dd: truncate to specified seek length

When specifying `seek=N` and *not* specifying `conv=notrunc`, truncate
the output file to `N` blocks instead of truncating it to zero before
starting to write output. For example

    $ printf "abc" > outfile
    $ printf "123" | dd bs=1 skip=1 seek=1 count=1 status=noxfer of=outfile
    1+0 records in
    1+0 records out
    $ cat outfile
    a2

Fixes #3068.

src/uu/dd/src/dd.rs

@@ -469,7 +469,6 @@ impl OutputTrait for Output<File> {
             let mut opts = OpenOptions::new();
             opts.write(true)
                 .create(!cflags.nocreat)
-                .truncate(!cflags.notrunc)
                 .create_new(cflags.excl)
                 .append(oflags.append);
 
@@ -489,13 +488,13 @@ impl OutputTrait for Output<File> {
             let mut dst = open_dst(Path::new(&fname), &cflags, &oflags)
                 .map_err_context(|| format!("failed to open {}", fname.quote()))?;
 
-            if let Some(amt) = seek {
+            let i = seek.unwrap_or(0).try_into().unwrap();
-                let amt: u64 = amt
+            if !cflags.notrunc {
-                    .try_into()
+                dst.set_len(i)
-                    .map_err(|_| USimpleError::new(1, "failed to parse seek amount"))?;
+                    .map_err_context(|| "failed to truncate output file".to_string())?;
-                dst.seek(io::SeekFrom::Start(amt))
-                    .map_err_context(|| "failed to seek in output file".to_string())?;
             }
+            dst.seek(io::SeekFrom::Start(i))
+                .map_err_context(|| "failed to seek in output file".to_string())?;
 
             Ok(Self { dst, obs, cflags })
         } else {

tests/by-util/test_dd.rs

@@ -604,5 +604,27 @@ fn test_seek_bytes() {
         .stdout_is("\0\0\0\0\0\0\0\0abcdefghijklm\n");
 }
 
+#[test]
+fn test_seek_do_not_overwrite() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let mut outfile = at.make_file("outfile");
+    outfile.write_all(b"abc").unwrap();
+    // Skip the first byte of the input, seek past the first byte of
+    // the output, and write only one byte to the output.
+    ucmd.args(&[
+        "bs=1",
+        "skip=1",
+        "seek=1",
+        "count=1",
+        "status=noxfer",
+        "of=outfile",
+    ])
+    .pipe_in("123")
+    .succeeds()
+    .stderr_is("1+0 records in\n1+0 records out\n")
+    .no_stdout();
+    assert_eq!(at.read("outfile"), "a2");
+}
+
 // conv=[ascii,ebcdic,ibm], conv=[ucase,lcase], conv=[block,unblock], conv=sync
 // TODO: Move conv tests from unit test module