// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Transport code's client connection pooling.

package http2

import (
	"net/http"
	"sync"
)

// ClientConnPool manages a pool of HTTP/2 client connections.
type ClientConnPool interface {
	GetClientConn(req *http.Request, addr string) (*ClientConn, error)
	MarkDead(*ClientConn)
}

type clientConnPool struct {
	t  *Transport
	mu sync.Mutex // TODO: maybe switch to RWMutex
	// TODO: add support for sharing conns based on cert names
	// (e.g. share conn for googleapis.com and appspot.com)
	conns   map[string][]*ClientConn // key is host:port
	dialing map[string]*dialCall     // currently in-flight dials
	keys    map[*ClientConn][]string
}

func (p *clientConnPool) GetClientConn(req *http.Request, addr string) (*ClientConn, error) {
	return p.getClientConn(req, addr, true)
}

func (p *clientConnPool) getClientConn(req *http.Request, addr string, dialOnMiss bool) (*ClientConn, error) {
	p.mu.Lock()
	for _, cc := range p.conns[addr] {
		if cc.CanTakeNewRequest() {
			p.mu.Unlock()
			return cc, nil
		}
	}
	if !dialOnMiss {
		p.mu.Unlock()
		return nil, ErrNoCachedConn
	}
	call := p.getStartDialLocked(addr)
	p.mu.Unlock()
	<-call.done
	return call.res, call.err
}

// dialCall is an in-flight Transport dial call to a host.
type dialCall struct {
	p    *clientConnPool
	done chan struct{} // closed when done
	res  *ClientConn   // valid after done is closed
	err  error         // valid after done is closed
}

// requires p.mu is held.
func (p *clientConnPool) getStartDialLocked(addr string) *dialCall {
	if call, ok := p.dialing[addr]; ok {
		// A dial is already in-flight. Don't start another.
		return call
	}
	call := &dialCall{p: p, done: make(chan struct{})}
	if p.dialing == nil {
		p.dialing = make(map[string]*dialCall)
	}
	p.dialing[addr] = call
	go call.dial(addr)
	return call
}

// run in its own goroutine.
func (c *dialCall) dial(addr string) {
	c.res, c.err = c.p.t.dialClientConn(addr)
	close(c.done)

	c.p.mu.Lock()
	delete(c.p.dialing, addr)
	if c.err == nil {
		c.p.addConnLocked(addr, c.res)
	}
	c.p.mu.Unlock()
}

func (p *clientConnPool) addConn(key string, cc *ClientConn) {
	p.mu.Lock()
	p.addConnLocked(key, cc)
	p.mu.Unlock()
}

// p.mu must be held
func (p *clientConnPool) addConnLocked(key string, cc *ClientConn) {
	for _, v := range p.conns[key] {
		if v == cc {
			return
		}
	}
	if p.conns == nil {
		p.conns = make(map[string][]*ClientConn)
	}
	if p.keys == nil {
		p.keys = make(map[*ClientConn][]string)
	}
	p.conns[key] = append(p.conns[key], cc)
	p.keys[cc] = append(p.keys[cc], key)
}

func (p *clientConnPool) MarkDead(cc *ClientConn) {
	p.mu.Lock()
	defer p.mu.Unlock()
	for _, key := range p.keys[cc] {
		vv, ok := p.conns[key]
		if !ok {
			continue
		}
		newList := filterOutClientConn(vv, cc)
		if len(newList) > 0 {
			p.conns[key] = newList
		} else {
			delete(p.conns, key)
		}
	}
	delete(p.keys, cc)
}

func (p *clientConnPool) closeIdleConnections() {
	p.mu.Lock()
	defer p.mu.Unlock()
	// TODO: don't close a cc if it was just added to the pool
	// milliseconds ago and has never been used. There's currently
	// a small race window with the HTTP/1 Transport's integration
	// where it can add an idle conn just before using it, and
	// somebody else can concurrently call CloseIdleConns and
	// break some caller's RoundTrip.
	for _, vv := range p.conns {
		for _, cc := range vv {
			cc.closeIfIdle()
		}
	}
}

func filterOutClientConn(in []*ClientConn, exclude *ClientConn) []*ClientConn {
	out := in[:0]
	for _, v := range in {
		if v != exclude {
			out = append(out, v)
		}
	}
	// If we filtered it out, zero out the last item to prevent
	// the GC from seeing it.
	if len(in) != len(out) {
		in[len(in)-1] = nil
	}
	return out
}