Update the example plugin to copy the template

The example plugin should be a better copy of the template (eg use two files,
connector.go and plugin.go) so that it's clearer to people trying to build
plugins exactly how to create and build them in practice

This reorganizes the content to better mimic the templates and updates
the Dockerfile to correctly build the plugin.

test/plugin/Dockerfile

@@ -7,7 +7,7 @@ COPY . .
 RUN mkdir -p /usr/local/lib/secretless && \
     go build -buildmode=plugin \
              -o /usr/local/lib/secretless/example-plugin.so \
-                 ./example/example_plugin.go
+                 ./example/
 
 # Do not remove this - we are intentionally trying to exercise
 # limited user functionality

test/plugin/example/connector.go

@@ -0,0 +1,70 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"net"
+
+	"github.com/cyberark/secretless-broker/pkg/secretless/log"
+	"github.com/cyberark/secretless-broker/pkg/secretless/plugin/connector"
+)
+
+// SingleUseConnector creates an authenticated connection to a target TCP service.
+type SingleUseConnector struct {
+	logger log.Logger
+}
+
+// Connect is the function that implements the tcp.Connector func signature in this
+// example plugin. It has access to the client connection and the credentials (as a map),
+// and is expected to return the target service connection.
+//
+// This example connector works as follows:
+// 1. Waits for the initial message from the client
+// 2. Connect to a target service whose address is the value of the credential identified
+// by the key "address"
+// 3. Inject credentials from a credential identified by the key "auth"
+// 4. Write the initial message from the client with some modification
+func (connector *SingleUseConnector) Connect(
+	clientConn net.Conn,
+	credentialValuesByID connector.CredentialValuesByID,
+) (net.Conn, error) {
+
+	connector.logger.Debugln("Waiting for initial write from client")
+	clientInitMsg, _, err := bufio.NewReader(clientConn).ReadLine()
+	if err != nil {
+		return nil, err
+	}
+
+	connector.logger.Debugln("Dialing target service")
+	conn, err := net.Dial("tcp", string(credentialValuesByID["address"]))
+	if err != nil {
+		return nil, err
+	}
+
+	connector.logger.Debugln("Sending packet with injected credentials to target service")
+	credInjectionPacket := []byte(
+		fmt.Sprintf(
+			"credential injection: %s\n",
+			string(credentialValuesByID["auth"]),
+		),
+	)
+	_, err = conn.Write(credInjectionPacket)
+	if err != nil {
+		return nil, err
+	}
+
+	connector.logger.Debugln("Sending modified client initial packet to target service")
+	initMsgPacket := []byte(
+		fmt.Sprintf(
+			"initial message from client: %s\n",
+			string(clientInitMsg),
+		),
+	)
+	_, err = conn.Write(initMsgPacket)
+	if err != nil {
+		return nil, err
+	}
+
+	connector.logger.Debugln("Successfully connected to target service")
+	return conn, nil
+}

test/plugin/example/plugin.go

@@ -0,0 +1,47 @@
+package main
+
+import (
+	"net"
+
+	"github.com/cyberark/secretless-broker/pkg/secretless/plugin/connector"
+	"github.com/cyberark/secretless-broker/pkg/secretless/plugin/connector/tcp"
+)
+
+// NewConnector is a required method on the tcp.Plugin interface. It returns a
+// tcp.Connector.
+//
+// The single argument passed in is of type connector.Resources. It contains
+// connector-specific config and a logger.
+func NewConnector(conRes connector.Resources) tcp.Connector {
+	connectorFunc := func(
+		clientConn net.Conn,
+		credentialValuesByID connector.CredentialValuesByID,
+	) (backendConn net.Conn, err error) {
+		// singleUseConnector is responsible for generating the authenticated connection
+		// to the target service for each incoming client connection
+		singleUseConnector := &SingleUseConnector{
+			logger: conRes.Logger(),
+		}
+
+		return singleUseConnector.Connect(clientConn, credentialValuesByID)
+	}
+
+	return tcp.ConnectorFunc(connectorFunc)
+}
+
+// PluginInfo is required as part of the Secretless plugin spec. It provides
+// important metadata about the plugin.
+func PluginInfo() map[string]string {
+	return map[string]string{
+		"pluginAPIVersion": "0.1.0",
+		"type":             "connector.tcp",
+		"id":               "example-tcp-connector",
+		"description":      "Example TCP Connector Plugin",
+	}
+}
+
+// GetTCPPlugin is required as part of the Secretless plugin spec for TCP connector
+// plugins. It returns the TCP plugin.
+func GetTCPPlugin() tcp.Plugin {
+	return tcp.ConnectorConstructor(NewConnector)
+}