tac: use temp file for stdin to respect TMPDIR and handle disk-full errors

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

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

src/uu/tac/Cargo.toml

@@ -27,6 +27,7 @@ clap = { workspace = true }
 uucore = { workspace = true }
 thiserror = { workspace = true }
 fluent = { workspace = true }
+tempfile = { workspace = true }
 
 [[bin]]
 name = "tac"

src/uu/tac/src/tac.rs

@@ -13,6 +13,7 @@ use std::ffi::OsString;
 use std::io::{BufWriter, Read, Write, stdin, stdout};
 use std::{
     fs::{File, read},
+    io::copy,
     path::Path,
 };
 use uucore::error::UError;
@@ -241,14 +242,22 @@ fn tac(filenames: &[OsString], before: bool, regex: bool, separator: &str) -> UR
                 mmap = mmap1;
                 &mmap
             } else {
-                let mut buf1 = Vec::new();
-                if let Err(e) = stdin().read_to_end(&mut buf1) {
-                    let e: Box<dyn UError> = TacError::ReadError(OsString::from("stdin"), e).into();
-                    show!(e);
-                    continue;
+                // Copy stdin to a temp file (respects TMPDIR), then mmap it.
+                // Falls back to Vec buffer if temp file creation fails (e.g., bad TMPDIR).
+                match buffer_stdin() {
+                    Ok(StdinData::Mmap(mmap1)) => {
+                        mmap = mmap1;
+                        &mmap
+                    }
+                    Ok(StdinData::Vec(buf1)) => {
+                        buf = buf1;
+                        &buf
+                    }
+                    Err(e) => {
+                        show!(TacError::ReadError(OsString::from("stdin"), e));
+                        continue;
+                    }
                 }
-                buf = buf1;
-                &buf
             }
         } else {
             let path = Path::new(filename);
@@ -304,6 +313,30 @@ fn try_mmap_stdin() -> Option<Mmap> {
     unsafe { Mmap::map(&stdin()).ok() }
 }
 
+enum StdinData {
+    Mmap(Mmap),
+    Vec(Vec<u8>),
+}
+
+/// Copy stdin to a temp file, then memory-map it.
+/// Falls back to reading directly into memory if temp file creation fails.
+fn buffer_stdin() -> std::io::Result<StdinData> {
+    // 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)?;
+        // 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.
+        let mmap = unsafe { Mmap::map(&tmp)? };
+        Ok(StdinData::Mmap(mmap))
+    } else {
+        // Fall back to reading directly into memory (e.g., bad TMPDIR)
+        let mut buf = Vec::new();
+        stdin().read_to_end(&mut buf)?;
+        Ok(StdinData::Vec(buf))
+    }
+}
+
 fn try_mmap_path(path: &Path) -> Option<Mmap> {
     let file = File::open(path).ok()?;
 

tests/by-util/test_tac.rs

@@ -335,3 +335,15 @@ fn test_failed_write_is_reported() {
         .fails()
         .stderr_is("tac: failed to write to stdout: No space left on device (os error 28)\n");
 }
+
+#[cfg(target_os = "linux")]
+#[test]
+fn test_stdin_bad_tmpdir_fallback() {
+    // When TMPDIR is invalid, tac falls back to reading stdin directly into memory
+    new_ucmd!()
+        .env("TMPDIR", "/nonexistent/dir")
+        .arg("-")
+        .pipe_in("a\nb\nc\n")
+        .succeeds()
+        .stdout_is("c\nb\na\n");
+}

util/fetch-gnu.sh

@@ -12,3 +12,6 @@ curl -L ${repo}/raw/refs/heads/master/tests/csplit/csplit-io-err.sh > tests/cspl
 curl -L ${repo}/raw/refs/heads/master/tests/stty/bad-speed.sh > tests/stty/bad-speed.sh
 # 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