Separate trace API components into own files (#5620)

This moves the `Span` (along with the `SpanKind` and `Link`), `Tracer`,
and `TracerProvider` out of the single trace.go file and into their own
files. This change is intended to help developers find these types when
looking at the directory and to break up the long trace.go file.

trace/provider.go

@@ -0,0 +1,59 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package trace // import "go.opentelemetry.io/otel/trace"
+
+import "go.opentelemetry.io/otel/trace/embedded"
+
+// TracerProvider provides Tracers that are used by instrumentation code to
+// trace computational workflows.
+//
+// A TracerProvider is the collection destination of all Spans from Tracers it
+// provides, it represents a unique telemetry collection pipeline. How that
+// pipeline is defined, meaning how those Spans are collected, processed, and
+// where they are exported, depends on its implementation. Instrumentation
+// authors do not need to define this implementation, rather just use the
+// provided Tracers to instrument code.
+//
+// Commonly, instrumentation code will accept a TracerProvider implementation
+// at runtime from its users or it can simply use the globally registered one
+// (see https://pkg.go.dev/go.opentelemetry.io/otel#GetTracerProvider).
+//
+// Warning: Methods may be added to this interface in minor releases. See
+// package documentation on API implementation for information on how to set
+// default behavior for unimplemented methods.
+type TracerProvider interface {
+	// Users of the interface can ignore this. This embedded type is only used
+	// by implementations of this interface. See the "API Implementations"
+	// section of the package documentation for more information.
+	embedded.TracerProvider
+
+	// Tracer returns a unique Tracer scoped to be used by instrumentation code
+	// to trace computational workflows. The scope and identity of that
+	// instrumentation code is uniquely defined by the name and options passed.
+	//
+	// The passed name needs to uniquely identify instrumentation code.
+	// Therefore, it is recommended that name is the Go package name of the
+	// library providing instrumentation (note: not the code being
+	// instrumented). Instrumentation libraries can have multiple versions,
+	// therefore, the WithInstrumentationVersion option should be used to
+	// distinguish these different codebases. Additionally, instrumentation
+	// libraries may sometimes use traces to communicate different domains of
+	// workflow data (i.e. using spans to communicate workflow events only). If
+	// this is the case, the WithScopeAttributes option should be used to
+	// uniquely identify Tracers that handle the different domains of workflow
+	// data.
+	//
+	// If the same name and options are passed multiple times, the same Tracer
+	// will be returned (it is up to the implementation if this will be the
+	// same underlying instance of that Tracer or not). It is not necessary to
+	// call this multiple times with the same name and options to get an
+	// up-to-date Tracer. All implementations will ensure any TracerProvider
+	// configuration changes are propagated to all provided Tracers.
+	//
+	// If name is empty, then an implementation defined default name will be
+	// used instead.
+	//
+	// This method is safe to call concurrently.
+	Tracer(name string, options ...TracerOption) Tracer
+}

