tail: fix stdin redirect when file is not at its beginning

Previously, if stdin redirect pointed to a regular file,
tailing started at the beginning of the file. However,
tailing needs to start at the current position because this
is expected by tests/tail-2/start-middle.sh.

This fixes the issue by taking the current offset into account
while going backwards through the stdin redirected file.

src/uu/tail/src/chunks.rs

@@ -30,7 +30,8 @@ pub struct ReverseChunks<'a> {
 
 impl<'a> ReverseChunks<'a> {
     pub fn new(file: &'a mut File) -> ReverseChunks<'a> {
-        let size = file.seek(SeekFrom::End(0)).unwrap();
+        let current = file.seek(SeekFrom::Current(0)).unwrap();
+        let size = file.seek(SeekFrom::End(0)).unwrap() - current;
         let max_blocks_to_read = (size as f64 / BLOCK_SIZE as f64).ceil() as usize;
         let block_idx = 0;
         ReverseChunks {

src/uu/tail/src/tail.rs

@@ -128,6 +128,8 @@ pub struct Settings {
     use_polling: bool,
     verbose: bool,
     stdin_is_pipe_or_fifo: bool,
+    stdin_offset: u64,
+    stdin_redirect: PathBuf,
 }
 
 impl Settings {
@@ -323,6 +325,15 @@ fn uu_tail(mut settings: Settings) -> UResult<()> {
         settings.paths.push_front(dash);
     }
 
+    if cfg!(unix) && settings.stdin_is_pipe_or_fifo {
+        settings.stdin_redirect = PathBuf::from(text::STDIN_HEADER).handle_redirect();
+        use std::os::unix::io::FromRawFd;
+        let mut stdin_handle = unsafe { std::fs::File::from_raw_fd(0) };
+        if let Ok(offset) = stdin_handle.seek(SeekFrom::Current(0)) {
+            settings.stdin_offset = offset;
+        }
+    }
+
     // TODO: is there a better way to check for a readable stdin?
     let mut buf = [0; 0]; // empty buffer to check if stdin().read().is_err()
     let stdin_read_possible = settings.stdin_is_pipe_or_fifo && stdin().read(&mut buf).is_ok();
@@ -341,7 +352,11 @@ fn uu_tail(mut settings: Settings) -> UResult<()> {
     // Iterate user provided `paths` and add them to Watcher.
     if let Some((ref mut watcher, _)) = watcher_rx {
         for path in &settings.paths {
-            let mut path = path.handle_redirect();
+            let mut path = if path.is_stdin() {
+                settings.stdin_redirect.to_owned()
+            } else {
+                path.to_owned()
+            };
             if path.is_stdin() {
                 continue;
             }
@@ -376,7 +391,11 @@ fn uu_tail(mut settings: Settings) -> UResult<()> {
         } else {
             path.to_owned()
         };
-        let mut path = path.handle_redirect();
+        let path = if path.is_stdin() {
+            settings.stdin_redirect.to_owned()
+        } else {
+            path.to_owned()
+        };
         let path_is_tailable = path.is_tailable();
 
         if !path.is_stdin() && !path_is_tailable {
@@ -479,9 +498,14 @@ fn uu_tail(mut settings: Settings) -> UResult<()> {
                         files.print_header(&display_name, !first_header);
                         first_header = false;
                     }
-                    let mut reader;
 
-                    if file.is_seekable() && metadata.as_ref().unwrap().get_block_size() > 0 {
+                    let mut reader;
+                    if file.is_seekable(if display_name.is_stdin() {
+                        settings.stdin_offset
+                    } else {
+                        0
+                    }) && metadata.as_ref().unwrap().get_block_size() > 0
+                    {
                         bounded_tail(&mut file, &settings);
                         reader = BufReader::new(file);
                     } else {
@@ -513,9 +537,11 @@ fn uu_tail(mut settings: Settings) -> UResult<()> {
                 }
             }
         } else if settings.retry && settings.follow.is_some() {
-            if path.is_relative() {
-                path = std::env::current_dir()?.join(&path);
-            }
+            let path = if path.is_relative() {
+                std::env::current_dir()?.join(&path)
+            } else {
+                path.to_owned()
+            };
             // Insert non-is_tailable() paths into `files.map`
             files.insert(
                 &path,
@@ -1544,14 +1570,16 @@ pub fn stdin_is_bad_fd() -> bool {
 
 trait FileExtTail {
     #[allow(clippy::wrong_self_convention)]
-    fn is_seekable(&mut self) -> bool;
+    fn is_seekable(&mut self, current_offset: u64) -> bool;
 }
 
 impl FileExtTail for File {
-    fn is_seekable(&mut self) -> bool {
+    /// Test if File is seekable.
+    /// Set the current position offset to `current_offset`.
+    fn is_seekable(&mut self, current_offset: u64) -> bool {
         self.seek(SeekFrom::Current(0)).is_ok()
             && self.seek(SeekFrom::End(0)).is_ok()
-            && self.seek(SeekFrom::Start(0)).is_ok()
+            && self.seek(SeekFrom::Start(current_offset)).is_ok()
     }
 }
 

tests/by-util/test_tail.rs

@@ -94,6 +94,26 @@ fn test_stdin_redirect_file() {
     assert!(buf_stderr.is_empty());
 }
 
+#[test]
+#[cfg(all(unix, not(any(target_os = "android", target_vendor = "apple"))))] // FIXME: make this work not just on Linux
+fn test_stdin_redirect_offset() {
+    // inspired by: "gnu/tests/tail-2/start-middle.sh"
+    use std::io::{Seek, SeekFrom};
+
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+
+    at.write("k", "1\n2\n");
+    let mut fh = std::fs::File::open(at.plus("k")).unwrap();
+    fh.seek(SeekFrom::Start(2)).unwrap();
+
+    ts.ucmd()
+        .set_stdin(fh)
+        .run()
+        .stdout_is("2\n")
+        .succeeded();
+}
+
 #[test]
 fn test_nc_0_wo_follow() {
     // verify that -[nc]0 without -f, exit without reading