PLT-7407: Back-end plugin mechanism (#7177)

* begin backend plugin wip

* flesh out rpcplugin. everything done except for minor supervisor stubs

* done with basic plugin infrastructure

* simplify tests

* remove unused test lines

1 files changed, 253 insertions, 0 deletions

diff --git a/plugin/rpcplugin/muxer.go b/plugin/rpcplugin/muxer.go
new file mode 100644
index 000000000..a2bfbf8b6
--- /dev/null
+++ b/plugin/rpcplugin/muxer.go
@@ -0,0 +1,253 @@
+package rpcplugin
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"sync"
+	"sync/atomic"
+)
+
+// Muxer allows multiple bidirectional streams to be transmitted over a single connection.
+//
+// Muxer is safe for use by multiple goroutines.
+//
+// Streams opened on the muxer must be periodically drained in order to reclaim read buffer memory.
+// In other words, readers must consume incoming data as it comes in.
+type Muxer struct {
+	// writeMutex guards conn writes
+	writeMutex sync.Mutex
+	conn       io.ReadWriteCloser
+
+	// didCloseConn is a boolean (0 or 1) used from multiple goroutines via atomic operations
+	didCloseConn int32
+
+	// streamsMutex guards streams and nextId
+	streamsMutex sync.Mutex
+	nextId       int64
+	streams      map[int64]*muxerStream
+
+	stream0Reader *io.PipeReader
+	stream0Writer *io.PipeWriter
+	result        chan error
+}
+
+// Creates a new Muxer.
+//
+// conn must be safe for simultaneous reads by one goroutine and writes by another.
+//
+// For two muxers communicating with each other via a connection, parity must be true for exactly
+// one of them.
+func NewMuxer(conn io.ReadWriteCloser, parity bool) *Muxer {
+	s0r, s0w := io.Pipe()
+	muxer := &Muxer{
+		conn:          conn,
+		streams:       make(map[int64]*muxerStream),
+		result:        make(chan error, 1),
+		nextId:        1,
+		stream0Reader: s0r,
+		stream0Writer: s0w,
+	}
+	if parity {
+		muxer.nextId = 2
+	}
+	go muxer.run()
+	return muxer
+}
+
+// Opens a new stream with a unique id.
+//
+// Writes made to the stream before the other end calls Connect will be discarded.
+func (m *Muxer) Serve() (int64, io.ReadWriteCloser) {
+	m.streamsMutex.Lock()
+	id := m.nextId
+	m.nextId += 2
+	m.streamsMutex.Unlock()
+	return id, m.Connect(id)
+}
+
+// Opens a remotely opened stream.
+func (m *Muxer) Connect(id int64) io.ReadWriteCloser {
+	m.streamsMutex.Lock()
+	defer m.streamsMutex.Unlock()
+	mutex := &sync.Mutex{}
+	stream := &muxerStream{
+		id:       id,
+		muxer:    m,
+		mutex:    mutex,
+		readWake: sync.NewCond(mutex),
+	}
+	m.streams[id] = stream
+	return stream
+}
+
+// Calling Read on the muxer directly performs a read on a dedicated, always-open channel.
+func (m *Muxer) Read(p []byte) (int, error) {
+	return m.stream0Reader.Read(p)
+}
+
+// Calling Write on the muxer directly performs a write on a dedicated, always-open channel.
+func (m *Muxer) Write(p []byte) (int, error) {
+	return m.write(p, 0)
+}
+
+// Closes the muxer.
+func (m *Muxer) Close() error {
+	if atomic.CompareAndSwapInt32(&m.didCloseConn, 0, 1) {
+		m.conn.Close()
+	}
+	m.stream0Reader.Close()
