dd: support the [io]flag=nocache option

Add support for the `iflag=nocache` and `oflag=nocache` to make `dd`
discard the filesystem cache for the processed portion of the input or
output file.

src/uu/dd/src/dd.rs

@@ -785,6 +785,25 @@ fn dd_copy(mut i: Input, mut o: Output) -> std::io::Result<()> {
     // Optimization: if no blocks are to be written, then don't
     // bother allocating any buffers.
     if let Some(Num::Blocks(0) | Num::Bytes(0)) = i.settings.count {
+        // Even though we are not reading anything from the input
+        // file, we still need to honor the `nocache` flag, which
+        // requests that we inform the system that we no longer
+        // need the contents of the input file in a system cache.
+        //
+        // TODO Better error handling for overflowing `len`.
+        if i.settings.iflags.nocache {
+            let offset = 0;
+            let len = i.src.len()?.try_into().unwrap();
+            i.discard_cache(offset, len);
+        }
+        // Similarly, discard the system cache for the output file.
+        //
+        // TODO Better error handling for overflowing `len`.
+        if i.settings.oflags.nocache {
+            let offset = 0;
+            let len = o.dst.len()?.try_into().unwrap();
+            o.discard_cache(offset, len);
+        }
         return finalize(&mut o, rstat, wstat, start, &prog_tx, output_thread);
     };
 
@@ -792,6 +811,13 @@ fn dd_copy(mut i: Input, mut o: Output) -> std::io::Result<()> {
     // This is the max size needed.
     let mut buf = vec![BUF_INIT_BYTE; bsize];
 
+    // Index in the input file where we are reading bytes and in
+    // the output file where we are writing bytes.
+    //
+    // These are updated on each iteration of the main loop.
+    let mut read_offset = 0;
+    let mut write_offset = 0;
+
     // The main read/write loop.
     //
     // Each iteration reads blocks from the input and writes
@@ -811,6 +837,30 @@ fn dd_copy(mut i: Input, mut o: Output) -> std::io::Result<()> {
         }
         let wstat_update = o.write_blocks(&buf)?;
 
+        // Discard the system file cache for the read portion of
+        // the input file.
+        //
+        // TODO Better error handling for overflowing `offset` and `len`.
+        let read_len = rstat_update.bytes_total;
+        if i.settings.iflags.nocache {
+            let offset = read_offset.try_into().unwrap();
+            let len = read_len.try_into().unwrap();
+            i.discard_cache(offset, len);
+        }
+        read_offset += read_len;
+
+        // Discard the system file cache for the written portion
+        // of the output file.
+        //
+        // TODO Better error handling for overflowing `offset` and `len`.
+        let write_len = wstat_update.bytes_total;
+        if o.settings.oflags.nocache {
+            let offset = write_offset.try_into().unwrap();
+            let len = write_len.try_into().unwrap();
+            o.discard_cache(offset, len);
+        }
+        write_offset += write_len;
+
         // Update the read/write stats and inform the progress thread once per second.
         //
         // If the receiver is disconnected, `send()` returns an

src/uu/dd/src/parseargs.rs

@@ -304,7 +304,7 @@ impl Parser {
                 "directory" => linux_only!(f, i.directory = true),
                 "dsync" => linux_only!(f, i.dsync = true),
                 "sync" => linux_only!(f, i.sync = true),
-                "nocache" => return Err(ParseError::Unimplemented(f.to_string())),
+                "nocache" => linux_only!(f, i.nocache = true),
                 "nonblock" => linux_only!(f, i.nonblock = true),
                 "noatime" => linux_only!(f, i.noatime = true),
                 "noctty" => linux_only!(f, i.noctty = true),
@@ -336,7 +336,7 @@ impl Parser {
                 "directory" => linux_only!(f, o.directory = true),
                 "dsync" => linux_only!(f, o.dsync = true),
                 "sync" => linux_only!(f, o.sync = true),
-                "nocache" => return Err(ParseError::Unimplemented(f.to_string())),
+                "nocache" => linux_only!(f, o.nocache = true),
                 "nonblock" => linux_only!(f, o.nonblock = true),
                 "noatime" => linux_only!(f, o.noatime = true),
                 "noctty" => linux_only!(f, o.noctty = true),

src/uu/dd/src/parseargs/unit_tests.rs

@@ -55,7 +55,7 @@ fn unimplemented_flags_should_error() {
     let mut succeeded = Vec::new();
 
     // The following flags are not implemented
-    for flag in ["cio", "nocache", "nolinks", "text", "binary"] {
+    for flag in ["cio", "nolinks", "text", "binary"] {
         let args = vec![format!("iflag={flag}")];
 
         if Parser::new()

tests/by-util/test_dd.rs

@@ -1536,3 +1536,25 @@ fn test_multiple_processes_reading_stdin() {
         .succeeds()
         .stdout_only("def\n");
 }
+
+/// Test that discarding system file cache fails for stdin.
+#[test]
+#[cfg(target_os = "linux")]
+fn test_nocache_stdin_error() {
+    new_ucmd!()
+        .args(&["iflag=nocache", "count=0", "status=noxfer"])
+        .fails()
+        .code_is(1)
+        .stderr_only("dd: failed to discard cache for: 'standard input': Illegal seek\n0+0 records in\n0+0 records out\n");
+}
+
+/// Test for discarding system file cache.
+#[test]
+#[cfg(target_os = "linux")]
+fn test_nocache_file() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.write_bytes("f", b"a".repeat(1 << 20).as_slice());
+    ucmd.args(&["if=f", "of=/dev/null", "iflag=nocache", "status=noxfer"])
+        .succeeds()
+        .stderr_only("2048+0 records in\n2048+0 records out\n");
+}