// Copyright The OpenTelemetry Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package internal

/*
This file contains the forwarding implementation of the trace.Provider used as
the default global instance. Prior to initialization of an SDK, Tracers
returned by the global Provider will provide no-op functionality. This means
that all Span created prior to initialization are no-op Spans.

Once an SDK has been initialized, all provided no-op Tracers are swapped for
Tracers provided by the SDK defined Provider. However, any Span started prior
to this initialization does not change its behavior. Meaning, the Span remains
a no-op Span.

The implementation to track and swap Tracers locks all new Tracer creation
until the swap is complete. This assumes that this operation is not
performance-critical. If that assumption is incorrect, be sure to configure an
SDK prior to any Tracer creation.
*/

import (
	"context"
	"sync"

	"go.opentelemetry.io/otel/api/trace"
)

// traceProvider is a placeholder for a configured SDK Provider.
//
// All Provider functionality is forwarded to a delegate once configured.
type traceProvider struct {
	mtx     sync.Mutex
	tracers []*tracer

	delegate trace.Provider
}

// Compile-time guarantee that traceProvider implements the trace.Provider interface.
var _ trace.Provider = &traceProvider{}

// setDelegate configures p to delegate all Provider functionality to provider.
//
// All Tracers provided prior to this function call are switched out to be
// Tracers provided by provider.
//
// Delegation only happens on the first call to this method. All subsequent
// calls result in no delegation changes.
func (p *traceProvider) setDelegate(provider trace.Provider) {
	if p.delegate != nil {
		return
	}

	p.mtx.Lock()
	defer p.mtx.Unlock()

	p.delegate = provider
	for _, t := range p.tracers {
		t.setDelegate(provider)
	}

	p.tracers = nil
}

// Tracer implements trace.Provider.
func (p *traceProvider) Tracer(name string, opts ...trace.TracerOption) trace.Tracer {
	p.mtx.Lock()
	defer p.mtx.Unlock()

	if p.delegate != nil {
		return p.delegate.Tracer(name)
	}

	t := &tracer{name: name, opts: opts}
	p.tracers = append(p.tracers, t)
	return t
}

// tracer is a placeholder for a trace.Tracer.
//
// All Tracer functionality is forwarded to a delegate once configured.
// Otherwise, all functionality is forwarded to a NoopTracer.
type tracer struct {
	once sync.Once
	name string
	opts []trace.TracerOption

	delegate trace.Tracer
}

// Compile-time guarantee that tracer implements the trace.Tracer interface.
var _ trace.Tracer = &tracer{}

// setDelegate configures t to delegate all Tracer functionality to Tracers
// created by provider.
//
// All subsequent calls to the Tracer methods will be passed to the delegate.
//
// Delegation only happens on the first call to this method. All subsequent
// calls result in no delegation changes.
func (t *tracer) setDelegate(provider trace.Provider) {
	t.once.Do(func() { t.delegate = provider.Tracer(t.name, t.opts...) })
}

// Start implements trace.Tracer by forwarding the call to t.delegate if
// set, otherwise it forwards the call to a NoopTracer.
func (t *tracer) Start(ctx context.Context, name string, opts ...trace.StartOption) (context.Context, trace.Span) {
	if t.delegate != nil {
		return t.delegate.Start(ctx, name, opts...)
	}
	return trace.NoopTracer{}.Start(ctx, name, opts...)
}
