log: add error field to Record and make SDK to emit exception attributes (#7924)

Fixes #7923


```sh
$ go test -run=^$ -bench=BenchmarkLoggerEmitExceptionAttributes -benchmem -count=5 -benchtime=500ms -cpu=1
goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/otel/sdk/log
cpu: Apple M4 Pro
BenchmarkLoggerSetErrAndEmit                 	  628162	      1023 ns/op	    5371 B/op	       1 allocs/op
BenchmarkLoggerSetErrAndEmit                 	  663955	       863.3 ns/op	    5105 B/op	       1 allocs/op
BenchmarkLoggerSetErrAndEmit                 	  653888	      1067 ns/op	    5177 B/op	       1 allocs/op
BenchmarkLoggerSetErrAndEmit                 	  716438	       824.8 ns/op	    4764 B/op	       1 allocs/op
BenchmarkLoggerSetErrAndEmit                 	  746902	       999.2 ns/op	    5630 B/op	       1 allocs/op
BenchmarkLoggerSetExceptionAttributesAndEmit 	  650696	      1042 ns/op	    5200 B/op	       1 allocs/op
BenchmarkLoggerSetExceptionAttributesAndEmit 	  574962	       980.7 ns/op	    4743 B/op	       1 allocs/op
BenchmarkLoggerSetExceptionAttributesAndEmit 	  536736	       989.2 ns/op	    5049 B/op	       1 allocs/op
BenchmarkLoggerSetExceptionAttributesAndEmit 	  558511	      1190 ns/op	    4870 B/op	       1 allocs/op
BenchmarkLoggerSetExceptionAttributesAndEmit 	  669452	       978.8 ns/op	    5067 B/op	       1 allocs/op
PASS
ok  	go.opentelemetry.io/otel/sdk/log	6.994s
```

TODO after merged:
-
https://github.com/open-telemetry/opentelemetry-go/pull/7924#discussion_r2832906451

---------

Signed-off-by: Israel Blancas <iblancasa@gmail.com>
Co-authored-by: Robert Pająk <pellared@hotmail.com>
Co-authored-by: Damien Mathieu <42@dmathieu.com>

CHANGELOG.md

@@ -13,6 +13,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Add `go.opentelemetry.io/otel/semconv/v1.40.0` package.
   The package contains semantic conventions from the `v1.40.0` version of the OpenTelemetry Semantic Conventions.
   See the [migration documentation](./semconv/v1.40.0/MIGRATION.md) for information on how to upgrade from `go.opentelemetry.io/otel/semconv/v1.39.0`. (#7985)
+- Add `Err` and `SetErr` on `Record` in `go.opentelemetry.io/otel/log` to attach an error and set record exception attributes in `go.opentelemetry.io/otel/log/sdk`. (#7924)
 
 ### Changed
 

log/record.go

@@ -26,6 +26,7 @@ type Record struct {
 	severity          Severity
 	severityText      string
 	body              Value
+	err               error
 
 	// The fields below are for optimizing the implementation of Attributes and
 	// AddAttributes. This design is borrowed from the slog Record type:
@@ -110,6 +111,16 @@ func (r *Record) SetBody(v Value) {
 	r.body = v
 }
 
+// Err returns the associated error if one has been set.
+func (r *Record) Err() error {
+	return r.err
+}
+
+// SetErr sets the associated error. Passing nil clears the error.
+func (r *Record) SetErr(err error) {
+	r.err = err
+}
+
 // WalkAttributes walks all attributes the log record holds by calling f for
 // each on each [KeyValue] in the [Record]. Iteration stops if f returns false.
 func (r *Record) WalkAttributes(f func(KeyValue) bool) {

