s3select should honour custom record delimiter (#6419)

Allow custom delimiters like `\r\n`, `a`, `\r` etc in input csv and 
replace with `\n`.

Fixes #6403

pkg/ioutil/delimited-reader.go

@@ -0,0 +1,87 @@
+/*
+ * Minio Cloud Storage, (C) 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package ioutil
+
+import (
+	"bufio"
+	"io"
+)
+
+var (
+	nByte byte = 10 // the byte that corresponds to the '\n' rune.
+	rByte byte = 13 // the byte that corresponds to the '\r' rune.
+)
+
+// DelimitedReader reduces the custom delimiter to `\n`.
+type DelimitedReader struct {
+	r           *bufio.Reader
+	delimiter   []rune // Select can have upto 2 characters as delimiter.
+	assignEmpty bool   // Decides whether the next read byte should be discarded.
+}
+
+// NewDelimitedReader detects the custom delimiter and replaces with `\n`.
+func NewDelimitedReader(r io.Reader, delimiter []rune) *DelimitedReader {
+	return &DelimitedReader{r: bufio.NewReader(r), delimiter: delimiter, assignEmpty: false}
+}
+
+// Reads and replaces the custom delimiter with `\n`.
+func (r *DelimitedReader) Read(p []byte) (n int, err error) {
+	n, err = r.r.Read(p)
+	if err != nil {
+		return
+	}
+	for i, b := range p {
+		if r.assignEmpty {
+			swapAndNullify(p, i)
+			r.assignEmpty = false
+			continue
+		}
+		if b == rByte && rune(b) != r.delimiter[0] {
+			// Replace the carriage returns with `\n`.
+			// Mac styled csv will have `\r` as their record delimiter.
+			p[i] = nByte
+		} else if rune(b) == r.delimiter[0] { // Eg, `\r\n`,`ab`,`a` are valid delimiters
+			if i+1 == len(p) && len(r.delimiter) > 1 {
+				// If the first delimiter match falls on the boundary,
+				// Peek the next byte and if it matches, discard it in the next byte read.
+				if nextByte, nerr := r.r.Peek(1); nerr == nil {
+					if rune(nextByte[0]) == r.delimiter[1] {
+						p[i] = nByte
+						// To Discard in the next read.
+						r.assignEmpty = true
+					}
+				}
+			} else if len(r.delimiter) > 1 && rune(p[i+1]) == r.delimiter[1] {
+				// The second delimiter falls in the same chunk.
+				p[i] = nByte
+				r.assignEmpty = true
+			} else if len(r.delimiter) == 1 {
+				// Replace with `\n` incase of single charecter delimiter match.
+				p[i] = nByte
+			}
+		}
+	}
+	return
+}
+
+// Occupy the first byte space and nullify the last byte.
+func swapAndNullify(p []byte, n int) {
+	for i := n; i < len(p)-1; i++ {
+		p[i] = p[i+1]
+	}
+	p[len(p)-1] = 0
+}