Fix read_to_end to not grow an exact size buffer

If you know how much data to expect and use `Vec::with_capacity` to
pre-allocate a buffer of that capacity, `Read::read_to_end` will still
double its capacity. It needs some space to perform a read, even though
that read ends up returning `0`.

It's a bummer to carefully pre-allocate 1GB to read a 1GB file into
memory and end up using 2GB.

This fixes that behavior by special casing a full buffer and reading
into a small "probe" buffer instead. If that read returns `0` then it's
confirmed that the buffer was the perfect size. If it doesn't, the probe
buffer is appended to the normal buffer and the read loop continues.

Fixing this allows several workarounds in the standard library to be
removed:

- `Take` no longer needs to override `Read::read_to_end`.
- The `reservation_size` callback that allowed `Take` to inhibit the
  previous over-allocation behavior isn't needed.
- `fs::read` doesn't need to reserve an extra byte in
  `initial_buffer_size`.

Curiously, there was a unit test that specifically checked that
`Read::read_to_end` *does* over-allocate. I removed that test, too.

library/std/src/io/mod.rs

@@ -362,22 +362,18 @@ where
 // Because we're extending the buffer with uninitialized data for trusted
 // readers, we need to make sure to truncate that if any of this panics.
 fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
-    read_to_end_with_reservation(r, buf, |_| 32)
-}
-
-fn read_to_end_with_reservation<R, F>(
-    r: &mut R,
-    buf: &mut Vec<u8>,
-    mut reservation_size: F,
-) -> Result<usize>
-where
-    R: Read + ?Sized,
-    F: FnMut(&R) -> usize,
-{
     let start_len = buf.len();
+    let start_cap = buf.capacity();
     let mut g = Guard { len: buf.len(), buf };
     loop {
-        if g.len == g.buf.len() {
+        // If we've read all the way up to the capacity, reserve more space.
+        if g.len == g.buf.capacity() {
+            g.buf.reserve(32);
+        }
+
+        // Initialize any excess capacity and adjust the length so we can write
+        // to it.
+        if g.buf.len() < g.buf.capacity() {
             unsafe {
                 // FIXME(danielhenrymantilla): #42788
                 //
@@ -387,7 +383,6 @@ where
                 //   - Only the standard library gets to soundly "ignore" this,
                 //     based on its privileged knowledge of unstable rustc
                 //     internals;
-                g.buf.reserve(reservation_size(r));
                 let capacity = g.buf.capacity();
                 g.buf.set_len(capacity);
                 r.initializer().initialize(&mut g.buf[g.len..]);
@@ -404,9 +399,30 @@ where
                 assert!(n <= buf.len());
                 g.len += n;
             }
-            Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
+            Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
             Err(e) => return Err(e),
         }
+
+        if g.len == g.buf.capacity() && g.buf.capacity() == start_cap {
+            // The buffer might be an exact fit. Let's read into a probe buffer
+            // and see if it returns `Ok(0)`. If so, we've avoided an
+            // unnecessary doubling of the capacity. But if not, append the
+            // probe buffer to the primary buffer and let its capacity grow.
+            let mut probe = [0u8; 32];
+
+            loop {
+                match r.read(&mut probe) {
+                    Ok(0) => return Ok(g.len - start_len),
+                    Ok(n) => {
+                        g.buf.extend_from_slice(&probe[..n]);
+                        g.len += n;
+                        break;
+                    }
+                    Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
+                    Err(e) => return Err(e),
+                }
+            }
+        }
     }
 }
 
@@ -2583,13 +2599,6 @@ impl<T: Read> Read for Take<T> {
     unsafe fn initializer(&self) -> Initializer {
         self.inner.initializer()
     }
-
-    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
-        // Pass in a reservation_size closure that respects the current value
-        // of limit for each read. If we hit the read limit, this prevents the
-        // final zero-byte read from allocating again.
-        read_to_end_with_reservation(self, buf, |self_| cmp::min(self_.limit, 32) as usize)
-    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]