fs: add set_max_buf_size to tokio::fs::File

tokio/src/fs/file.rs

@@ -3,7 +3,7 @@
 //! [`File`]: File
 
 use crate::fs::{asyncify, OpenOptions};
-use crate::io::blocking::Buf;
+use crate::io::blocking::{Buf, DEFAULT_MAX_BUF_SIZE};
 use crate::io::{AsyncRead, AsyncSeek, AsyncWrite, ReadBuf};
 use crate::sync::Mutex;
 
@@ -90,6 +90,7 @@ use std::fs::File as StdFile;
 pub struct File {
     std: Arc<StdFile>,
     inner: Mutex<Inner>,
+    max_buf_size: usize,
 }
 
 struct Inner {
@@ -241,6 +242,7 @@ impl File {
                 last_write_err: None,
                 pos: 0,
             }),
+            max_buf_size: DEFAULT_MAX_BUF_SIZE,
         }
     }
 
@@ -508,6 +510,34 @@ impl File {
         let std = self.std.clone();
         asyncify(move || std.set_permissions(perm)).await
     }
+
+    /// Set the maximum buffer size for the underlying [`AsyncRead`] / [`AsyncWrite`] operation.
+    ///
+    /// Although Tokio uses a sensible default value for this buffer size, this function would be
+    /// useful for changing that default depending on the situation.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::io::AsyncWriteExt;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::open("foo.txt").await?;
+    ///
+    /// // Set maximum buffer size to 8 MiB
+    /// file.set_max_buf_size(8 * 1024 * 1024);
+    ///
+    /// let mut buf = vec![1; 1024 * 1024 * 1024];
+    ///
+    /// // Write the 1 GiB buffer in chunks up to 8 MiB each.
+    /// file.write_all(&mut buf).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn set_max_buf_size(&mut self, max_buf_size: usize) {
+        self.max_buf_size = max_buf_size;
+    }
 }
 
 impl AsyncRead for File {
@@ -531,7 +561,7 @@ impl AsyncRead for File {
                         return Poll::Ready(Ok(()));
                     }
 
-                    buf.ensure_capacity_for(dst);
+                    buf.ensure_capacity_for(dst, me.max_buf_size);
                     let std = me.std.clone();
 
                     inner.state = State::Busy(spawn_blocking(move || {
@@ -668,7 +698,7 @@ impl AsyncWrite for File {
                         None
                     };
 
-                    let n = buf.copy_from(src);
+                    let n = buf.copy_from(src, me.max_buf_size);
                     let std = me.std.clone();
 
                     let blocking_task_join_handle = spawn_mandatory_blocking(move || {
@@ -739,7 +769,7 @@ impl AsyncWrite for File {
                         None
                     };
 
-                    let n = buf.copy_from_bufs(bufs);
+                    let n = buf.copy_from_bufs(bufs, me.max_buf_size);
                     let std = me.std.clone();
 
                     let blocking_task_join_handle = spawn_mandatory_blocking(move || {

tokio/src/fs/file/tests.rs

@@ -231,7 +231,7 @@ fn flush_while_idle() {
 #[cfg_attr(miri, ignore)] // takes a really long time with miri
 fn read_with_buffer_larger_than_max() {
     // Chunks
-    let chunk_a = crate::io::blocking::MAX_BUF;
+    let chunk_a = crate::io::blocking::DEFAULT_MAX_BUF_SIZE;
     let chunk_b = chunk_a * 2;
     let chunk_c = chunk_a * 3;
     let chunk_d = chunk_a * 4;
@@ -303,7 +303,7 @@ fn read_with_buffer_larger_than_max() {
 #[cfg_attr(miri, ignore)] // takes a really long time with miri
 fn write_with_buffer_larger_than_max() {
     // Chunks
-    let chunk_a = crate::io::blocking::MAX_BUF;
+    let chunk_a = crate::io::blocking::DEFAULT_MAX_BUF_SIZE;
     let chunk_b = chunk_a * 2;
     let chunk_c = chunk_a * 3;
     let chunk_d = chunk_a * 4;

tokio/src/fs/mocks.rs

@@ -30,6 +30,7 @@ mock! {
         pub fn open(pb: PathBuf) -> io::Result<Self>;
         pub fn set_len(&self, size: u64) -> io::Result<()>;
         pub fn set_permissions(&self, _perm: Permissions) -> io::Result<()>;
+        pub fn set_max_buf_size(&self, max_buf_size: usize);
         pub fn sync_all(&self) -> io::Result<()>;
         pub fn sync_data(&self) -> io::Result<()>;
         pub fn try_clone(&self) -> io::Result<Self>;

tokio/src/io/blocking.rs

@@ -23,7 +23,7 @@ pub(crate) struct Buf {
     pos: usize,
 }
 
-pub(crate) const MAX_BUF: usize = 2 * 1024 * 1024;
+pub(crate) const DEFAULT_MAX_BUF_SIZE: usize = 2 * 1024 * 1024;
 
