cp: implement --copy-contents option for fifos

Implement the `--copy-contents` option when the source is a FIFO, so
that the contents of the FIFO are copied (when the bytes become
available for reading) instead of the FIFO object itself. For example,

    $ mkfifo fifo
    $ cp --copy-contents fifo outfile &
    [1] 1614080
    $ echo foo > fifo
    $ cat outfile
    foo
    [1]+  Done                    cp --copy-contents fifo outfile

tests/by-util/test_cp.rs

@@ -1,4 +1,4 @@
-// spell-checker:ignore (flags) reflink (fs) tmpfs (linux) rlimit Rlim NOFILE clob btrfs ROOTDIR USERDIR procfs
+// spell-checker:ignore (flags) reflink (fs) tmpfs (linux) rlimit Rlim NOFILE clob btrfs ROOTDIR USERDIR procfs outfile
 
 use crate::common::util::*;
 #[cfg(not(windows))]
@@ -2226,3 +2226,32 @@ fn test_copy_dir_preserve_permissions_inaccessible_file() {
     let metadata2 = at.metadata("d2");
     assert_metadata_eq!(metadata1, metadata2);
 }
+
+/// Test for copying the contents of a FIFO as opposed to the FIFO object itself.
+#[cfg(unix)]
+#[test]
+fn test_copy_contents_fifo() {
+    let scenario = TestScenario::new(util_name!());
+    let at = &scenario.fixtures;
+
+    // Start the `cp` process, reading the contents of `fifo` and
+    // writing to regular file `outfile`.
+    at.mkfifo("fifo");
+    let mut ucmd = scenario.ucmd();
+    let child = ucmd
+        .args(&["--copy-contents", "fifo", "outfile"])
+        .run_no_wait();
+
+    // Write some bytes to the `fifo`. We expect these bytes to get
+    // copied through to `outfile`.
+    std::fs::write(at.plus("fifo"), "foo").unwrap();
+
+    // At this point the child process should have terminated
+    // successfully with no output. The `outfile` should have the
+    // contents of `fifo` copied into it.
+    let output = child.wait_with_output().unwrap();
+    assert!(output.status.success());
+    assert!(output.stdout.is_empty());
+    assert!(output.stderr.is_empty());
+    assert_eq!(at.read("outfile"), "foo");
+}