trace/span.go

@@ -0,0 +1,177 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package trace // import "go.opentelemetry.io/otel/trace"
+
+import (
+	"context"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/codes"
+	"go.opentelemetry.io/otel/trace/embedded"
+)
+
+// Span is the individual component of a trace. It represents a single named
+// and timed operation of a workflow that is traced. A Tracer is used to
+// create a Span and it is then up to the operation the Span represents to
+// properly end the Span when the operation itself ends.
+//
+// Warning: Methods may be added to this interface in minor releases. See
+// package documentation on API implementation for information on how to set
+// default behavior for unimplemented methods.
+type Span interface {
+	// Users of the interface can ignore this. This embedded type is only used
+	// by implementations of this interface. See the "API Implementations"
+	// section of the package documentation for more information.
+	embedded.Span
+
+	// End completes the Span. The Span is considered complete and ready to be
+	// delivered through the rest of the telemetry pipeline after this method
+	// is called. Therefore, updates to the Span are not allowed after this
+	// method has been called.
+	End(options ...SpanEndOption)
+
+	// AddEvent adds an event with the provided name and options.
+	AddEvent(name string, options ...EventOption)
+
+	// AddLink adds a link.
+	// Adding links at span creation using WithLinks is preferred to calling AddLink
+	// later, for contexts that are available during span creation, because head
+	// sampling decisions can only consider information present during span creation.
+	AddLink(link Link)
+
+	// IsRecording returns the recording state of the Span. It will return
+	// true if the Span is active and events can be recorded.
+	IsRecording() bool
+
+	// RecordError will record err as an exception span event for this span. An
+	// additional call to SetStatus is required if the Status of the Span should
+	// be set to Error, as this method does not change the Span status. If this
+	// span is not being recorded or err is nil then this method does nothing.
+	RecordError(err error, options ...EventOption)
+
+	// SpanContext returns the SpanContext of the Span. The returned SpanContext
+	// is usable even after the End method has been called for the Span.
+	SpanContext() SpanContext
+
+	// SetStatus sets the status of the Span in the form of a code and a
+	// description, provided the status hasn't already been set to a higher
+	// value before (OK > Error > Unset). The description is only included in a
+	// status when the code is for an error.
+	SetStatus(code codes.Code, description string)
+
+	// SetName sets the Span name.
+	SetName(name string)
+
+	// SetAttributes sets kv as attributes of the Span. If a key from kv
+	// already exists for an attribute of the Span it will be overwritten with
+	// the value contained in kv.
+	SetAttributes(kv ...attribute.KeyValue)
+
+	// TracerProvider returns a TracerProvider that can be used to generate
+	// additional Spans on the same telemetry pipeline as the current Span.
+	TracerProvider() TracerProvider
+}
+
+// Link is the relationship between two Spans. The relationship can be within
+// the same Trace or across different Traces.
+//
+// For example, a Link is used in the following situations:
+//
+//  1. Batch Processing: A batch of operations may contain operations
+//     associated with one or more traces/spans. Since there can only be one
+//     parent SpanContext, a Link is used to keep reference to the
+//     SpanContext of all operations in the batch.
+//  2. Public Endpoint: A SpanContext for an in incoming client request on a
+//     public endpoint should be considered untrusted. In such a case, a new
+//     trace with its own identity and sampling decision needs to be created,
+//     but this new trace needs to be related to the original trace in some
+//     form. A Link is used to keep reference to the original SpanContext and
+//     track the relationship.
+type Link struct {
+	// SpanContext of the linked Span.
+	SpanContext SpanContext
+
+	// Attributes describe the aspects of the link.
+	Attributes []attribute.KeyValue
+}
+
+// LinkFromContext returns a link encapsulating the SpanContext in the provided
+// ctx.
+func LinkFromContext(ctx context.Context, attrs ...attribute.KeyValue) Link {
+	return Link{
+		SpanContext: SpanContextFromContext(ctx),
+		Attributes:  attrs,
+	}
+}
+
+// SpanKind is the role a Span plays in a Trace.
+type SpanKind int
+
+// As a convenience, these match the proto definition, see
+// https://github.com/open-telemetry/opentelemetry-proto/blob/30d237e1ff3ab7aa50e0922b5bebdd93505090af/opentelemetry/proto/trace/v1/trace.proto#L101-L129
+//
+// The unspecified value is not a valid `SpanKind`. Use `ValidateSpanKind()`
+// to coerce a span kind to a valid value.
+const (
+	// SpanKindUnspecified is an unspecified SpanKind and is not a valid
+	// SpanKind. SpanKindUnspecified should be replaced with SpanKindInternal
+	// if it is received.
+	SpanKindUnspecified SpanKind = 0
+	// SpanKindInternal is a SpanKind for a Span that represents an internal
+	// operation within an application.
+	SpanKindInternal SpanKind = 1
+	// SpanKindServer is a SpanKind for a Span that represents the operation
+	// of handling a request from a client.
+	SpanKindServer SpanKind = 2
+	// SpanKindClient is a SpanKind for a Span that represents the operation
+	// of client making a request to a server.
+	SpanKindClient SpanKind = 3
+	// SpanKindProducer is a SpanKind for a Span that represents the operation
+	// of a producer sending a message to a message broker. Unlike
+	// SpanKindClient and SpanKindServer, there is often no direct
+	// relationship between this kind of Span and a SpanKindConsumer kind. A
+	// SpanKindProducer Span will end once the message is accepted by the
+	// message broker which might not overlap with the processing of that
+	// message.
+	SpanKindProducer SpanKind = 4
+	// SpanKindConsumer is a SpanKind for a Span that represents the operation
+	// of a consumer receiving a message from a message broker. Like
+	// SpanKindProducer Spans, there is often no direct relationship between
+	// this Span and the Span that produced the message.
+	SpanKindConsumer SpanKind = 5
+)
+
+// ValidateSpanKind returns a valid span kind value.  This will coerce
+// invalid values into the default value, SpanKindInternal.
+func ValidateSpanKind(spanKind SpanKind) SpanKind {
+	switch spanKind {
+	case SpanKindInternal,
+		SpanKindServer,
+		SpanKindClient,
+		SpanKindProducer,
+		SpanKindConsumer:
+		// valid
+		return spanKind
+	default:
+		return SpanKindInternal
+	}
+}
+
+// String returns the specified name of the SpanKind in lower-case.
+func (sk SpanKind) String() string {
+	switch sk {
+	case SpanKindInternal:
+		return "internal"
+	case SpanKindServer:
+		return "server"
+	case SpanKindClient:
+		return "client"
+	case SpanKindProducer:
+		return "producer"
+	case SpanKindConsumer:
+		return "consumer"
+	default:
+		return "unspecified"
+	}
+}

