dd: interpret Stdin as File to seek it

src/uu/dd/src/dd.rs

@@ -37,7 +37,7 @@ use clap::{crate_version, Arg, ArgMatches, Command};
 use gcd::Gcd;
 use uucore::display::Quotable;
 use uucore::error::{FromIo, UResult};
-use uucore::{show_error, InvalidEncodingHandling};
+use uucore::InvalidEncodingHandling;
 
 const ABOUT: &str = "copy, and optionally convert, a file system resource";
 const BUF_INIT_BYTE: u8 = 0xDD;
@@ -61,8 +61,7 @@ impl Input<io::Stdin> {
         let iseek =
             parseargs::parse_seek_skip_amt(&ibs, iflags.skip_bytes, matches, options::ISEEK)?;
         let count = parseargs::parse_count(&iflags, matches)?;
-
-        let mut i = Self {
+        let i = Self {
             src: io::stdin(),
             ibs,
             print_level,
@@ -71,17 +70,23 @@ impl Input<io::Stdin> {
             iflags,
         };
 
-        // The --skip and --iseek flags are additive. On a stream, they discard bytes.
+        // there is way to seek stdin, so we interpret stdin as a File
+        // to make it seekable
+        #[cfg(any(target_family = "unix", target_family = "wasi"))]
+        let mut seekable_src = unsafe {
+            use std::os::unix::io::{AsRawFd, FromRawFd};
+            std::fs::File::from_raw_fd(std::io::stdin().as_raw_fd())
+        };
+        #[cfg(target_family = "windows")]
+        let mut seekable_src = unsafe {
+            use std::os::windows::io::{AsRawHandle, FromRawHandle};
+            std::fs::File::from_raw_handle(std::io::stdin().as_raw_handle())
+        };
         let amt = skip.unwrap_or(0) + iseek.unwrap_or(0);
         if amt > 0 {
-            if let Err(e) = i.read_skip(amt) {
-                if let io::ErrorKind::UnexpectedEof = e.kind() {
-                    show_error!("'standard input': cannot skip to specified offset");
-                } else {
-                    return io::Result::Err(e)
-                        .map_err_context(|| "I/O error while skipping".to_string());
-                }
-            }
+            seekable_src
+                .seek(io::SeekFrom::Start(amt))
+                .map_err_context(|| "failed to seek in input file".to_string())?;
         }
 
         Ok(i)
@@ -261,20 +266,6 @@ impl<R: Read> Input<R> {
             records_truncated: 0,
         })
     }
-
-    /// Skips amount_to_read bytes from the Input by copying into a sink
-    fn read_skip(&mut self, amount_to_read: u64) -> std::io::Result<()> {
-        let copy_result = io::copy(&mut self.src.by_ref().take(amount_to_read), &mut io::sink());
-        if let Ok(n) = copy_result {
-            if n != amount_to_read {
-                io::Result::Err(io::Error::new(io::ErrorKind::UnexpectedEof, ""))
-            } else {
-                Ok(())
-            }
-        } else {
-            io::Result::Err(copy_result.unwrap_err())
-        }
-    }
 }
 
 trait OutputTrait: Sized + Write {

tests/by-util/test_dd.rs

@@ -1186,3 +1186,16 @@ fn test_final_stats_si_iec() {
     let s = result.stderr_str();
     assert!(s.starts_with("2+0 records in\n2+0 records out\n1024 bytes (1 KB, 1024 B) copied,"));
 }
+
+#[test]
+fn test_partial_skip_stdin() {
+    let input = "abc\ndef\n";
+    // build_test_file!("infile.txt", input.as_bytes());
+    new_ucmd!()
+        .args(&["status=none", "bs=1", "skip=1", "count=0"])
+        .pipe_in(input)
+        .succeeds()
+        .no_stderr()
+        .stdout_is("bc")
+        .success();
+}