truncated while we map it, SIGBUS will be raised
+    // and our process will be terminated, thus preventing access of invalid memory.
+    let mmap = unsafe { Mmap::map(&file).ok()? };
+
+    Some(mmap)
+}