log/record_test.go

@@ -221,3 +221,39 @@ func TestRecordClone(t *testing.T) {
 	assert.Contains(t, r1Attrs, attr0)
 	assert.Contains(t, r1Attrs, attr1)
 }
+
+func TestRecordErr(t *testing.T) {
+	tests := []struct {
+		name string
+		fn   func(*log.Record)
+		want error
+	}{
+		{
+			name: "zero value",
+		},
+		{
+			name: "set error",
+			fn: func(r *log.Record) {
+				r.SetErr(assert.AnError)
+			},
+			want: assert.AnError,
+		},
+		{
+			name: "clear error",
+			fn: func(r *log.Record) {
+				r.SetErr(assert.AnError)
+				r.SetErr(nil)
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var r log.Record
+			if tt.fn != nil {
+				tt.fn(&r)
+			}
+			assert.Equal(t, tt.want, r.Err())
+		})
+	}
+}

sdk/log/logger.go

@@ -5,17 +5,25 @@ package log // import "go.opentelemetry.io/otel/sdk/log"
 
 import (
 	"context"
+	"reflect"
 	"time"
 
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/log"
 	"go.opentelemetry.io/otel/log/embedded"
 	"go.opentelemetry.io/otel/sdk/instrumentation"
+	semconv "go.opentelemetry.io/otel/semconv/v1.40.0"
 	"go.opentelemetry.io/otel/trace"
 )
 
 var now = time.Now
 
+const (
+	exceptionTypeKey       = string(semconv.ExceptionTypeKey)
+	exceptionMessageKey    = string(semconv.ExceptionMessageKey)
+	exceptionStacktraceKey = string(semconv.ExceptionStacktraceKey)
+)
+
 // Compile-time check logger implements log.Logger.
 var _ log.Logger = (*logger)(nil)
 
@@ -108,10 +116,70 @@ func (l *logger) newRecord(ctx context.Context, r log.Record) Record {
 		newRecord.observedTimestamp = now()
 	}
 
+	hasExceptionAttr := false
 	r.WalkAttributes(func(kv log.KeyValue) bool {
+		switch kv.Key {
+		case exceptionTypeKey, exceptionMessageKey, exceptionStacktraceKey:
+			hasExceptionAttr = true
+		}
 		newRecord.AddAttributes(kv)
 		return true
 	})
 
+	if err := r.Err(); err != nil && !hasExceptionAttr {
+		addExceptionAttrs(&newRecord, err)
+	}
+
 	return newRecord
 }
+
+func addExceptionAttrs(r *Record, err error) {
+	var attrs [2]log.KeyValue
+	n := 0
+	if msg := err.Error(); msg != "" {
+		if r.attributeCountLimit > 0 && r.attributeCountLimit-r.AttributesLen() < n+1 {
+			goto flush
+		}
+		attrs[n] = log.String(exceptionMessageKey, msg)
+		n++
+	}
+	if errType := errorType(err); errType != "" {
+		if r.attributeCountLimit > 0 && r.attributeCountLimit-r.AttributesLen() < n+1 {
+			goto flush
+		}
+		attrs[n] = log.String(exceptionTypeKey, errType)
+		n++
+	}
+
+flush:
+	if n > 0 {
+		r.addAttrs(attrs[:n])
+	}
+}
+
+func errorType(err error) string {
+	if et, ok := err.(interface{ ErrorType() string }); ok {
+		if s := et.ErrorType(); s != "" {
+			return s
+		}
+	}
+
+	t := reflect.TypeOf(err)
+	if t == nil {
+		return ""
+	}
+
+	pkg, name := t.PkgPath(), t.Name()
+	if pkg != "" && name != "" {
+		return pkg + "." + name
+	}
+
+	// The type has no package path or name (predeclared, not-defined,
+	// or alias for a not-defined type).
+	//
+	// The type has no package path or name (predeclared, not-defined,
+	// or alias for a not-defined type).
+	//
+	// This is not guaranteed to be unique, but is a best effort.
+	return t.String()
+}

sdk/log/logger_bench_test.go

@@ -4,6 +4,7 @@
 package log // import "go.opentelemetry.io/otel/sdk/log"
 
 import (
+	"errors"
 	"testing"
 	"time"
 
