package clientv3

import (
    "fmt"
    "sync"
    "time"

    v3rpc "github.com/coreos/etcd/etcdserver/api/v3rpc/rpctypes"
    pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
    mvccpb "github.com/coreos/etcd/mvcc/mvccpb"
    "golang.org/x/net/context"
    "google.golang.org/grpc"
)

const (
    EventTypeDelete = mvccpb.DELETE
    EventTypePut    = mvccpb.PUT

    closeSendErrTimeout = 250 * time.Millisecond
)

type Event mvccpb.Event

type WatchChan <-chan WatchResponse

type Watcher interface {
    // Watch watches on a key or prefix. The watched events will be returned
    // through the returned channel.
    // If the watch is slow or the required rev is compacted, the watch request
    // might be canceled from the server-side and the chan will be closed.
    // 'opts' can be: 'WithRev' and/or 'WithPrefix'.
    Watch(ctx context.Context, key string, opts ...OpOption) WatchChan

    // Close closes the watcher and cancels all watch requests.
    Close() error
}

type WatchResponse struct {
    Header pb.ResponseHeader
    Events []*Event

    // CompactRevision is the minimum revision the watcher may receive.
    CompactRevision int64

    // Canceled is used to indicate watch failure.
    // If the watch failed and the stream was about to close, before the channel is closed,
    // the channel sends a final response that has Canceled set to true with a non-nil Err().
    Canceled bool

    // Created is used to indicate the creation of the watcher.
    Created bool

    closeErr error
}

// IsCreate returns true if the event tells that the key is newly created.
func (e *Event) IsCreate() bool {
    return e.Type == EventTypePut && e.Kv.CreateRevision == e.Kv.ModRevision
}

// IsModify returns true if the event tells that a new value is put on existing key.
func (e *Event) IsModify() bool {
    return e.Type == EventTypePut && e.Kv.CreateRevision != e.Kv.ModRevision
}

// Err is the error value if this WatchResponse holds an error.
func (wr *WatchResponse) Err() error {
    switch {
    case wr.closeErr != nil:
        return v3rpc.Error(wr.closeErr)
    case wr.CompactRevision != 0:
        return v3rpc.ErrCompacted
    case wr.Canceled:
        return v3rpc.ErrFutureRev
    }
    return nil
}

// IsProgressNotify returns true if the WatchResponse is progress notification.
func (wr *WatchResponse) IsProgressNotify() bool {
    return len(wr.Events) == 0 && !wr.Canceled && !wr.Created && wr.CompactRevision == 0 && wr.Header.Revision != 0
}

// watcher implements the Watcher interface
type watcher struct {
    remote pb.WatchClient

    // mu protects the grpc streams map
    mu sync.RWMutex

    // streams holds all the active grpc streams keyed by ctx value.
    streams map[string]*watchGrpcStream
}

// watchGrpcStream tracks all watch resources attached to a single grpc stream.
type watchGrpcStream struct {
    owner  *watcher
    remote pb.WatchClient

    // ctx controls internal remote.Watch requests
    ctx context.Context
    // ctxKey is the key used when looking up this stream's context
    ctxKey string
    cancel context.CancelFunc

    // substreams holds all active watchers on this grpc stream
    substreams map[int64]*watcherStream
    // resuming holds all resuming watchers on this grpc stream
    resuming []*watcherStream

    // reqc sends a watch request from Watch() to the main goroutine
    reqc chan *watchRequest
    // respc receives data from the watch client
    respc chan *pb.WatchResponse
    // donec closes to broadcast shutdown
    donec chan struct{}
    // errc transmits errors from grpc Recv to the watch stream reconn logic
    errc chan error
    // closingc gets the watcherStream of closing watchers
    closingc chan *watcherStream

    // resumec closes to signal that all substreams should begin resuming
    resumec chan struct{}
    // closeErr is the error that closed the watch stream
    closeErr error
}

// watchRequest is issued by the subscriber to start a new watcher
type watchRequest struct {
    ctx context.Context
    key string
    end string
    rev int64
    // send created notification event if this field is true
    createdNotify bool
    // progressNotify is for progress updates
    progressNotify bool
    // filters is the list of events to filter out
    filters []pb.WatchCreateRequest_FilterType
    // get the previous key-value pair before the event happens
    prevKV bool
    // retc receives a chan WatchResponse once the watcher is established
    retc chan chan WatchResponse
}

