add AUTH & EMPTY to SmtpCommand (#9999)

Motivation:
AUTH command is used to login to a SMTP server.
EMPTY command is for request with only parameter.

Modifications:
Add AUTH & EMPTY to SmtpCommand & SmtpRequests
Update SmtpRequestEncoder#writeParameters, handle SP according to
command
Add unit test

Result:
fix #9995

codec-smtp/src/main/java/io/netty/handler/codec/smtp/SmtpCommand.java

@@ -32,6 +32,7 @@ import java.util.Map;
 public final class SmtpCommand {
     public static final SmtpCommand EHLO = new SmtpCommand(AsciiString.cached("EHLO"));
     public static final SmtpCommand HELO = new SmtpCommand(AsciiString.cached("HELO"));
+    public static final SmtpCommand AUTH = new SmtpCommand(AsciiString.cached("AUTH"));
     public static final SmtpCommand MAIL = new SmtpCommand(AsciiString.cached("MAIL"));
     public static final SmtpCommand RCPT = new SmtpCommand(AsciiString.cached("RCPT"));
     public static final SmtpCommand DATA = new SmtpCommand(AsciiString.cached("DATA"));
@@ -41,11 +42,13 @@ public final class SmtpCommand {
     public static final SmtpCommand VRFY = new SmtpCommand(AsciiString.cached("VRFY"));
     public static final SmtpCommand HELP = new SmtpCommand(AsciiString.cached("HELP"));
     public static final SmtpCommand QUIT = new SmtpCommand(AsciiString.cached("QUIT"));
+    public static final SmtpCommand EMPTY = new SmtpCommand(AsciiString.cached(""));
 
     private static final Map<String, SmtpCommand> COMMANDS = new HashMap<>();
     static {
         COMMANDS.put(EHLO.name().toString(), EHLO);
         COMMANDS.put(HELO.name().toString(), HELO);
+        COMMANDS.put(AUTH.name().toString(), AUTH);
         COMMANDS.put(MAIL.name().toString(), MAIL);
         COMMANDS.put(RCPT.name().toString(), RCPT);
         COMMANDS.put(DATA.name().toString(), DATA);
@@ -55,6 +58,7 @@ public final class SmtpCommand {
         COMMANDS.put(VRFY.name().toString(), VRFY);
         COMMANDS.put(HELP.name().toString(), HELP);
         COMMANDS.put(QUIT.name().toString(), QUIT);
+        COMMANDS.put(EMPTY.name().toString(), EMPTY);
     }
 
     /**

codec-smtp/src/main/java/io/netty/handler/codec/smtp/SmtpRequestEncoder.java

@@ -58,7 +58,8 @@ public final class SmtpRequestEncoder extends MessageToMessageEncoder<Object> {
             final ByteBuf buffer = ctx.alloc().buffer();
             try {
                 req.command().encode(buffer);
-                writeParameters(req.parameters(), buffer);
+                boolean notEmpty = req.command() != SmtpCommand.EMPTY;
+                writeParameters(req.parameters(), buffer, notEmpty);
                 ByteBufUtil.writeShortBE(buffer, CRLF_SHORT);
                 out.add(buffer);
                 release = false;
@@ -85,11 +86,13 @@ public final class SmtpRequestEncoder extends MessageToMessageEncoder<Object> {
         }
     }
 
-    private static void writeParameters(List<CharSequence> parameters, ByteBuf out) {
+    private static void writeParameters(List<CharSequence> parameters, ByteBuf out, boolean commandNotEmpty) {
         if (parameters.isEmpty()) {
             return;
         }
-        out.writeByte(SP);
+        if (commandNotEmpty) {
+            out.writeByte(SP);
+        }
         if (parameters instanceof RandomAccess) {
             final int sizeMinusOne = parameters.size() - 1;
             for (int i = 0; i < sizeMinusOne; i++) {

codec-smtp/src/main/java/io/netty/handler/codec/smtp/SmtpRequests.java

@@ -51,6 +51,20 @@ public final class SmtpRequests {
         return new DefaultSmtpRequest(SmtpCommand.EHLO, hostname);
     }
 
+    /**
+     * Creates a {@code EMPTY} request.
+     */
+    public static SmtpRequest empty(CharSequence... parameter) {
+        return new DefaultSmtpRequest(SmtpCommand.EMPTY, parameter);
+    }
+
+    /**
+     * Creates a {@code AUTH} request.
+     */
+    public static SmtpRequest auth(CharSequence... parameter) {
+        return new DefaultSmtpRequest(SmtpCommand.AUTH, parameter);
+    }
+
     /**
      * Creates a {@code NOOP} request.
      */

codec-smtp/src/test/java/io/netty/handler/codec/smtp/SmtpRequestEncoderTest.java

@@ -38,6 +38,21 @@ public class SmtpRequestEncoderTest {
         testEncode(SmtpRequests.helo("localhost"), "HELO localhost\r\n");
     }
 
+    @Test
+    public void testEncodeAuth() {
+        testEncode(SmtpRequests.auth("LOGIN"), "AUTH LOGIN\r\n");
+    }
+
+    @Test
+    public void testEncodeAuthWithParameter() {
+        testEncode(SmtpRequests.auth("PLAIN", "dGVzdAB0ZXN0ADEyMzQ="), "AUTH PLAIN dGVzdAB0ZXN0ADEyMzQ=\r\n");
+    }
+
+    @Test
+    public void testEncodeEmpty() {
+        testEncode(SmtpRequests.empty("dGVzdAB0ZXN0ADEyMzQ="),  "dGVzdAB0ZXN0ADEyMzQ=\r\n");
+    }
+
     @Test
     public void testEncodeMail() {
         testEncode(SmtpRequests.mail("me@netty.io"), "MAIL FROM:<me@netty.io>\r\n");