re-use existing fd for stdout even if its a seek-able file

this is important as the fd holds the file offset we need to use

src/uu/dd/src/dd.rs

@@ -3,7 +3,7 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
-// spell-checker:ignore fname, ftype, tname, fpath, specfile, testfile, unspec, ifile, ofile, outfile, fullblock, urand, fileio, atoe, atoibm, behaviour, bmax, bremain, cflags, creat, ctable, ctty, datastructures, doesnt, etoa, fileout, fname, gnudd, iconvflags, iseek, nocache, noctty, noerror, nofollow, nolinks, nonblock, oconvflags, oseek, outfile, parseargs, rlen, rmax, rremain, rsofar, rstat, sigusr, wlen, wstat seekable oconv canonicalized fadvise Fadvise FADV DONTNEED ESPIPE bufferedoutput
+// spell-checker:ignore fname, ftype, tname, fpath, specfile, testfile, unspec, ifile, ofile, outfile, fullblock, urand, fileio, atoe, atoibm, behaviour, bmax, bremain, cflags, creat, ctable, ctty, datastructures, doesnt, etoa, fileout, fname, gnudd, iconvflags, iseek, nocache, noctty, noerror, nofollow, nolinks, nonblock, oconvflags, oseek, outfile, parseargs, rlen, rmax, rremain, rsofar, rstat, sigusr, wlen, wstat seekable oconv canonicalized fadvise Fadvise FADV DONTNEED ESPIPE bufferedoutput, SETFL
 
 mod blocks;
 mod bufferedoutput;
@@ -16,6 +16,10 @@ mod progress;
 use crate::bufferedoutput::BufferedOutput;
 use blocks::conv_block_unblock_helper;
 use datastructures::*;
+#[cfg(any(target_os = "linux", target_os = "android"))]
+use nix::fcntl::FcntlArg::F_SETFL;
+#[cfg(any(target_os = "linux", target_os = "android"))]
+use nix::fcntl::OFlag;
 use parseargs::Parser;
 use progress::{gen_prog_updater, ProgUpdate, ReadStat, StatusLevel, WriteStat};
 
@@ -24,6 +28,8 @@ use std::env;
 use std::ffi::OsString;
 use std::fs::{File, OpenOptions};
 use std::io::{self, Read, Seek, SeekFrom, Stdout, Write};
+use std::os::fd::AsFd;
+use std::os::fd::OwnedFd;
 #[cfg(any(target_os = "linux", target_os = "android"))]
 use std::os::unix::fs::OpenOptionsExt;
 #[cfg(unix)]
@@ -557,7 +563,7 @@ impl Dest {
                         return Ok(len);
                     }
                 }
-                f.seek(io::SeekFrom::Start(n))
+                f.seek(io::SeekFrom::Current(n.try_into().unwrap()))
             }
             #[cfg(unix)]
             Self::Fifo(f) => {
@@ -699,6 +705,11 @@ impl<'a> Output<'a> {
         if !settings.oconv.notrunc {
             dst.set_len(settings.seek).ok();
         }
+
+        Self::prepare_file(dst, settings)
+    }
+
+    fn prepare_file(dst: File, settings: &'a Settings) -> UResult<Self> {
         let density = if settings.oconv.sparse {
             Density::Sparse
         } else {
@@ -710,6 +721,28 @@ impl<'a> Output<'a> {
         Ok(Self { dst, settings })
     }
 
+    /// Instantiate this struct with file descriptor as a destination.
+    ///
+    /// This is useful e.g. for the case when the file descriptor was
+    /// already opened by the system (stdout) and has a state
+    /// (current position) that shall be used.
+    fn new_file_from_fd(fd: OwnedFd, settings: &'a Settings) -> UResult<Self> {
+        fn open_dst(fd: OwnedFd, oflags: &OFlags) -> Result<File, io::Error> {
+            #[cfg(any(target_os = "linux", target_os = "android"))]
+            if let Some(libc_flags) = make_linux_oflags(oflags) {
+                nix::fcntl::fcntl(fd.as_raw_fd(), F_SETFL(OFlag::from_bits_retain(libc_flags)))?;
+            }
+
+            Ok(File::from(fd))
+        }
+
+        let raw_fd = fd.as_raw_fd();
+        let dst = open_dst(fd, &settings.oflags)
+            .map_err_context(|| format!("failed to open fd {}", raw_fd))?;
+
+        Self::prepare_file(dst, settings)
+    }
+
     /// Instantiate this struct with the given named pipe as a destination.
     #[cfg(unix)]
     fn new_fifo(filename: &Path, settings: &'a Settings) -> UResult<Self> {
@@ -1288,7 +1321,8 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         Some(ref outfile) if is_fifo(outfile) => Output::new_fifo(Path::new(&outfile), &settings)?,
         Some(ref outfile) => Output::new_file(Path::new(&outfile), &settings)?,
         None if is_stdout_redirected_to_seekable_file() => {
-            Output::new_file(Path::new(&stdout_canonicalized()), &settings)?
+            let fd = io::stdout().as_fd().try_clone_to_owned()?;
+            Output::new_file_from_fd(fd, &settings)?
         }
         None => Output::new_stdout(&settings)?,
     };