// watcherStream represents a registered watcher
type watcherStream struct {
    // initReq is the request that initiated this request
    initReq watchRequest

    // outc publishes watch responses to subscriber
    outc chan WatchResponse
    // recvc buffers watch responses before publishing
    recvc chan *WatchResponse
    // donec closes when the watcherStream goroutine stops.
    donec chan struct{}
    // closing is set to true when stream should be scheduled to shutdown.
    closing bool
    // id is the registered watch id on the grpc stream
    id int64

    // buf holds all events received from etcd but not yet consumed by the client
    buf []*WatchResponse
}

func NewWatcher(c *Client) Watcher {
    return NewWatchFromWatchClient(pb.NewWatchClient(c.conn))
}

func NewWatchFromWatchClient(wc pb.WatchClient) Watcher {
    return &watcher{
        remote:  wc,
        streams: make(map[string]*watchGrpcStream),
    }
}

// never closes
var valCtxCh = make(chan struct{})
var zeroTime = time.Unix(0, 0)

// ctx with only the values; never Done
type valCtx struct{ context.Context }

func (vc *valCtx) Deadline() (time.Time, bool) { return zeroTime, false }
func (vc *valCtx) Done() <-chan struct{}       { return valCtxCh }
func (vc *valCtx) Err() error                  { return nil }

func (w *watcher) newWatcherGrpcStream(inctx context.Context) *watchGrpcStream {
    ctx, cancel := context.WithCancel(&valCtx{inctx})
    wgs := &watchGrpcStream{
        owner:      w,
        remote:     w.remote,
        ctx:        ctx,
        ctxKey:     fmt.Sprintf("%v", inctx),
        cancel:     cancel,
        substreams: make(map[int64]*watcherStream),

        respc:    make(chan *pb.WatchResponse),
        reqc:     make(chan *watchRequest),
        donec:    make(chan struct{}),
        errc:     make(chan error, 1),
        closingc: make(chan *watcherStream),
        resumec:  make(chan struct{}),
    }
    go wgs.run()
    return wgs
}

// Watch posts a watch request to run() and waits for a new watcher channel
func (w *watcher) Watch(ctx context.Context, key string, opts ...OpOption) WatchChan {
    ow := opWatch(key, opts...)

    var filters []pb.WatchCreateRequest_FilterType
    if ow.filterPut {
        filters = append(filters, pb.WatchCreateRequest_NOPUT)
    }
    if ow.filterDelete {
        filters = append(filters, pb.WatchCreateRequest_NODELETE)
    }

    wr := &watchRequest{
        ctx:            ctx,
        createdNotify:  ow.createdNotify,
        key:            string(ow.key),
        end:            string(ow.end),
        rev:            ow.rev,
        progressNotify: ow.progressNotify,
        filters:        filters,
        prevKV:         ow.prevKV,
        retc:           make(chan chan WatchResponse, 1),
    }

    ok := false
    ctxKey := fmt.Sprintf("%v", ctx)

    // find or allocate appropriate grpc watch stream
    w.mu.Lock()
    if w.streams == nil {
        // closed
        w.mu.Unlock()
        ch := make(chan WatchResponse)
        close(ch)
        return ch
    }
    wgs := w.streams[ctxKey]
    if wgs == nil {
        wgs = w.newWatcherGrpcStream(ctx)
        w.streams[ctxKey] = wgs
    }
    donec := wgs.donec
    reqc := wgs.reqc
    w.mu.Unlock()

    // couldn't create channel; return closed channel
    closeCh := make(chan WatchResponse, 1)

    // submit request
    select {
    case reqc <- wr:
        ok = true
    case <-wr.ctx.Done():
    case <-donec:
        if wgs.closeErr != nil {
            closeCh <- WatchResponse{closeErr: wgs.closeErr}
            break
        }
        // retry; may have dropped stream from no ctxs
        return w.Watch(ctx, key, opts...)
    }