 #[derive(Debug)]
 enum State<T> {
@@ -64,7 +64,7 @@ where
                         return Poll::Ready(Ok(()));
                     }
 
-                    buf.ensure_capacity_for(dst);
+                    buf.ensure_capacity_for(dst, DEFAULT_MAX_BUF_SIZE);
                     let mut inner = self.inner.take().unwrap();
 
                     self.state = State::Busy(sys::run(move || {
@@ -111,7 +111,7 @@ where
 
                     assert!(buf.is_empty());
 
-                    let n = buf.copy_from(src);
+                    let n = buf.copy_from(src, DEFAULT_MAX_BUF_SIZE);
                     let mut inner = self.inner.take().unwrap();
 
                     self.state = State::Busy(sys::run(move || {
@@ -214,10 +214,10 @@ impl Buf {
         n
     }
 
-    pub(crate) fn copy_from(&mut self, src: &[u8]) -> usize {
+    pub(crate) fn copy_from(&mut self, src: &[u8], max_buf_size: usize) -> usize {
         assert!(self.is_empty());
 
-        let n = cmp::min(src.len(), MAX_BUF);
+        let n = cmp::min(src.len(), max_buf_size);
 
         self.buf.extend_from_slice(&src[..n]);
         n
@@ -227,10 +227,10 @@ impl Buf {
         &self.buf[self.pos..]
     }
 
-    pub(crate) fn ensure_capacity_for(&mut self, bytes: &ReadBuf<'_>) {
+    pub(crate) fn ensure_capacity_for(&mut self, bytes: &ReadBuf<'_>, max_buf_size: usize) {
         assert!(self.is_empty());
 
-        let len = cmp::min(bytes.remaining(), MAX_BUF);
+        let len = cmp::min(bytes.remaining(), max_buf_size);
 
         if self.buf.len() < len {
             self.buf.reserve(len - self.buf.len());
@@ -274,10 +274,10 @@ cfg_fs! {
             ret
         }
 
-        pub(crate) fn copy_from_bufs(&mut self, bufs: &[io::IoSlice<'_>]) -> usize {
+        pub(crate) fn copy_from_bufs(&mut self, bufs: &[io::IoSlice<'_>], max_buf_size: usize) -> usize {
             assert!(self.is_empty());
 
-            let mut rem = MAX_BUF;
+            let mut rem = max_buf_size;
             for buf in bufs {
                 if rem == 0 {
                     break
@@ -288,7 +288,7 @@ cfg_fs! {
                 rem -= len;
             }
 
-            MAX_BUF - rem
+            max_buf_size - rem
         }
     }
 }

tokio/src/io/stdio_common.rs

@@ -4,7 +4,7 @@ use std::pin::Pin;
 use std::task::{Context, Poll};
 /// # Windows
 /// [`AsyncWrite`] adapter that finds last char boundary in given buffer and does not write the rest,
-/// if buffer contents seems to be `utf8`. Otherwise it only trims buffer down to `MAX_BUF`.
+/// if buffer contents seems to be `utf8`. Otherwise it only trims buffer down to `DEFAULT_MAX_BUF_SIZE`.
 /// That's why, wrapped writer will always receive well-formed utf-8 bytes.
 /// # Other platforms
 /// Passes data to `inner` as is.
@@ -45,12 +45,13 @@ where
         // 2. If buffer is small, it will not be shrunk.
         // That's why, it's "textness" will not change, so we don't have
         // to fixup it.
-        if cfg!(not(any(target_os = "windows", test))) || buf.len() <= crate::io::blocking::MAX_BUF
+        if cfg!(not(any(target_os = "windows", test)))
+            || buf.len() <= crate::io::blocking::DEFAULT_MAX_BUF_SIZE
         {
             return call_inner(buf);
         }
 
