attribute: change INVALID Type to EMPTY and mark INVALID as deprecated

CHANGELOG.md

@@ -11,6 +11,16 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ### Added
 
 - Add service detection with `WithService` in `go.opentelemetry.io/otel/sdk/resource`. (#7642)
+- Support attributes with empty value (`attribute.EMPTY`) in OTLP exporters (`go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc`, `go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc`, `go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploggrpc`, `go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp`, `go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp`, `go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploghttp`). (#8038)
+- Support attributes with empty value (`attribute.EMPTY`) in `go.opentelemetry.io/otel/sdk/metric/metricdata/metricdatatest`. (#8038)
+
+### Changed
+
+- Introduce the `EMPTY` Type in `go.opentelemetry.io/otel/attribute` to reflect that an empty value is now a valid value, with `INVALID` remaining as a deprecated alias of `EMPTY`. (#8038)
+
+### Deprecated
+
+- Deprecate `INVALID` in `go.opentelemetry.io/otel/attribute`. Use `EMPTY` instead. (#8038)
 
 <!-- Released section -->
 <!-- Don't change this section unless doing release -->

attribute/hash.go

@@ -27,6 +27,7 @@ const (
 	int64SliceID   uint64 = 3762322556277578591 // "_[]int64" (little endian)
 	float64SliceID uint64 = 7308324551835016539 // "[]double" (little endian)
 	stringSliceID  uint64 = 7453010373645655387 // "[]string" (little endian)
+	emptyID        uint64 = 7305809155345288421 // "__empty_" (little endian)
 )
 
 // hashKVs returns a new xxHash64 hash of kvs.
@@ -80,7 +81,8 @@ func hashKV(h xxhash.Hash, kv KeyValue) xxhash.Hash {
 		for i := 0; i < rv.Len(); i++ {
 			h = h.String(rv.Index(i).String())
 		}
-	case INVALID:
+	case EMPTY:
+		h = h.Uint64(emptyID)
 	default:
 		// Logging is an alternative, but using the internal logger here
 		// causes an import cycle so it is not done.

attribute/hash_test.go

@@ -36,6 +36,7 @@ var keyVals = []func(string) KeyValue{
 	func(k string) KeyValue { return String(k, "bar") },
 	func(k string) KeyValue { return StringSlice(k, []string{"foo", "bar", "baz"}) },
 	func(k string) KeyValue { return StringSlice(k, []string{"[]i1"}) },
+	func(k string) KeyValue { return KeyValue{Key: Key(k)} }, // Empty value.
 }
 
 func TestHashKVsEquality(t *testing.T) {
@@ -155,8 +156,8 @@ func FuzzHashKVs(f *testing.F) {
 		0, int64(0), math.Inf(1), false, uint8(2))
 
 	f.Fuzz(func(t *testing.T, k1, k2, k3, k4, k5, s string, i int, i64 int64, fVal float64, b bool, sliceType uint8) {
-		// Test variable number of attributes (0-10).
-		numAttrs := len(k1) % 11 // Use key length to determine number of attributes.
+		// Test variable number of attributes (0-11).
+		numAttrs := len(k1) % 12 // Use key length to determine number of attributes.
 		if numAttrs == 0 && k1 == "" {
 			// Test empty set.
 			h := hashKVs(nil)
@@ -250,6 +251,11 @@ func FuzzHashKVs(f *testing.F) {
 			kvs = append(kvs, String("empty", ""))
 		}
 
+		// Add empty value.
+		if numAttrs > 10 {
+			kvs = append(kvs, KeyValue{Key: Key("empty_value")})
+		}
+
 		// Sort to ensure consistent ordering (as Set would do).
 		slices.SortFunc(kvs, func(a, b KeyValue) int {
 			return cmp.Compare(string(a.Key), string(b.Key))
@@ -301,6 +307,8 @@ func FuzzHashKVs(f *testing.F) {
 					if !math.IsNaN(val) && !math.IsInf(val, 0) {
 						modifiedKvs[0] = Float64(string(modifiedKvs[0].Key), val+1.0)
 					}
+				case EMPTY:
+					modifiedKvs[0] = String(string(modifiedKvs[0].Key), "not_empty")
 				}
 
 				h3 := hashKVs(modifiedKvs)

attribute/key_test.go

@@ -108,6 +108,11 @@ func TestEmit(t *testing.T) {
 			v:    attribute.StringSliceValue([]string{"foo", "bar"}),
 			want: `["foo","bar"]`,
 		},
+		{
+			name: `test Key.Emit() can emit a string representing self.EMPTY`,
+			v:    attribute.Value{},
+			want: "",
+		},
 	} {
 		t.Run(testcase.name, func(t *testing.T) {
 			// proto: func (v attribute.Value) Emit() string {

attribute/kv.go

@@ -15,7 +15,7 @@ type KeyValue struct {
 
 // Valid reports whether kv is a valid OpenTelemetry attribute.
 func (kv KeyValue) Valid() bool {
-	return kv.Key.Defined() && kv.Value.Type() != INVALID
+	return kv.Key.Defined()
 }
 
