io: make read_to_end not grow unnecessarily

tokio/src/io/util/read_to_end.rs

@@ -1,11 +1,11 @@
 use crate::io::util::vec_with_initialized::{into_read_buf_parts, VecU8, VecWithInitialized};
-use crate::io::AsyncRead;
+use crate::io::{AsyncRead, ReadBuf};
 
 use pin_project_lite::pin_project;
 use std::future::Future;
 use std::io;
 use std::marker::PhantomPinned;
-use std::mem;
+use std::mem::{self, MaybeUninit};
 use std::pin::Pin;
 use std::task::{Context, Poll};
 
@@ -67,16 +67,47 @@ fn poll_read_to_end<V: VecU8, R: AsyncRead + ?Sized>(
     // has 4 bytes while still making large reads if the reader does have a ton
     // of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
     // time is 4,500 times (!) slower than this if the reader has a very small
-    // amount of data to return.
-    buf.reserve(32);
+    // amount of data to return. When the vector is full with its starting
+    // capacity, we first try to read into a small buffer to see if we reached
+    // an EOF. This only happens when the starting capacity is >= NUM_BYTES, since
+    // we allocate at least NUM_BYTES each time. This avoids the unnecessary
+    // allocation that we attempt before reading into the vector.
+
+    const NUM_BYTES: usize = 32;
+    let try_small_read = buf.try_small_read_first(NUM_BYTES);
 
     // Get a ReadBuf into the vector.
-    let mut read_buf = buf.get_read_buf();
+    let mut read_buf;
+    let poll_result;
+
+    let n = if try_small_read {
+        // Read some bytes using a small read.
+        let mut small_buf: [MaybeUninit<u8>; NUM_BYTES] = [MaybeUninit::uninit(); NUM_BYTES];
+        let mut small_read_buf = ReadBuf::uninit(&mut small_buf);
+        poll_result = read.poll_read(cx, &mut small_read_buf);
+        let to_write = small_read_buf.filled();
+
+        // Ensure we have enough space to fill our vector with what we read.
+        read_buf = buf.get_read_buf();
+        if to_write.len() > read_buf.remaining() {
+            buf.reserve(NUM_BYTES);
+            read_buf = buf.get_read_buf();
+        }
+        read_buf.put_slice(to_write);
+
+        to_write.len()
+    } else {
+        // Ensure we have enough space for reading.
+        buf.reserve(NUM_BYTES);
+        read_buf = buf.get_read_buf();
+
+        // Read data directly into vector.
+        let filled_before = read_buf.filled().len();
+        poll_result = read.poll_read(cx, &mut read_buf);
 
-    let filled_before = read_buf.filled().len();
-    let poll_result = read.poll_read(cx, &mut read_buf);
-    let filled_after = read_buf.filled().len();
-    let n = filled_after - filled_before;
+        // Compute the number of bytes read.
+        read_buf.filled().len() - filled_before
+    };
 
     // Update the length of the vector using the result of poll_read.
     let read_buf_parts = into_read_buf_parts(read_buf);
@@ -87,11 +118,11 @@ fn poll_read_to_end<V: VecU8, R: AsyncRead + ?Sized>(
             // In this case, nothing should have been read. However we still
             // update the vector in case the poll_read call initialized parts of
             // the vector's unused capacity.
-            debug_assert_eq!(filled_before, filled_after);
+            debug_assert_eq!(n, 0);
             Poll::Pending
         }
         Poll::Ready(Err(err)) => {
-            debug_assert_eq!(filled_before, filled_after);
+            debug_assert_eq!(n, 0);
             Poll::Ready(Err(err))
         }
         Poll::Ready(Ok(())) => Poll::Ready(Ok(n)),

tokio/src/io/util/vec_with_initialized.rs

@@ -28,6 +28,7 @@ pub(crate) struct VecWithInitialized<V> {
     // The number of initialized bytes in the vector.
     // Always between `vec.len()` and `vec.capacity()`.
     num_initialized: usize,
+    starting_capacity: usize,
 }
 
 impl VecWithInitialized<Vec<u8>> {
@@ -47,6 +48,7 @@ where
         // to its length are initialized.
         Self {
             num_initialized: vec.as_mut().len(),
+            starting_capacity: vec.as_ref().capacity(),
             vec,
         }
     }
@@ -111,6 +113,15 @@ where
             vec.set_len(parts.len);
         }
     }
+
+    // Returns a boolean telling the caller to try reading into a small local buffer first if true.
+    // Doing so would avoid overallocating when vec is filled to capacity and we reached EOF.
+    pub(crate) fn try_small_read_first(&self, num_bytes: usize) -> bool {
+        let vec = self.vec.as_ref();
+        vec.capacity() - vec.len() < num_bytes
+            && self.starting_capacity == vec.capacity()
+            && self.starting_capacity >= num_bytes
+    }
 }
 
 pub(crate) struct ReadBufParts {

tokio/tests/io_read_to_end.rs

@@ -5,6 +5,7 @@ use std::pin::Pin;
 use std::task::{Context, Poll};
 use tokio::io::{AsyncRead, AsyncReadExt, ReadBuf};
 use tokio_test::assert_ok;
+use tokio_test::io::Builder;
 
 #[tokio::test]
 async fn read_to_end() {
@@ -76,3 +77,48 @@ async fn read_to_end_uninit() {
     test.read_to_end(&mut buf).await.unwrap();
     assert_eq!(buf.len(), 33);
 }
+
+#[tokio::test]
+async fn read_to_end_doesnt_grow_with_capacity() {
+    let arr: Vec<u8> = (0..100).collect();
+
+    // We only test from 32 since we allocate at least 32 bytes each time
+    for len in 32..100 {
+        let bytes = &arr[..len];
+        for split in 0..len {
+            for cap in 0..101 {
+                let mut mock = if split == 0 {
+                    Builder::new().read(bytes).build()
+                } else {
+                    Builder::new()
+                        .read(&bytes[..split])
+                        .read(&bytes[split..])
+                        .build()
+                };
+                let mut buf = Vec::with_capacity(cap);
+                AsyncReadExt::read_to_end(&mut mock, &mut buf)
+                    .await
+                    .unwrap();
+                // It has the right data.
+                assert_eq!(buf.as_slice(), bytes);
+                // Unless cap was smaller than length, then we did not reallocate.
+                if cap >= len {
+                    assert_eq!(buf.capacity(), cap);
+                }
+            }
+        }
+    }
+}
+
+#[tokio::test]
+async fn read_to_end_grows_capacity_if_unfit() {
+    let bytes = b"the_vector_startingcap_will_be_smaller";
+    let mut mock = Builder::new().read(bytes).build();
+    let initial_capacity = bytes.len() - 4;
+    let mut buf = Vec::with_capacity(initial_capacity);
+    AsyncReadExt::read_to_end(&mut mock, &mut buf)
+        .await
+        .unwrap();
+    // *4 since it doubles when it doesn't fit and again when reaching EOF
+    assert_eq!(buf.capacity(), initial_capacity * 4);
+}