move to GET for internal stream READs instead of POST (#20160)

the main reason is to let Go net/http perform necessary
book keeping properly, and in essential from consistency
point of view its GETs all the way.

Deprecate sendFile() as its buggy inside Go runtime.

internal/ioutil/ioutil.go

@@ -34,8 +34,9 @@ import (
 
 // Block sizes constant.
 const (
-	SmallBlock = 32 * humanize.KiByte // Default r/w block size for smaller objects.
-	LargeBlock = 1 * humanize.MiByte  // Default r/w block size for normal objects.
+	SmallBlock  = 32 * humanize.KiByte  // Default r/w block size for smaller objects.
+	MediumBlock = 128 * humanize.KiByte // Default r/w block size for medium sized objects.
+	LargeBlock  = 1 * humanize.MiByte   // Default r/w block size for normal objects.
 )
 
 // aligned sync.Pool's
@@ -46,6 +47,12 @@ var (
 			return &b
 		},
 	}
+	ODirectPoolMedium = sync.Pool{
+		New: func() interface{} {
+			b := disk.AlignedBlock(MediumBlock)
+			return &b
+		},
+	}
 	ODirectPoolSmall = sync.Pool{
 		New: func() interface{} {
 			b := disk.AlignedBlock(SmallBlock)
@@ -294,13 +301,19 @@ func NewSkipReader(r io.Reader, n int64) io.Reader {
 	return &SkipReader{r, n}
 }
 
+// writerOnly hides an io.Writer value's optional ReadFrom method
+// from io.Copy.
+type writerOnly struct {
+	io.Writer
+}
+
 // Copy is exactly like io.Copy but with reusable buffers.
 func Copy(dst io.Writer, src io.Reader) (written int64, err error) {
-	bufp := ODirectPoolLarge.Get().(*[]byte)
+	bufp := ODirectPoolMedium.Get().(*[]byte)
+	defer ODirectPoolMedium.Put(bufp)
 	buf := *bufp
-	defer ODirectPoolLarge.Put(bufp)
 
-	return io.CopyBuffer(dst, src, buf)
+	return io.CopyBuffer(writerOnly{dst}, src, buf)
 }
 
 // SameFile returns if the files are same.