+	m.stream0Writer.Close()
+	<-m.result
+	return nil
+}
+
+func (m *Muxer) IsClosed() bool {
+	return atomic.LoadInt32(&m.didCloseConn) > 0
+}
+
+func (m *Muxer) write(p []byte, sid int64) (int, error) {
+	m.writeMutex.Lock()
+	defer m.writeMutex.Unlock()
+	if m.IsClosed() {
+		return 0, fmt.Errorf("muxer closed")
+	}
+	buf := make([]byte, 10)
+	n := binary.PutVarint(buf, sid)
+	if _, err := m.conn.Write(buf[:n]); err != nil {
+		m.shutdown(err)
+		return 0, err
+	}
+	n = binary.PutVarint(buf, int64(len(p)))
+	if _, err := m.conn.Write(buf[:n]); err != nil {
+		m.shutdown(err)
+		return 0, err
+	}
+	if _, err := m.conn.Write(p); err != nil {
+		m.shutdown(err)
+		return 0, err
+	}
+	return len(p), nil
+}
+
+func (m *Muxer) rm(sid int64) {
+	m.streamsMutex.Lock()
+	defer m.streamsMutex.Unlock()
+	delete(m.streams, sid)
+}
+
+func (m *Muxer) run() {
+	m.shutdown(m.loop())
+}
+
+func (m *Muxer) loop() error {
+	reader := bufio.NewReader(m.conn)
+
+	for {
+		sid, err := binary.ReadVarint(reader)
+		if err != nil {
+			return err
+		}
+		len, err := binary.ReadVarint(reader)
+		if err != nil {
+			return err
+		}
+
+		if sid == 0 {
+			if _, err := io.CopyN(m.stream0Writer, reader, len); err != nil {
+				return err
+			}
+			continue
+		}
+
+		m.streamsMutex.Lock()
+		stream, ok := m.streams[sid]
+		m.streamsMutex.Unlock()
+		if !ok {
+			if _, err := reader.Discard(int(len)); err != nil {
+				return err
+			}
+			continue
+		}
+
+		stream.mutex.Lock()
+		if stream.isClosed {
+			stream.mutex.Unlock()
+			if _, err := reader.Discard(int(len)); err != nil {
+				return err
+			}
+			continue
+		}
+		_, err = io.CopyN(&stream.readBuf, reader, len)
+		stream.mutex.Unlock()
+		if err != nil {
+			return err
+		}
+		stream.readWake.Signal()
+	}
+}
+
+func (m *Muxer) shutdown(err error) {
+	if atomic.CompareAndSwapInt32(&m.didCloseConn, 0, 1) {
+		m.conn.Close()
+	}
+	go func() {
+		m.streamsMutex.Lock()
+		for _, stream := range m.streams {
+			stream.mutex.Lock()
+			stream.readWake.Signal()
+			stream.mutex.Unlock()
+		}
+		m.streams = make(map[int64]*muxerStream)
+		m.streamsMutex.Unlock()
+	}()
+	m.result <- err
+}
+
+type muxerStream struct {
+	id       int64
+	muxer    *Muxer
+	readBuf  bytes.Buffer
+	mutex    *sync.Mutex
+	readWake *sync.Cond
+	isClosed bool
+	closeErr error
+}
+
+func (s *muxerStream) Read(p []byte) (int, error) {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	for {
+		if s.muxer.IsClosed() {
+			return 0, fmt.Errorf("muxer closed")
+		} else if s.isClosed {
+			return 0, io.EOF
+		} else if s.readBuf.Len() > 0 {
+			n, err := s.readBuf.Read(p)
+			return n, err
+		}
+		s.readWake.Wait()
+	}
+}
+
+func (s *muxerStream) Write(p []byte) (int, error) {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	if s.isClosed {
+		return 0, fmt.Errorf("stream closed")
+	}
+	return s.muxer.write(p, s.id)
+}
+
+func (s *muxerStream) Close() error {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	if !s.isClosed {
+		s.isClosed = true
+		s.muxer.rm(s.id)
+	}
+	s.readWake.Signal()
+	return nil
+}