s3: Provide enough buffer when the object final size is unknown (#20847)

When compression is enabled, the final object size is not calculated in
that case, we need to make sure that the provided buffer is always
more significant than the shard size, the bitrot will always calculate 
the hash of blocks with shard size, except the last block.

cmd/bitrot-streaming.go

@@ -20,6 +20,7 @@ package cmd
 import (
 	"bytes"
 	"context"
+	"errors"
 	"hash"
 	"io"
 	"sync"
@@ -37,12 +38,22 @@ type streamingBitrotWriter struct {
 	shardSize    int64
 	canClose     *sync.WaitGroup
 	byteBuf      []byte
+	finished     bool
 }
 
 func (b *streamingBitrotWriter) Write(p []byte) (int, error) {
 	if len(p) == 0 {
 		return 0, nil
 	}
+	if b.finished {
+		return 0, errors.New("bitrot write not allowed")
+	}
+	if int64(len(p)) > b.shardSize {
+		return 0, errors.New("unexpected bitrot buffer size")
+	}
+	if int64(len(p)) < b.shardSize {
+		b.finished = true
+	}
 	b.h.Reset()
 	b.h.Write(p)
 	hashBytes := b.h.Sum(nil)

cmd/erasure-multipart.go

@@ -626,17 +626,13 @@ func (er erasureObjects) PutObjectPart(ctx context.Context, bucket, object, uplo
 	switch size := data.Size(); {
 	case size == 0:
 		buffer = make([]byte, 1) // Allocate at least a byte to reach EOF
-	case size == -1:
+	case size >= fi.Erasure.BlockSize || size == -1:
-		if size := data.ActualSize(); size > 0 && size < fi.Erasure.BlockSize {
+		if int64(globalBytePoolCap.Load().Width()) < fi.Erasure.BlockSize {
-			// Account for padding and forced compression overhead and encryption.
+			buffer = make([]byte, fi.Erasure.BlockSize, 2*fi.Erasure.BlockSize)
-			buffer = make([]byte, data.ActualSize()+256+32+32, data.ActualSize()*2+512)
 		} else {
 			buffer = globalBytePoolCap.Load().Get()
 			defer globalBytePoolCap.Load().Put(buffer)
 		}
-	case size >= fi.Erasure.BlockSize:
-		buffer = globalBytePoolCap.Load().Get()
-		defer globalBytePoolCap.Load().Put(buffer)
 	case size < fi.Erasure.BlockSize:
 		// No need to allocate fully fi.Erasure.BlockSize buffer if the incoming data is smaller.
 		buffer = make([]byte, size, 2*size+int64(fi.Erasure.ParityBlocks+fi.Erasure.DataBlocks-1))