 // Bool creates a KeyValue with a BOOL Value type.

attribute/kv_test.go

@@ -86,11 +86,11 @@ func TestKeyValueValid(t *testing.T) {
 			kv:    attribute.Key("").Bool(true),
 		},
 		{
-			desc:  "INVALID value type should be invalid",
-			valid: false,
+			desc:  "EMPTY value type should be valid",
+			valid: true,
 			kv: attribute.KeyValue{
 				Key: attribute.Key("valid key"),
-				// Default type is INVALID.
+				// Default type is EMPTY.
 				Value: attribute.Value{},
 			},
 		},
@@ -152,6 +152,10 @@ func TestIncorrectCast(t *testing.T) {
 			name: "StringSlice",
 			val:  attribute.BoolSliceValue([]bool{true}),
 		},
+		{
+			name: "Empty",
+			val:  attribute.Value{},
+		},
 	}
 	for _, tt := range testCases {
 		t.Run(tt.name, func(t *testing.T) {

attribute/value.go

@@ -18,6 +18,8 @@ import (
 type Type int // nolint: revive  // redefines builtin Type.
 
 // Value represents the value part in key-value pairs.
+//
+// Note that the zero value is a valid empty value.
 type Value struct {
 	vtype    Type
 	numeric  uint64
@@ -26,8 +28,8 @@ type Value struct {
 }
 
 const (
-	// INVALID is used for a Value with no value set.
-	INVALID Type = iota
+	// EMPTY is used for a Value with no value set.
+	EMPTY Type = iota
 	// BOOL is a boolean Type Value.
 	BOOL
 	// INT64 is a 64-bit signed integral Type Value.
@@ -44,6 +46,10 @@ const (
 	FLOAT64SLICE
 	// STRINGSLICE is a slice of strings Type Value.
 	STRINGSLICE
+	// INVALID is used for a Value with no value set.
+	//
+	// Deprecated: Use EMPTY instead as an empty value is a valid value.
+	INVALID = EMPTY
 )
 
 // BoolValue creates a BOOL Value.
@@ -217,6 +223,8 @@ func (v Value) AsInterface() any {
 		return v.stringly
 	case STRINGSLICE:
 		return v.asStringSlice()
+	case EMPTY:
+		return nil
 	}
 	return unknownValueType{}
 }
@@ -252,6 +260,8 @@ func (v Value) Emit() string {
 		return string(j)
 	case STRING:
 		return v.stringly
+	case EMPTY:
+		return ""
 	default:
 		return "unknown"
 	}

attribute/value_test.go

@@ -86,14 +86,17 @@ func TestValue(t *testing.T) {
 			wantType:  attribute.STRINGSLICE,
 			wantValue: []string{"forty-two", "negative three", "twelve"},
 		},
+		{
+			name:      "empty value",
+			value:     attribute.Value{},
+			wantType:  attribute.EMPTY,
+			wantValue: nil,
+		},
 	} {
 		t.Logf("Running test case %s", testcase.name)
 		if testcase.value.Type() != testcase.wantType {
 			t.Errorf("wrong value type, got %#v, expected %#v", testcase.value.Type(), testcase.wantType)
 		}
-		if testcase.wantType == attribute.INVALID {
-			continue
-		}
 		got := testcase.value.AsInterface()
 		if diff := cmp.Diff(testcase.wantValue, got); diff != "" {
 			t.Errorf("+got, -want: %s", diff)
@@ -143,6 +146,10 @@ func TestEquivalence(t *testing.T) {
 			attribute.StringSlice("StringSlice", []string{"one", "two", "three"}),
 			attribute.StringSlice("StringSlice", []string{"one", "two", "three"}),
 		},
+		{
+			attribute.KeyValue{Key: "Empty"},
+			attribute.KeyValue{Key: "Empty"},
+		},
 	}
 
 	t.Run("Distinct", func(t *testing.T) {
@@ -234,6 +241,10 @@ func TestNotEquivalence(t *testing.T) {
 			attribute.StringSlice("StringSlice", []string{"one", "two", "three"}),
 			attribute.StringSlice("StringSlice", []string{"one", "two"}),
 		},
+		{
+			attribute.KeyValue{Key: "Empty"},
+			attribute.String("Empty", ""),
+		},
 	}
 
 	t.Run("Distinct", func(t *testing.T) {