From cac4a63ac439f2b183803a52e6e650ad8fefef5c Mon Sep 17 00:00:00 2001
From: Christopher Dryden <christopher.paul.dryden@gmail.com>
Date: Sat, 7 Feb 2026 17:44:22 +0000
Subject: [PATCH] tac: fix stdin piping on Windows by skipping empty mmap
 results

---
 src/uu/tac/src/tac.rs     |  6 +++++-
 tests/by-util/test_tac.rs | 27 ---------------------------
 2 files changed, 5 insertions(+), 28 deletions(-)

diff --git a/src/uu/tac/src/tac.rs b/src/uu/tac/src/tac.rs
index ec8ae450344..164cd58aa5b 100644
--- a/src/uu/tac/src/tac.rs
+++ b/src/uu/tac/src/tac.rs
@@ -411,7 +411,11 @@ fn tac(filenames: &[OsString], before: bool, regex: bool, separator: &str) -> UR
 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() }
+    let mmap = unsafe { Mmap::map(&stdin()).ok()? };
+    // On Windows, mmap on a pipe handle can "succeed" but return 0 bytes
+    // (the file size of a pipe is reported as 0). When that happens, return
+    // None so we fall through to buffer_stdin() which reads the pipe properly.
+    if mmap.is_empty() { None } else { Some(mmap) }
 }
 
 enum StdinData {
diff --git a/tests/by-util/test_tac.rs b/tests/by-util/test_tac.rs
index 1fc42d1c83f..dee01967749 100644
--- a/tests/by-util/test_tac.rs
+++ b/tests/by-util/test_tac.rs
@@ -29,8 +29,6 @@ fn test_invalid_arg() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_stdin_default() {
     new_ucmd!()
         .pipe_in("100\n200\n300\n400\n500")
@@ -39,8 +37,6 @@ fn test_stdin_default() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_stdin_non_newline_separator() {
     new_ucmd!()
         .args(&["-s", ":"])
@@ -50,8 +46,6 @@ fn test_stdin_non_newline_separator() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_stdin_non_newline_separator_before() {
     new_ucmd!()
         .args(&["-b", "-s", ":"])
@@ -104,14 +98,11 @@ fn test_invalid_input() {
 }
 
 #[test]
-#[cfg(not(windows))] // FIXME: https://github.com/uutils/coreutils/issues/4204
 fn test_no_line_separators() {
     new_ucmd!().pipe_in("a").succeeds().stdout_is("a");
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_before_trailing_separator_no_leading_separator() {
     new_ucmd!()
         .arg("-b")
@@ -121,8 +112,6 @@ fn test_before_trailing_separator_no_leading_separator() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_before_trailing_separator_and_leading_separator() {
     new_ucmd!()
         .arg("-b")
@@ -132,8 +121,6 @@ fn test_before_trailing_separator_and_leading_separator() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_before_leading_separator_no_trailing_separator() {
     new_ucmd!()
         .arg("-b")
@@ -143,8 +130,6 @@ fn test_before_leading_separator_no_trailing_separator() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_before_no_separator() {
     new_ucmd!()
         .arg("-b")
@@ -154,15 +139,11 @@ fn test_before_no_separator() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_before_empty_file() {
     new_ucmd!().arg("-b").pipe_in("").succeeds().stdout_is("");
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_multi_char_separator() {
     new_ucmd!()
         .args(&["-s", "xx"])
@@ -204,8 +185,6 @@ fn test_multi_char_separator_overlap() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_multi_char_separator_overlap_before() {
     // With the "-b" option, the line separator is assumed to be at the
     // beginning of the line. In this case, That is, "axxx" is
@@ -248,8 +227,6 @@ fn test_multi_char_separator_overlap_before() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_null_separator() {
     new_ucmd!()
         .args(&["-s", ""])
@@ -259,8 +236,6 @@ fn test_null_separator() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_regex() {
     new_ucmd!()
         .args(&["-r", "-s", "[xyz]+"])
@@ -289,8 +264,6 @@ fn test_regex() {
 }
 
 #[test]
-// FIXME: See https://github.com/uutils/coreutils/issues/4204
-#[cfg(not(windows))]
 fn test_regex_before() {
     new_ucmd!()
         .args(&["-b", "-r", "-s", "[xyz]+"])