@@ -14,6 +15,7 @@ import (
 	"go.opentelemetry.io/otel/sdk/instrumentation"
 	"go.opentelemetry.io/otel/sdk/metric"
 	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+	semconv "go.opentelemetry.io/otel/semconv/v1.39.0"
 )
 
 func BenchmarkLoggerEmit(b *testing.B) {
@@ -119,6 +121,33 @@ func BenchmarkLoggerEnabled(b *testing.B) {
 	_ = enabled
 }
 
+func BenchmarkLoggerSetErrAndEmit(b *testing.B) {
+	logger := newTestLogger(b)
+	err := errors.New("boom")
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for b.Loop() {
+		r := log.Record{}
+		r.SetErr(err)
+		logger.Emit(b.Context(), r)
+	}
+}
+
+func BenchmarkLoggerSetExceptionAttributesAndEmit(b *testing.B) {
+	logger := newTestLogger(b)
+	err := errors.New("boom")
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for b.Loop() {
+		r := log.Record{}
+		r.AddAttributes(log.String(string(semconv.ExceptionMessageKey), err.Error()))
+		r.AddAttributes(log.String(string(semconv.ExceptionTypeKey), errorType(err)))
+		logger.Emit(b.Context(), r)
+	}
+}
+
 func newTestLogger(t testing.TB) log.Logger {
 	provider := NewLoggerProvider(
 		WithProcessor(newFltrProcessor("0", false)),