    // receive channel
    if ok {
        select {
        case ret := <-wr.retc:
            return ret
        case <-ctx.Done():
        case <-donec:
            if wgs.closeErr != nil {
                closeCh <- WatchResponse{closeErr: wgs.closeErr}
                break
            }
            // retry; may have dropped stream from no ctxs
            return w.Watch(ctx, key, opts...)
        }
    }

    close(closeCh)
    return closeCh
}

func (w *watcher) Close() (err error) {
    w.mu.Lock()
    streams := w.streams
    w.streams = nil
    w.mu.Unlock()
    for _, wgs := range streams {
        if werr := wgs.Close(); werr != nil {
            err = werr
        }
    }
    return err
}

func (w *watchGrpcStream) Close() (err error) {
    w.cancel()
    <-w.donec
    select {
    case err = <-w.errc:
    default:
    }
    return toErr(w.ctx, err)
}

func (w *watcher) closeStream(wgs *watchGrpcStream) {
    w.mu.Lock()
    close(wgs.donec)
    wgs.cancel()
    if w.streams != nil {
        delete(w.streams, wgs.ctxKey)
    }
    w.mu.Unlock()
}

func (w *watchGrpcStream) addSubstream(resp *pb.WatchResponse, ws *watcherStream) {
    if resp.WatchId == -1 {
        // failed; no channel
        close(ws.recvc)
        return
    }
    ws.id = resp.WatchId
    w.substreams[ws.id] = ws
}

func (w *watchGrpcStream) sendCloseSubstream(ws *watcherStream, resp *WatchResponse) {
    select {
    case ws.outc <- *resp:
    case <-ws.initReq.ctx.Done():
    case <-time.After(closeSendErrTimeout):
    }
    close(ws.outc)
}

func (w *watchGrpcStream) closeSubstream(ws *watcherStream) {
    // send channel response in case stream was never established
    select {
    case ws.initReq.retc <- ws.outc:
    default:
    }
    // close subscriber's channel
    if closeErr := w.closeErr; closeErr != nil && ws.initReq.ctx.Err() == nil {
        go w.sendCloseSubstream(ws, &WatchResponse{closeErr: w.closeErr})
    } else if ws.outc != nil {
        close(ws.outc)
    }
    if ws.id != -1 {
        delete(w.substreams, ws.id)
        return
    }
    for i := range w.resuming {
        if w.resuming[i] == ws {
            w.resuming[i] = nil
            return
        }
    }
}

// run is the root of the goroutines for managing a watcher client
func (w *watchGrpcStream) run() {
    var wc pb.Watch_WatchClient
    var closeErr error

    // substreams marked to close but goroutine still running; needed for
    // avoiding double-closing recvc on grpc stream teardown
    closing := make(map[*watcherStream]struct{})

    defer func() {
        w.closeErr = closeErr
        // shutdown substreams and resuming substreams
        for _, ws := range w.substreams {
            if _, ok := closing[ws]; !ok {
                close(ws.recvc)
                closing[ws] = struct{}{}
            }
        }
        for _, ws := range w.resuming {
            if _, ok := closing[ws]; ws != nil && !ok {
                close(ws.recvc)
                closing[ws] = struct{}{}
            }
        }
        w.joinSubstreams()
        for range closing {
            w.closeSubstream(<-w.closingc)
        }

        w.owner.closeStream(w)
    }()

    // start a stream with the etcd grpc server
    if wc, closeErr = w.newWatchClient(); closeErr != nil {
        return
    }

    cancelSet := make(map[int64]struct{})

    for {
        select {
        // Watch() requested
        case wreq := <-w.reqc:
            outc := make(chan WatchResponse, 1)
            ws := &watcherStream{
                initReq: *wreq,
                id:      -1,
                outc:    outc,
                // unbufffered so resumes won't cause repeat events
                recvc: make(chan *WatchResponse),
            }

            ws.donec = make(chan struct{})
            go w.serveSubstream(ws, w.resumec)

