tr: Fix regression causing read error with sockets (#8083)

* tr: Fix regression causing read error with sockets

The test used for determining if a file descriptor
tested with fstat points to a directory is traditionally
done by using the S_IFMT mask[^1][^2], e.g.:
    #define S_ISDIR(m)	(((m) & S_IFMT) == S_IFDIR)
In Rust, this translates to 'm & libc::S_IFMT == libc::S_IFDIR'.
is_stdin_directory() uses 'has!(mode, S_IFDIR)', which is
**not** equivalent and causes non-directory sockets to be
incorrectly recognized as directories. This causes uu-tr
to break when used with all sockets created with socketpair,
including ksh93 pipes[^3]. Below is an example Rust program
demonstrating why the current check is bogus:
  fn main() {
     use nix::sys::socket::{socketpair, SockFlag, AddressFamily, SockType};
     use nix::sys::stat::fstat;
     use libc::{S_IFDIR, S_IFMT, mode_t};
     let (_fd1, fd2) = socketpair(AddressFamily::Unix, SockType::Stream, None, SockFlag::empty()).unwrap();
     let mode = fstat(&fd2).unwrap().st_mode as mode_t;
     if mode & S_IFMT == S_IFDIR {   // Equivalent to S_ISDIR()
         println!("Not reached");    // Not a dir, so unreachable
     }
     if mode & S_IFDIR == S_IFDIR {  // Bogus check for S_IFDIR
         println!("BAD");
     }
  }

- is_stdin_directory(): Fix the regression introduced in 3e4221a4
  by replacing the bogus check with one equivalent to S_ISDIR().
- test_tr.rs: Add a regression test for this bug.

[^1]: https://sourceware.org/git/?p=glibc.git;a=blob;f=io/sys/stat.h;h=4bea9e9a#l123
[^2]: https://git.musl-libc.org/cgit/musl/tree/include/sys/stat.h?id=047a1639#n51
[^3]: cc5e0692/src/cmd/ksh93/sh/io.c (L98-L102)

Fixes https://github.com/uutils/coreutils/issues/7658

* Add comment explaining rationale

Also fix cargo clippy lint.
(In case it isn't obvious, I'm fairly new to Rust)

* Fix more lint

.vscode/cspell.dictionaries/workspace.wordlist.txt

@@ -154,6 +154,7 @@ IFSOCK
 IRGRP
 IROTH
 IRUSR
+ISDIR
 ISGID
 ISUID
 ISVTX
@@ -205,6 +206,7 @@ setgid
 setgroups
 settime
 setuid
+socketpair
 socktype
 statfs
 statp

Cargo.lock

@@ -1692,6 +1692,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "memoffset"
+version = "0.9.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "488016bfae457b036d996092f6cb448677611ce4449e970ceaf42695203f218a"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "minimal-lexical"
 version = "0.2.1"
@@ -1729,6 +1738,7 @@ dependencies = [
  "cfg-if",
  "cfg_aliases",
  "libc",
+ "memoffset",
 ]
 
 [[package]]

Cargo.toml

@@ -537,7 +537,13 @@ hex-literal = "1.0.0"
 rstest.workspace = true
 
 [target.'cfg(unix)'.dev-dependencies]
-nix = { workspace = true, features = ["process", "signal", "user", "term"] }
+nix = { workspace = true, features = [
+  "process",
+  "signal",
+  "socket",
+  "user",
+  "term",
+] }
 rlimit = "0.10.1"
 xattr.workspace = true
 

src/uucore/src/lib/features/fs.rs

@@ -720,7 +720,9 @@ pub fn is_stdin_directory(stdin: &Stdin) -> bool {
     {
         use nix::sys::stat::fstat;
         let mode = fstat(stdin.as_fd()).unwrap().st_mode as mode_t;
-        has!(mode, S_IFDIR)
+        // We use the S_IFMT mask ala S_ISDIR() to avoid mistaking
+        // sockets for directories.
+        mode & S_IFMT == S_IFDIR
     }
 
     #[cfg(windows)]

tests/by-util/test_tr.rs

@@ -1563,3 +1563,24 @@ fn test_broken_pipe_no_error() {
         .run_stdout_starts_with(b"")
         .fails_silently();
 }
+
+#[cfg(not(windows))]
+#[test]
+fn test_stdin_is_socket() {
+    use nix::sys::socket::{AddressFamily, SockFlag, SockType, socketpair};
+    use nix::unistd::write;
+
+    let (fd1, fd2) = socketpair(
+        AddressFamily::Unix,
+        SockType::Stream,
+        None,
+        SockFlag::empty(),
+    )
+    .unwrap();
+    write(fd1, b"::").unwrap();
+    new_ucmd!()
+        .args(&[":", ";"])
+        .set_stdin(fd2)
+        .succeeds()
+        .stdout_is(";;");
+}