Labels: simpler/faster stringlabels encoding (#16069)

Instead of using varint to encode the size of each label, use a single
byte for size 0-254, or a flag value of 255 followed by the size in
3 bytes little-endian.

This reduces the amount of code, and also the number of branches in
commonly-executed code, so it runs faster.

The maximum allowed label name or value length is now 2^24 or 16MB.

Memory used by labels changes as follows:
* Labels from 0 to 127 bytes length: same
* From 128 to 254: 1 byte less
* From 255 to 16383: 2 bytes more
* From 16384 to 2MB: 1 byte more
* From 2MB to 16MB: same

Labels: panic on string too long.

Slightly more user-friendly than encoding bad data and finding out when
we decode.

Clarify that Labels.Bytes() encoding can change

---------

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

model/labels/labels.go

@@ -32,8 +32,8 @@ func (ls Labels) Len() int           { return len(ls) }
 func (ls Labels) Swap(i, j int)      { ls[i], ls[j] = ls[j], ls[i] }
 func (ls Labels) Less(i, j int) bool { return ls[i].Name < ls[j].Name }
 
-// Bytes returns ls as a byte slice.
-// It uses an byte invalid character as a separator and so should not be used for printing.
+// Bytes returns an opaque, not-human-readable, encoding of ls, usable as a map key.
+// Encoding may change over time or between runs of Prometheus.
 func (ls Labels) Bytes(buf []byte) []byte {
 	b := bytes.NewBuffer(buf[:0])
 	b.WriteByte(labelSep)

model/labels/labels_dedupelabels.go

@@ -140,8 +140,8 @@ func decodeString(t *nameTable, data string, index int) (string, int) {
 	return t.ToName(num), index
 }
 
-// Bytes returns ls as a byte slice.
-// It uses non-printing characters and so should not be used for printing.
+// Bytes returns an opaque, not-human-readable, encoding of ls, usable as a map key.
+// Encoding may change over time or between runs of Prometheus.
 func (ls Labels) Bytes(buf []byte) []byte {
 	b := bytes.NewBuffer(buf[:0])
 	for i := 0; i < len(ls.data); {

model/labels/labels_stringlabels.go

@@ -24,31 +24,25 @@ import (
 )
 
 // Labels is implemented by a single flat string holding name/value pairs.
-// Each name and value is preceded by its length in varint encoding.
+// Each name and value is preceded by its length, encoded as a single byte
+// for size 0-254, or the following 3 bytes little-endian, if the first byte is 255.
+// Maximum length allowed is 2^24 or 16MB.
 // Names are in order.
 type Labels struct {
 	data string
 }
 
 func decodeSize(data string, index int) (int, int) {
-	// Fast-path for common case of a single byte, value 0..127.
 	b := data[index]
 	index++
-	if b < 0x80 {
-		return int(b), index
-	}
-	size := int(b & 0x7F)
-	for shift := uint(7); ; shift += 7 {
+	if b == 255 {
+		// Larger numbers are encoded as 3 bytes little-endian.
 		// Just panic if we go of the end of data, since all Labels strings are constructed internally and
 		// malformed data indicates a bug, or memory corruption.
-		b := data[index]
-		index++
-		size |= int(b&0x7F) << shift
-		if b < 0x80 {
-			break
+		return int(data[index]) + (int(data[index+1]) << 8) + (int(data[index+2]) << 16), index + 3
 	}
-	}
-	return size, index
+	// More common case of a single byte, value 0..254.
+	return int(b), index
 }
 
 func decodeString(data string, index int) (string, int) {
@@ -57,8 +51,8 @@ func decodeString(data string, index int) (string, int) {
 	return data[index : index+size], index + size
 }
 
-// Bytes returns ls as a byte slice.
-// It uses non-printing characters and so should not be used for printing.
+// Bytes returns an opaque, not-human-readable, encoding of ls, usable as a map key.
+// Encoding may change over time or between runs of Prometheus.
 func (ls Labels) Bytes(buf []byte) []byte {
 	if cap(buf) < len(ls.data) {
 		buf = make([]byte, len(ls.data))
@@ -527,48 +521,27 @@ func marshalLabelToSizedBuffer(m *Label, data []byte) int {
 	return len(data) - i
 }
 
-func sizeVarint(x uint64) (n int) {
-	// Most common case first
-	if x < 1<<7 {
+func sizeWhenEncoded(x uint64) (n int) {
+	if x < 255 {
 		return 1
+	} else if x <= 1<<24 {
+		return 4
 	}
-	if x >= 1<<56 {
-		return 9
-	}
-	if x >= 1<<28 {
-		x >>= 28
-		n = 4
-	}
-	if x >= 1<<14 {
-		x >>= 14
-		n += 2
-	}
-	if x >= 1<<7 {
-		n++
-	}
-	return n + 1
+	panic("String too long to encode as label.")
 }
 
-func encodeVarint(data []byte, offset int, v uint64) int {
-	offset -= sizeVarint(v)
-	base := offset
-	for v >= 1<<7 {
-		data[offset] = uint8(v&0x7f | 0x80)
-		v >>= 7
-		offset++
-	}
-	data[offset] = uint8(v)
-	return base
-}
-
-// Special code for the common case that a size is less than 128
 func encodeSize(data []byte, offset, v int) int {
-	if v < 1<<7 {
+	if v < 255 {
 		offset--
 		data[offset] = uint8(v)
 		return offset
 	}
-	return encodeVarint(data, offset, uint64(v))
+	offset -= 4
+	data[offset] = 255
+	data[offset+1] = byte(v)
+	data[offset+2] = byte((v >> 8))
+	data[offset+3] = byte((v >> 16))
+	return offset
 }
 
 func labelsSize(lbls []Label) (n int) {
@@ -582,9 +555,9 @@ func labelsSize(lbls []Label) (n int) {
 func labelSize(m *Label) (n int) {
 	// strings are encoded as length followed by contents.
 	l := len(m.Name)
-	n += l + sizeVarint(uint64(l))
+	n += l + sizeWhenEncoded(uint64(l))
 	l = len(m.Value)
-	n += l + sizeVarint(uint64(l))
+	n += l + sizeWhenEncoded(uint64(l))
 	return n
 }
 

model/labels/labels_test.go

@@ -27,6 +27,8 @@ import (
 )
 
 func TestLabels_String(t *testing.T) {
+	s254 := strings.Repeat("x", 254) // Edge cases for stringlabels encoding.
+	s255 := strings.Repeat("x", 255)
 	cases := []struct {
 		labels   Labels
 		expected string
@@ -43,6 +45,14 @@ func TestLabels_String(t *testing.T) {
 			labels:   FromStrings("service.name", "t1", "whatever\\whatever", "t2"),
 			expected: `{"service.name"="t1", "whatever\\whatever"="t2"}`,
 		},
+		{
+			labels:   FromStrings("aaa", "111", "xx", s254),
+			expected: `{aaa="111", xx="` + s254 + `"}`,
+		},
+		{
+			labels:   FromStrings("aaa", "111", "xx", s255),
+			expected: `{aaa="111", xx="` + s255 + `"}`,
+		},
 	}
 	for _, c := range cases {
 		str := c.labels.String()