add kafka.SeekDontCheck (segmentio#295)

* add kafka.SeekDontCheck

* fix logic

* touch less code

* use SeekDontCheck in ReadBatch

* add test

conn.go

@@ -595,6 +595,12 @@ const (
 	SeekAbsolute = 1 // Seek to an absolute offset.
 	SeekEnd      = 2 // Seek relative to the last offset available in the partition.
 	SeekCurrent  = 3 // Seek relative to the current offset.
+
+	// This flag may be combined to any of the SeekAbsolute and SeekCurrent
+	// constants to skip the bound check that the connection would do otherwise.
+	// Programs can use this flag to avoid making a metadata request to the kafka
+	// broker to read the current first and last offsets of the partition.
+	SeekDontCheck = 1 << 31
 )
 
 // Seek sets the offset for the next read or write operation according to whence, which
@@ -604,12 +610,32 @@ const (
 // as in lseek(2) or os.Seek.
 // The method returns the new absolute offset of the connection.
 func (c *Conn) Seek(offset int64, whence int) (int64, error) {
+	seekDontCheck := (whence & SeekDontCheck) != 0
+	whence &= ^SeekDontCheck
+
 	switch whence {
 	case SeekStart, SeekAbsolute, SeekEnd, SeekCurrent:
 	default:
 		return 0, fmt.Errorf("whence must be one of 0, 1, 2, or 3. (whence = %d)", whence)
 	}
 
+	if seekDontCheck {
+		if whence == SeekAbsolute {
+			c.mutex.Lock()
+			c.offset = offset
+			c.mutex.Unlock()
+			return offset, nil
+		}
+
+		if whence == SeekCurrent {
+			c.mutex.Lock()
+			c.offset += offset
+			offset = c.offset
+			c.mutex.Unlock()
+			return offset, nil
+		}
+	}
+
 	if whence == SeekAbsolute {
 		c.mutex.Lock()
 		unchanged := offset == c.offset
@@ -618,6 +644,7 @@ func (c *Conn) Seek(offset int64, whence int) (int64, error) {
 			return offset, nil
 		}
 	}
+
 	if whence == SeekCurrent {
 		c.mutex.Lock()
 		offset = c.offset + offset
@@ -726,7 +753,9 @@ func (c *Conn) ReadBatchWith(cfg ReadBatchConfig) *Batch {
 		return &Batch{err: fmt.Errorf("kafka.(*Conn).ReadBatch: minBytes (%d) > maxBytes (%d)", cfg.MinBytes, cfg.MaxBytes)}
 	}
 
-	offset, err := c.Seek(c.Offset())
+	offset, whence := c.Offset()
+
+	offset, err := c.Seek(offset, whence|SeekDontCheck)
 	if err != nil {
 		return &Batch{err: dontExpectEOF(err)}
 	}

conn_test.go

@@ -152,6 +152,11 @@ func TestConn(t *testing.T) {
 			function: testConnSeekRandomOffset,
 		},
 
+		{
+			scenario: "unchecked seeks allow the connection to be positionned outside the boundaries of the partition",
+			function: testConnSeekDontCheck,
+		},
+
 		{
 			scenario: "writing and reading messages sequentially should preserve the order",
 			function: testConnWriteReadSequentially,
@@ -439,6 +444,27 @@ func testConnSeekRandomOffset(t *testing.T, conn *Conn) {
 	}
 }
 
+func testConnSeekDontCheck(t *testing.T, conn *Conn) {
+	for i := 0; i != 10; i++ {
+		if _, err := conn.Write([]byte(strconv.Itoa(i))); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	offset, err := conn.Seek(42, SeekAbsolute|SeekDontCheck)
+	if err != nil {
+		t.Error(err)
+	}
+
+	if offset != 42 {
+		t.Error("bad offset:", offset)
+	}
+
+	if _, err := conn.ReadMessage(1024); err != OffsetOutOfRange {
+		t.Error("unexpected error:", err)
+	}
+}
+
 func testConnWriteReadSequentially(t *testing.T, conn *Conn) {
 	for i := 0; i != 10; i++ {
 		if _, err := conn.Write([]byte(strconv.Itoa(i))); err != nil {

go.mod

@@ -9,4 +9,5 @@ require (
 	github.com/xdg/scram v0.0.0-20180814205039-7eeb5667e42c
 	github.com/xdg/stringprep v1.0.0 // indirect
 	golang.org/x/crypto v0.0.0-20190506204251-e1dfcc566284 // indirect
+	golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3
 )

go.sum

@@ -11,6 +11,7 @@ github.com/xdg/stringprep v1.0.0/go.mod h1:Jhud4/sHMO4oL310DaZAKk9ZaJ08SJfe+sJh0
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20190506204251-e1dfcc566284 h1:rlLehGeYg6jfoyz/eDqDU1iRXLKfR42nnNh57ytKEWo=
 golang.org/x/crypto v0.0.0-20190506204251-e1dfcc566284/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
+golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3 h1:0GoQqolDA55aaLxZyTzK/Y2ePZzZTUrRacwib7cNsYQ=
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=