Set S3 Select record message length to 128KiB (#7475)

- Previously this limit was a little more than 1MiB, and it broke
  compatibility with AWS SDK Java causing a buffer overflow error.

pkg/s3select/message.go

@@ -61,6 +61,14 @@ var recordsHeader = []byte{
 	11, ':', 'e', 'v', 'e', 'n', 't', '-', 't', 'y', 'p', 'e', 7, 0, 7, 'R', 'e', 'c', 'o', 'r', 'd', 's',
 }
 
+const (
+	maxRecordMessageLength = 128 * 1024 // Chosen for compatibility with AWS JAVA SDK
+)
+
+var (
+	bufLength = payloadLenForMsgLen(maxRecordMessageLength)
+)
+
 // newRecordsMessage - creates new Records Message which can contain a single record, partial records,
 // or multiple records. Depending on the size of the result, a response can contain one or more of these messages.
 //
@@ -74,6 +82,14 @@ func newRecordsMessage(payload []byte) []byte {
 	return genMessage(recordsHeader, payload)
 }
 
+// payloadLenForMsgLen computes the length of the payload in a record
+// message given the length of the message.
+func payloadLenForMsgLen(messageLength int) int {
+	headerLength := len(recordsHeader)
+	payloadLength := messageLength - 4 - 4 - 4 - headerLength - 4
+	return payloadLength
+}
+
 // continuationMessage - S3 periodically sends this message to keep the TCP connection open.
 // These messages appear in responses at random. The client must detect the message type and process accordingly.
 //
@@ -292,16 +308,20 @@ func (writer *messageWriter) start() {
 				}
 				writer.write(endMessage)
 			} else {
-				// Write record payload to staging buffer
+				for len(payload) > 0 {
+					copiedLen := copy(writer.payloadBuffer[writer.payloadBufferIndex:], payload)
+					writer.payloadBufferIndex += copiedLen
+					payload = payload[copiedLen:]
+
+					// If buffer is filled, flush it now!
 					freeSpace := bufLength - writer.payloadBufferIndex
-				if len(payload) > freeSpace {
+					if freeSpace == 0 {
 						if !writer.flushRecords() {
 							quitFlag = true
 							break
 						}
 					}
-				copy(writer.payloadBuffer[writer.payloadBufferIndex:], payload)
+				}
-				writer.payloadBufferIndex += len(payload)
 			}
 
 		case <-recordStagingTicker.C:
@@ -331,10 +351,6 @@ func (writer *messageWriter) start() {
 	}
 }
 
-const (
-	bufLength = maxRecordSize
-)
-
 // Sends a single whole record.
 func (writer *messageWriter) SendRecord(payload []byte) error {
 	select {