            // queue up for watcher creation/resume
            w.resuming = append(w.resuming, ws)
            if len(w.resuming) == 1 {
                // head of resume queue, can register a new watcher
                wc.Send(ws.initReq.toPB())
            }
        // New events from the watch client
        case pbresp := <-w.respc:
            switch {
            case pbresp.Created:
                // response to head of queue creation
                if ws := w.resuming[0]; ws != nil {
                    w.addSubstream(pbresp, ws)
                    w.dispatchEvent(pbresp)
                    w.resuming[0] = nil
                }
                if ws := w.nextResume(); ws != nil {
                    wc.Send(ws.initReq.toPB())
                }
            case pbresp.Canceled:
                delete(cancelSet, pbresp.WatchId)
                if ws, ok := w.substreams[pbresp.WatchId]; ok {
                    // signal to stream goroutine to update closingc
                    close(ws.recvc)
                    closing[ws] = struct{}{}
                }
            default:
                // dispatch to appropriate watch stream
                if ok := w.dispatchEvent(pbresp); ok {
                    break
                }
                // watch response on unexpected watch id; cancel id
                if _, ok := cancelSet[pbresp.WatchId]; ok {
                    break
                }
                cancelSet[pbresp.WatchId] = struct{}{}
                cr := &pb.WatchRequest_CancelRequest{
                    CancelRequest: &pb.WatchCancelRequest{
                        WatchId: pbresp.WatchId,
                    },
                }
                req := &pb.WatchRequest{RequestUnion: cr}
                wc.Send(req)
            }
        // watch client failed to recv; spawn another if possible
        case err := <-w.errc:
            if isHaltErr(w.ctx, err) || toErr(w.ctx, err) == v3rpc.ErrNoLeader {
                closeErr = err
                return
            }
            if wc, closeErr = w.newWatchClient(); closeErr != nil {
                return
            }
            if ws := w.nextResume(); ws != nil {
                wc.Send(ws.initReq.toPB())
            }
            cancelSet = make(map[int64]struct{})
        case <-w.ctx.Done():
            return
        case ws := <-w.closingc:
            w.closeSubstream(ws)
            delete(closing, ws)
            if len(w.substreams)+len(w.resuming) == 0 {
                // no more watchers on this stream, shutdown
                return
            }
        }
    }
}

// nextResume chooses the next resuming to register with the grpc stream. Abandoned
// streams are marked as nil in the queue since the head must wait for its inflight registration.
func (w *watchGrpcStream) nextResume() *watcherStream {
    for len(w.resuming) != 0 {
        if w.resuming[0] != nil {
            return w.resuming[0]
        }
        w.resuming = w.resuming[1:len(w.resuming)]
    }
    return nil
}

// dispatchEvent sends a WatchResponse to the appropriate watcher stream
func (w *watchGrpcStream) dispatchEvent(pbresp *pb.WatchResponse) bool {
    ws, ok := w.substreams[pbresp.WatchId]
    if !ok {
        return false
    }
    events := make([]*Event, len(pbresp.Events))
    for i, ev := range pbresp.Events {
        events[i] = (*Event)(ev)
    }
    wr := &WatchResponse{
        Header:          *pbresp.Header,
        Events:          events,
        CompactRevision: pbresp.CompactRevision,
        Created:         pbresp.Created,
        Canceled:        pbresp.Canceled,
    }
    select {
    case ws.recvc <- wr:
    case <-ws.donec:
        return false
    }
    return true
}

// serveWatchClient forwards messages from the grpc stream to run()
func (w *watchGrpcStream) serveWatchClient(wc pb.Watch_WatchClient) {
    for {
        resp, err := wc.Recv()
        if err != nil {
            select {
            case w.errc <- err:
            case <-w.donec:
            }
            return
        }
        select {
        case w.respc <- resp:
        case <-w.donec:
            return
        }
    }
}

// serveSubstream forwards watch responses from run() to the subscriber
func (w *watchGrpcStream) serveSubstream(ws *watcherStream, resumec chan struct{}) {
    if ws.closing {
        panic("created substream goroutine but substream is closing")
    }

    // nextRev is the minimum expected next revision
    nextRev := ws.initReq.rev
    resuming := false
    defer func() {
        if !resuming {
            ws.closing = true
        }
        close(ws.donec)
        if !resuming {
            w.closingc <- ws
        }
    }()

    emptyWr := &WatchResponse{}
    for {
        curWr := emptyWr
        outc := ws.outc

        if len(ws.buf) > 0 {
            curWr = ws.buf[0]
        } else {
            outc = nil
        }
        select {
        case outc <- *curWr:
            if ws.buf[0].Err() != nil {
                return
            }
            ws.buf[0] = nil
            ws.buf = ws.buf[1:]
        case wr, ok := <-ws.recvc:
            if !ok {
                // shutdown from closeSubstream
                return
            }

            if wr.Created {
                if ws.initReq.retc != nil {
                    ws.initReq.retc <- ws.outc
                    // to prevent next write from taking the slot in buffered channel
                    // and posting duplicate create events
                    ws.initReq.retc = nil