-        buf = &buf[..crate::io::blocking::MAX_BUF];
+        buf = &buf[..crate::io::blocking::DEFAULT_MAX_BUF_SIZE];
 
         // Now there are two possibilities.
         // If caller gave is binary buffer, we **should not** shrink it
@@ -108,7 +109,7 @@ where
 #[cfg(test)]
 #[cfg(not(loom))]
 mod tests {
-    use crate::io::blocking::MAX_BUF;
+    use crate::io::blocking::DEFAULT_MAX_BUF_SIZE;
     use crate::io::AsyncWriteExt;
     use std::io;
     use std::pin::Pin;
@@ -123,7 +124,7 @@ mod tests {
             _cx: &mut Context<'_>,
             buf: &[u8],
         ) -> Poll<Result<usize, io::Error>> {
-            assert!(buf.len() <= MAX_BUF);
+            assert!(buf.len() <= DEFAULT_MAX_BUF_SIZE);
             assert!(std::str::from_utf8(buf).is_ok());
             Poll::Ready(Ok(buf.len()))
         }
@@ -158,7 +159,7 @@ mod tests {
             _cx: &mut Context<'_>,
             buf: &[u8],
         ) -> Poll<Result<usize, io::Error>> {
-            assert!(buf.len() <= MAX_BUF);
+            assert!(buf.len() <= DEFAULT_MAX_BUF_SIZE);
             self.write_history.push(buf.len());
             Poll::Ready(Ok(buf.len()))
         }
@@ -178,7 +179,7 @@ mod tests {
     #[test]
     #[cfg_attr(miri, ignore)]
     fn test_splitter() {
-        let data = str::repeat("█", MAX_BUF);
+        let data = str::repeat("█", DEFAULT_MAX_BUF_SIZE);
         let mut wr = super::SplitByUtf8BoundaryIfWindows::new(TextMockWriter);
         let fut = async move {
             wr.write_all(data.as_bytes()).await.unwrap();
@@ -197,7 +198,7 @@ mod tests {
         // was not shrunk too much.
         let checked_count = super::MAGIC_CONST * super::MAX_BYTES_PER_CHAR;
         let mut data: Vec<u8> = str::repeat("a", checked_count).into();
-        data.extend(std::iter::repeat(0b1010_1010).take(MAX_BUF - checked_count + 1));
+        data.extend(std::iter::repeat(0b1010_1010).take(DEFAULT_MAX_BUF_SIZE - checked_count + 1));
         let mut writer = LoggingMockWriter::new();
         let mut splitter = super::SplitByUtf8BoundaryIfWindows::new(&mut writer);
         crate::runtime::Builder::new_current_thread()
@@ -214,7 +215,7 @@ mod tests {
             data.len()
         );
         // Check that at most MAX_BYTES_PER_CHAR + 1 (i.e. 5) bytes were shrunk
-        // from the buffer: one because it was outside of MAX_BUF boundary, and
+        // from the buffer: one because it was outside of DEFAULT_MAX_BUF_SIZE boundary, and
         // up to one "utf8 code point".
         assert!(data.len() - writer.write_history[0] <= super::MAX_BYTES_PER_CHAR + 1);
     }

tokio/tests/fs_file.rs

@@ -180,6 +180,28 @@ fn tempfile() -> NamedTempFile {
     NamedTempFile::new().unwrap()
 }
 
+#[tokio::test]
+async fn set_max_buf_size_read() {
+    let mut tempfile = tempfile();
+    tempfile.write_all(HELLO).unwrap();
+    let mut file = File::open(tempfile.path()).await.unwrap();
+    let mut buf = [0; 1024];
+    file.set_max_buf_size(1);
+
+    // A single read operation reads a maximum of 1 byte.
+    assert_eq!(file.read(&mut buf).await.unwrap(), 1);
+}
+
+#[tokio::test]
+async fn set_max_buf_size_write() {
+    let tempfile = tempfile();
+    let mut file = File::create(tempfile.path()).await.unwrap();
+    file.set_max_buf_size(1);
+
+    // A single write operation writes a maximum of 1 byte.
+    assert_eq!(file.write(HELLO).await.unwrap(), 1);
+}
+
 #[tokio::test]
 #[cfg(unix)]
 async fn file_debug_fmt() {