sdk/log/logger_test.go

@@ -314,6 +314,163 @@ func TestLoggerEmit(t *testing.T) {
 	}
 }
 
+func TestNewRecordAddsExceptionAttrs(t *testing.T) {
+	l := newLogger(NewLoggerProvider(), instrumentation.Scope{})
+
+	t.Run("AddsMissing", func(t *testing.T) {
+		var in log.Record
+		in.SetBody(log.StringValue("boom"))
+		in.SetSeverity(log.SeverityError)
+		in.SetErr(errors.New("boom"))
+		got := l.newRecord(t.Context(), in)
+
+		var gotAttrs []log.KeyValue
+		got.WalkAttributes(func(kv log.KeyValue) bool {
+			gotAttrs = append(gotAttrs, kv)
+			return true
+		})
+
+		assert.Len(t, gotAttrs, 2)
+		assert.Contains(t, gotAttrs, log.String(string(semconv.ExceptionTypeKey), "*errors.errorString"))
+		assert.Contains(t, gotAttrs, log.String(string(semconv.ExceptionMessageKey), "boom"))
+	})
+
+	t.Run("ShortCircuitsAtAttributeLimit", func(t *testing.T) {
+		var in log.Record
+		in.SetBody(log.StringValue("boom"))
+		in.SetSeverity(log.SeverityError)
+		in.SetErr(errors.New("boom"))
+		in.AddAttributes(log.String("k1", "v1"))
+
+		lLimited := newLogger(NewLoggerProvider(WithAttributeCountLimit(2)), instrumentation.Scope{})
+		got := lLimited.newRecord(t.Context(), in)
+
+		var gotType, gotMessage string
+		got.WalkAttributes(func(kv log.KeyValue) bool {
+			switch kv.Key {
+			case string(semconv.ExceptionTypeKey):
+				gotType = kv.Value.AsString()
+			case string(semconv.ExceptionMessageKey):
+				gotMessage = kv.Value.AsString()
+			}
+			return true
+		})
+
+		assert.Empty(t, gotType)
+		assert.Equal(t, "boom", gotMessage)
+	})
+
+	t.Run("NoSlotsLeft", func(t *testing.T) {
+		var in log.Record
+		in.SetBody(log.StringValue("boom"))
+		in.SetSeverity(log.SeverityError)
+		in.SetErr(errors.New("boom"))
+		in.AddAttributes(log.String("k1", "v1"))
+		lLimited := newLogger(NewLoggerProvider(WithAttributeCountLimit(1)), instrumentation.Scope{})
+		got := lLimited.newRecord(t.Context(), in)
+
+		var gotType, gotMessage string
+		got.WalkAttributes(func(kv log.KeyValue) bool {
+			switch kv.Key {
+			case string(semconv.ExceptionTypeKey):
+				gotType = kv.Value.AsString()
+			case string(semconv.ExceptionMessageKey):
+				gotMessage = kv.Value.AsString()
+			}
+			return true
+		})
+
+		assert.Empty(t, gotType)
+		assert.Empty(t, gotMessage)
+	})
+}
+
+func TestErrorType(t *testing.T) {
+	t.Run("UsesErrorTypeMethod", func(t *testing.T) {
+		err := errWithType{msg: "boom", typ: "custom.type"}
+		assert.Equal(t, "custom.type", errorType(err))
+	})
+
+	t.Run("FallsBackWhenErrorTypeEmpty", func(t *testing.T) {
+		err := errWithType{msg: "boom", typ: ""}
+		assert.Equal(t, "go.opentelemetry.io/otel/sdk/log.errWithType", errorType(err))
+	})
+
+	t.Run("NilError", func(t *testing.T) {
+		assert.Empty(t, errorType(nil))
+	})
+
+	t.Run("UnnamedType", func(t *testing.T) {
+		var err error = struct{ baseErr }{}
+		assert.Contains(t, errorType(err), "struct")
+	})
+}
+
+type errWithType struct {
+	msg string
+	typ string
+}
+
+func (e errWithType) Error() string { return e.msg }
+
+func (e errWithType) ErrorType() string { return e.typ }
+
+type baseErr struct{}
+
+func (baseErr) Error() string { return "boom" }
+
+func TestNewRecordSkipsExceptionWhenPresent(t *testing.T) {
+	l := newLogger(NewLoggerProvider(), instrumentation.Scope{})
+
+	t.Run("ExistingMessage", func(t *testing.T) {
+		var r log.Record
+		r.SetBody(log.StringValue("boom"))
+		r.SetSeverity(log.SeverityError)
+		r.SetErr(errors.New("boom"))
+		r.AddAttributes(log.String(string(semconv.ExceptionMessageKey), "existing.message"))
+
+		got := l.newRecord(t.Context(), r)
+
+		var gotType, gotMessage string
+		got.WalkAttributes(func(kv log.KeyValue) bool {
+			switch kv.Key {
+			case string(semconv.ExceptionTypeKey):
+				gotType = kv.Value.AsString()
+			case string(semconv.ExceptionMessageKey):
+				gotMessage = kv.Value.AsString()
+			}
+			return true
+		})
+
+		assert.Equal(t, "existing.message", gotMessage)
+		assert.Empty(t, gotType)
+	})
+
+	t.Run("ExistingType", func(t *testing.T) {
+		var r log.Record
+		r.SetBody(log.StringValue("boom"))
+		r.SetSeverity(log.SeverityError)
+		r.SetErr(errors.New("boom"))
+		r.AddAttributes(log.String(string(semconv.ExceptionTypeKey), "existing.type"))
+
+		got := l.newRecord(t.Context(), r)
+
+		var gotType, gotMessage string
+		got.WalkAttributes(func(kv log.KeyValue) bool {
+			switch kv.Key {
+			case string(semconv.ExceptionTypeKey):
+				gotType = kv.Value.AsString()
+			case string(semconv.ExceptionMessageKey):
+				gotMessage = kv.Value.AsString()
+			}
+			return true
+		})
+
+		assert.Equal(t, "existing.type", gotType)
+		assert.Empty(t, gotMessage)
+	})
+}
+
 func TestLoggerEnabled(t *testing.T) {
 	p0 := newFltrProcessor("0", true)
 	p1 := newFltrProcessor("1", true)