tac: fall back to memory when TMPDIR is invalid, add to root tests

Author: ChrisDryden

.vscode/cspell.dictionaries/jargon.wordlist.txt

@@ -215,3 +215,4 @@ TUNABLES
 tunables
 VMULL
 vmull
+tmpfs

src/uu/tac/src/tac.rs

@@ -310,13 +310,22 @@ fn try_mmap_stdin() -> Option<Mmap> {
 }
 
 /// Copy stdin to a temp file, then read it into a buffer.
+/// Falls back to reading directly into memory if temp file creation fails.
 fn read_stdin_to_buf() -> std::io::Result<Vec<u8>> {
-    let mut tmp = tempfile::tempfile()?;
-    copy(&mut stdin(), &mut tmp)?;
-    tmp.rewind()?;
-    let mut buf = Vec::new();
-    tmp.read_to_end(&mut buf)?;
-    Ok(buf)
+    // Try to create a temp file (respects TMPDIR)
+    if let Ok(mut tmp) = tempfile::tempfile() {
+        // Temp file created - copy stdin to it, then read back
+        copy(&mut stdin(), &mut tmp)?;
+        tmp.rewind()?;
+        let mut buf = Vec::new();
+        tmp.read_to_end(&mut buf)?;
+        Ok(buf)
+    } else {
+        // Fall back to reading directly into memory (e.g., bad TMPDIR)
+        let mut buf = Vec::new();
+        stdin().read_to_end(&mut buf)?;
+        Ok(buf)
+    }
 }
 
 fn try_mmap_path(path: &Path) -> Option<Mmap> {

tests/by-util/test_tac.rs

@@ -338,14 +338,12 @@ fn test_failed_write_is_reported() {
 
 #[cfg(target_os = "linux")]
 #[test]
-fn test_stdin_tempfile_error_continues() {
-    // Set TMPDIR to a non-writable location to force temp file creation to fail
+fn test_stdin_bad_tmpdir_fallback() {
+    // When TMPDIR is invalid, tac falls back to reading stdin directly into memory
     new_ucmd!()
-        .env("TMPDIR", "/proc")
+        .env("TMPDIR", "/nonexistent/dir")
         .arg("-")
-        .arg("prime_per_line.txt")
         .pipe_in("a\nb\nc\n")
-        .fails()
-        .stderr_contains("stdin")
-        .stdout_is_fixture("prime_per_line.expected");
+        .succeeds()
+        .stdout_is("c\nb\na\n");
 }

util/fetch-gnu.sh

@@ -11,3 +11,5 @@ curl -L ${repo}/raw/refs/heads/master/tests/csplit/csplit-io-err.sh > tests/cspl
 # Avoid incorrect PASS
 curl -L ${repo}/raw/refs/heads/master/tests/runcon/runcon-compute.sh > tests/runcon/runcon-compute.sh
 curl -L ${repo}/raw/refs/heads/master/tests/tac/tac-continue.sh > tests/tac/tac-continue.sh
+# Add tac-continue.sh to root tests (it requires root to mount tmpfs)
+sed -i 's|tests/split/l-chunk-root.sh.*|tests/split/l-chunk-root.sh\t\t\t\\\n  tests/tac/tac-continue.sh\t\t\t\\|' tests/local.mk