Reading cancelation

Author: DmitryAstafyev

src/lib.rs

@@ -203,6 +203,9 @@ pub struct BufReader<R, P = StdPolicy>{
     buf: Buffer,
     inner: R,
     policy: P,
+    // We need field "empty" to return empty &[u8] in case of reader is paused
+    // This field is never used, never filled.
+    empty: Vec<u8>,
 }
 
 impl<R> BufReader<R, StdPolicy> {
@@ -262,7 +265,7 @@ impl<R> BufReader<R, StdPolicy> {
     /// then it will be returned in `read()` and `fill_buf()` ahead of any data from `inner`.
     pub fn with_buffer(buf: Buffer, inner: R) -> Self {
         BufReader {
-            buf, inner, policy: StdPolicy
+            buf, inner, policy: StdPolicy, empty: vec![]
         }
     }
 }
@@ -273,6 +276,7 @@ impl<R, P> BufReader<R, P> {
         BufReader {
             inner: self.inner,
             buf: self.buf,
+            empty: self.empty,
             policy
         }
     }
@@ -357,6 +361,11 @@ impl<R, P: ReaderPolicy> BufReader<R, P> {
     fn should_read(&mut self) -> bool {
         self.policy.before_read(&mut self.buf).0
     }
+
+    #[inline]
+    fn is_paused(&mut self) -> bool {
+        self.policy.is_paused()
+    }
 }
 
 impl<R: Read, P> BufReader<R, P> {
@@ -377,12 +386,17 @@ impl<R: Read, P> BufReader<R, P> {
             inner,
             buf: self.buf,
             policy: self.policy,
+            empty: self.empty,
         }
     }
 }
 
 impl<R: Read, P: ReaderPolicy> Read for BufReader<R, P> {
     fn read(&mut self, out: &mut [u8]) -> io::Result<usize> {
+        // If reading is paused, returning 0 to send end reading signal
+        if self.is_paused() {
+            return Ok(0);
+        }
         // If we don't have any buffered data and we're doing a read matching
         // or exceeding the internal buffer's capacity, bypass the buffer.
         if self.buf.is_empty() && out.len() >= self.buf.capacity() {
@@ -397,6 +411,10 @@ impl<R: Read, P: ReaderPolicy> Read for BufReader<R, P> {
 
 impl<R: Read, P: ReaderPolicy> BufRead for BufReader<R, P> {
     fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        // If reading is paused, we are sending empty buffer to send signal - "no data any more"
+        if self.is_paused() {
+            return Ok(&self.empty);
+        }
         // If we've reached the end of our internal buffer then we need to fetch
         // some more data from the underlying reader.
         // This execution order is important; the policy may want to resize the buffer or move data

src/policy.rs

@@ -64,6 +64,23 @@ pub trait ReaderPolicy {
     ///
     /// This is a no-op by default.
     fn after_consume(&mut self, _buffer: &mut Buffer, _amt: usize) {}
+
+    /// Consulted with `read` and `fill_buf` methods.
+    /// Return `bool` to continue (`true`) reading or pause it (`false`).
+    ///
+    /// ### Note
+    /// As soon as it was paused, the current position in the buffer isn't dropped.
+    /// The reader still can continue reading with the next iteration if the flag will
+    /// be changed again to `true`.
+    ///
+    /// Possible use-case. For example, we have a huge file, which we would like to
+    /// read and somehow manipulate with content (search in it for example). If we
+    /// are passing the reader into the searcher, we are losing control of it. Withing
+    /// `pausing` policy we can pass to a reader some kind of token and keep control
+    /// of the reading process.
+    fn is_paused(&mut self) -> bool {
+        false
+    }
 }
 
 /// Behavior of `std::io::BufReader`: the buffer will only be read into if it is empty.