tests/tail: Add a test for tail'ing large files

This tests both large files and iterating backwards through the file when we
need to search backwards further than our BUFFER_SIZE.

tests/tail.rs

@@ -1,3 +1,5 @@
+use std::io::Write;
+
 #[macro_use]
 mod common;
 
@@ -7,6 +9,9 @@ static UTIL_NAME: &'static str = "tail";
 
 static INPUT: &'static str = "foobar.txt";
 
+static BIG: &'static str = "big.txt";
+
+static BIG_EXPECTED: &'static str = "big_single_big_args.expected";
 
 #[test]
 fn test_stdin_default() {
@@ -21,3 +26,34 @@ fn test_single_default() {
     let result = ucmd.arg(INPUT).run();
     assert_eq!(result.stdout, at.read("foobar_single_default.expected"));
 }
+
+const BIG_LINES: usize = 1_000_000;
+const BIG_N_ARG: usize = 100_000;
+
+fn generate_big_test_files(at: &AtPath) {
+    let mut big_input = at.make_file(BIG);
+    for i in 0..BIG_LINES {
+        write!(&mut big_input, "Line {}\n", i).expect("Could not write to BIG file");
+    }
+    big_input.flush().expect("Could not flush BIG file");
+
+    let mut big_expected = at.make_file(BIG_EXPECTED);
+    for i in (BIG_LINES - BIG_N_ARG)..BIG_LINES {
+        write!(&mut big_expected, "Line {}\n", i).expect("Could not write to BIG_EXPECTED file");
+    }
+    big_expected.flush().expect("Could not flush BIG_EXPECTED file");
+}
+
+fn cleanup_big_test_files(at: &AtPath) {
+    at.cleanup(BIG);
+    at.cleanup(BIG_EXPECTED);
+}
+
+#[test]
+fn test_single_big_args() {
+    let (at, mut ucmd) = testing(UTIL_NAME);
+    generate_big_test_files(&at);
+    let result = ucmd.arg(BIG).arg("-n").arg(format!("{}", BIG_N_ARG)).run();
+    assert_eq!(result.stdout, at.read(BIG_EXPECTED));
+    cleanup_big_test_files(&at);
+}