dd: use ibs/obs-sized buffer for skip/seek on non-seekable files

src/uu/dd/src/dd.rs

@@ -183,6 +183,32 @@ impl Num {
     }
 }
 
+/// Read and discard `n` bytes from `reader` using a buffer of size `buf_size`.
+///
+/// This is more efficient than `io::copy` with `BufReader` because it reads
+/// directly in `buf_size`-sized chunks, matching GNU dd's behavior.
+/// Returns the total number of bytes actually read.
+fn read_and_discard<R: Read>(reader: &mut R, n: u64, buf_size: usize) -> io::Result<u64> {
+    let mut buf = vec![0u8; buf_size];
+    let mut total = 0u64;
+    let mut remaining = n;
+
+    while remaining > 0 {
+        let to_read = cmp::min(remaining, buf_size as u64) as usize;
+        match reader.read(&mut buf[..to_read]) {
+            Ok(0) => break, // EOF
+            Ok(bytes_read) => {
+                total += bytes_read as u64;
+                remaining -= bytes_read as u64;
+            }
+            Err(e) if e.kind() == io::ErrorKind::Interrupted => continue,
+            Err(e) => return Err(e),
+        }
+    }
+
+    Ok(total)
+}
+
 /// Data sources.
 ///
 /// Use [`Source::stdin_as_file`] if available to enable more
@@ -219,20 +245,19 @@ impl Source {
         Self::StdinFile(f)
     }
 
-    fn skip(&mut self, n: u64) -> io::Result<u64> {
+    fn skip(&mut self, n: u64, ibs: usize) -> io::Result<u64> {
         match self {
             #[cfg(not(unix))]
-            Self::Stdin(stdin) => match io::copy(&mut stdin.take(n), &mut io::sink()) {
-                Ok(m) if m < n => {
+            Self::Stdin(stdin) => {
+                let m = read_and_discard(stdin, n, ibs)?;
+                if m < n {
                     show_error!(
                         "{}",
                         translate!("dd-error-cannot-skip-offset", "file" => "standard input")
                     );
-                    Ok(m)
                 }
-                Ok(m) => Ok(m),
-                Err(e) => Err(e),
-            },
+                Ok(m)
+            }
             #[cfg(unix)]
             Self::StdinFile(f) => {
                 if let Ok(Some(len)) = try_get_len_of_block_device(f) {
@@ -247,21 +272,18 @@ impl Source {
                         return Ok(len);
                     }
                 }
-                match io::copy(&mut f.take(n), &mut io::sink()) {
-                    Ok(m) if m < n => {
-                        show_error!(
-                            "{}",
-                            translate!("dd-error-cannot-skip-offset", "file" => "standard input")
-                        );
-                        Ok(m)
-                    }
-                    Ok(m) => Ok(m),
-                    Err(e) => Err(e),
+                let m = read_and_discard(f, n, ibs)?;
+                if m < n {
+                    show_error!(
+                        "{}",
+                        translate!("dd-error-cannot-skip-offset", "file" => "standard input")
+                    );
                 }
+                Ok(m)
             }
             Self::File(f) => f.seek(SeekFrom::Current(n.try_into().unwrap())),
             #[cfg(unix)]
-            Self::Fifo(f) => io::copy(&mut f.take(n), &mut io::sink()),
+            Self::Fifo(f) => read_and_discard(f, n, ibs),
         }
     }
 
@@ -346,7 +368,7 @@ impl<'a> Input<'a> {
             }
         }
         if settings.skip > 0 {
-            src.skip(settings.skip)?;
+            src.skip(settings.skip, settings.ibs)?;
         }
         Ok(Self { src, settings })
     }
@@ -369,7 +391,7 @@ impl<'a> Input<'a> {
 
         let mut src = Source::File(src);
         if settings.skip > 0 {
-            src.skip(settings.skip)?;
+            src.skip(settings.skip, settings.ibs)?;
         }
         Ok(Self { src, settings })
     }
@@ -383,7 +405,7 @@ impl<'a> Input<'a> {
         opts.custom_flags(make_linux_iflags(&settings.iflags).unwrap_or(0));
         let mut src = Source::Fifo(opts.open(filename)?);
         if settings.skip > 0 {
-            src.skip(settings.skip)?;
+            src.skip(settings.skip, settings.ibs)?;
         }
         Ok(Self { src, settings })
     }
@@ -605,7 +627,8 @@ impl Dest {
         }
     }
 
-    fn seek(&mut self, n: u64) -> io::Result<u64> {
+    #[cfg_attr(not(unix), allow(unused_variables))]
+    fn seek(&mut self, n: u64, obs: usize) -> io::Result<u64> {
         match self {
             Self::Stdout(stdout) => io::copy(&mut io::repeat(0).take(n), stdout),
             Self::File(f, _) => {
@@ -627,7 +650,7 @@ impl Dest {
             #[cfg(unix)]
             Self::Fifo(f) => {
                 // Seeking in a named pipe means *reading* from the pipe.
-                io::copy(&mut f.take(n), &mut io::sink())
+                read_and_discard(f, n, obs)
             }
             #[cfg(unix)]
             Self::Sink => Ok(0),
@@ -781,7 +804,7 @@ impl<'a> Output<'a> {
     /// Instantiate this struct with stdout as a destination.
     fn new_stdout(settings: &'a Settings) -> UResult<Self> {
         let mut dst = Dest::Stdout(io::stdout());
-        dst.seek(settings.seek)
+        dst.seek(settings.seek, settings.obs)
             .map_err_context(|| translate!("dd-error-write-error"))?;
         Ok(Self { dst, settings })
     }
@@ -829,7 +852,7 @@ impl<'a> Output<'a> {
             Density::Dense
         };
         let mut dst = Dest::File(dst, density);
-        dst.seek(settings.seek)
+        dst.seek(settings.seek, settings.obs)
             .map_err_context(|| translate!("dd-error-failed-to-seek"))?;
         Ok(Self { dst, settings })
     }
@@ -859,7 +882,7 @@ impl<'a> Output<'a> {
         // file for reading. But then we need to close the file and
         // re-open it for writing.
         if settings.seek > 0 {
-            Dest::Fifo(File::open(filename)?).seek(settings.seek)?;
+            Dest::Fifo(File::open(filename)?).seek(settings.seek, settings.obs)?;
         }
         // If `count=0`, then we don't bother opening the file for
         // writing because that would cause this process to block