                    // send first creation event only if requested
                    if ws.initReq.createdNotify {
                        ws.outc <- *wr
                    }
                }
            }

            nextRev = wr.Header.Revision
            if len(wr.Events) > 0 {
                nextRev = wr.Events[len(wr.Events)-1].Kv.ModRevision + 1
            }
            ws.initReq.rev = nextRev

            // created event is already sent above,
            // watcher should not post duplicate events
            if wr.Created {
                continue
            }

            // TODO pause channel if buffer gets too large
            ws.buf = append(ws.buf, wr)
        case <-w.ctx.Done():
            return
        case <-ws.initReq.ctx.Done():
            return
        case <-resumec:
            resuming = true
            return
        }
    }
    // lazily send cancel message if events on missing id
}

func (w *watchGrpcStream) newWatchClient() (pb.Watch_WatchClient, error) {
    // mark all substreams as resuming
    close(w.resumec)
    w.resumec = make(chan struct{})
    w.joinSubstreams()
    for _, ws := range w.substreams {
        ws.id = -1
        w.resuming = append(w.resuming, ws)
    }
    // strip out nils, if any
    var resuming []*watcherStream
    for _, ws := range w.resuming {
        if ws != nil {
            resuming = append(resuming, ws)
        }
    }
    w.resuming = resuming
    w.substreams = make(map[int64]*watcherStream)

    // connect to grpc stream while accepting watcher cancelation
    stopc := make(chan struct{})
    donec := w.waitCancelSubstreams(stopc)
    wc, err := w.openWatchClient()
    close(stopc)
    <-donec

    // serve all non-closing streams, even if there's a client error
    // so that the teardown path can shutdown the streams as expected.
    for _, ws := range w.resuming {
        if ws.closing {
            continue
        }
        ws.donec = make(chan struct{})
        go w.serveSubstream(ws, w.resumec)
    }

    if err != nil {
        return nil, v3rpc.Error(err)
    }

    // receive data from new grpc stream
    go w.serveWatchClient(wc)
    return wc, nil
}

func (w *watchGrpcStream) waitCancelSubstreams(stopc <-chan struct{}) <-chan struct{} {
    var wg sync.WaitGroup
    wg.Add(len(w.resuming))
    donec := make(chan struct{})
    for i := range w.resuming {
        go func(ws *watcherStream) {
            defer wg.Done()
            if ws.closing {
                return
            }
            select {
            case <-ws.initReq.ctx.Done():
                // closed ws will be removed from resuming
                ws.closing = true
                close(ws.outc)
                ws.outc = nil
                go func() { w.closingc <- ws }()
            case <-stopc:
            }
        }(w.resuming[i])
    }
    go func() {
        defer close(donec)
        wg.Wait()
    }()
    return donec
}

// joinSubstream waits for all substream goroutines to complete
func (w *watchGrpcStream) joinSubstreams() {
    for _, ws := range w.substreams {
        <-ws.donec
    }
    for _, ws := range w.resuming {
        if ws != nil {
            <-ws.donec
        }
    }
}

// openWatchClient retries opening a watchclient until retryConnection fails
func (w *watchGrpcStream) openWatchClient() (ws pb.Watch_WatchClient, err error) {
    for {
        select {
        case <-w.ctx.Done():
            if err == nil {
                return nil, w.ctx.Err()
            }
            return nil, err
        default:
        }
        if ws, err = w.remote.Watch(w.ctx, grpc.FailFast(false)); ws != nil && err == nil {
            break
        }
        if isHaltErr(w.ctx, err) {
            return nil, v3rpc.Error(err)
        }
    }
    return ws, nil
}

// toPB converts an internal watch request structure to its protobuf messagefunc (wr *watchRequest)
func (wr *watchRequest) toPB() *pb.WatchRequest {
    req := &pb.WatchCreateRequest{
        StartRevision:  wr.rev,
        Key:            []byte(wr.key),
        RangeEnd:       []byte(wr.end),
        ProgressNotify: wr.progressNotify,
        Filters:        wr.filters,
        PrevKv:         wr.prevKV,
    }
    cr := &pb.WatchRequest_CreateRequest{CreateRequest: req}
    return &pb.WatchRequest{RequestUnion: cr}
}

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