trace/span_test.go

@@ -0,0 +1,101 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package trace
+
+import (
+	"context"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/stretchr/testify/assert"
+
+	"go.opentelemetry.io/otel/attribute"
+)
+
+func TestValidateSpanKind(t *testing.T) {
+	tests := []struct {
+		in   SpanKind
+		want SpanKind
+	}{
+		{
+			SpanKindUnspecified,
+			SpanKindInternal,
+		},
+		{
+			SpanKindInternal,
+			SpanKindInternal,
+		},
+		{
+			SpanKindServer,
+			SpanKindServer,
+		},
+		{
+			SpanKindClient,
+			SpanKindClient,
+		},
+		{
+			SpanKindProducer,
+			SpanKindProducer,
+		},
+		{
+			SpanKindConsumer,
+			SpanKindConsumer,
+		},
+	}
+	for _, test := range tests {
+		if got := ValidateSpanKind(test.in); got != test.want {
+			t.Errorf("ValidateSpanKind(%#v) = %#v, want %#v", test.in, got, test.want)
+		}
+	}
+}
+
+func TestSpanKindString(t *testing.T) {
+	tests := []struct {
+		in   SpanKind
+		want string
+	}{
+		{
+			SpanKindUnspecified,
+			"unspecified",
+		},
+		{
+			SpanKindInternal,
+			"internal",
+		},
+		{
+			SpanKindServer,
+			"server",
+		},
+		{
+			SpanKindClient,
+			"client",
+		},
+		{
+			SpanKindProducer,
+			"producer",
+		},
+		{
+			SpanKindConsumer,
+			"consumer",
+		},
+	}
+	for _, test := range tests {
+		if got := test.in.String(); got != test.want {
+			t.Errorf("%#v.String() = %#v, want %#v", test.in, got, test.want)
+		}
+	}
+}
+
+func TestLinkFromContext(t *testing.T) {
+	k1v1 := attribute.String("key1", "value1")
+	spanCtx := SpanContext{traceID: TraceID([16]byte{1}), remote: true}
+
+	receiverCtx := ContextWithRemoteSpanContext(context.Background(), spanCtx)
+	link := LinkFromContext(receiverCtx, k1v1)
+
+	if !assertSpanContextEqual(link.SpanContext, spanCtx) {
+		t.Fatalf("LinkFromContext: Unexpected context created: %s", cmp.Diff(link.SpanContext, spanCtx))
+	}
+	assert.Equal(t, link.Attributes[0], k1v1)
+}