Allow no SP after status-code for HTTP/1.x responses (#2247)

Motivation:

While RFC7230, section 3.1.2
(https://datatracker.ietf.org/doc/html/rfc7230#section-3.1.2) requires a
`SP` before a `reason-phrase`, even if it's an empty phrase, many
implementations don't include that `SP` character. Many clients parse a
response without trailing `SP` character with no error. Those
implementations are:
 - Apache HTTP Client
 - JDK HttpClient
 - Netty
 - Tomcat (https://bz.apache.org/bugzilla/show_bug.cgi?id=60362)
 - Go HttpClient (golang/go@d71d08a#diff-0e6945dfde3e4dcaba7cfd394a85328b8137c42e152b4486dbfb6756ad69a779R145-R146)
 - Rust HttpClient (hyperium/http#345)
 - httpd with Passenger (https://bugzilla.redhat.com/show_bug.cgi?id=1032733)

Moreover, neither HTTP/2 nor HTTP/3 support reason-phrase. For this
reasons, many HTTP/1.x implementations (Tomcat, Rust, etc.) decided to
drop including the reason-phrase in serialized response for performance
reasons. Instead, they use the reason-phrase defined by RFC for all
known status codes and use `Unknown` for non-standard status codes.

Modifications:

- `HttpObjectDecoder`: second `SP` is optional for `HttpResponseDecoder`;
- Enhance tests to verify new behavior;

Result:

ServiceTalk HttpClient parses responses with no `SP` after status code.
Example: `HTTP/1.1 200\r\n`.

servicetalk-http-netty/src/main/java/io/servicetalk/http/netty/HttpObjectDecoder.java

@@ -263,16 +263,26 @@ protected final void decode(final ChannelHandlerContext ctx, final ByteBuf buffe
                 }
 
                 final int bStart = aEnd + 1;    // Expect a single WS
-                final int bEnd;
+                int bEnd;
                 try {
                     bEnd = buffer.forEachByte(bStart, nonControlIndex - bStart + 1,
                             isDecodingRequest() ? FIND_VCHAR_END : FIND_WS);
                 } catch (IllegalCharacterException cause) {
                     throw new StacklessDecoderException(
                             "Invalid start-line: HTTP request-target contains an illegal character", cause);
                 }
-                if (bEnd < 0 || bEnd == bStart) {
-                    throw newStartLineError("second");
+                if (bEnd < 0) {
+                    if (isDecodingRequest()) {
+                        throw newStartLineError("second");
+                    } else {    // Response can be without a reason-phrase: "HTTP/1.1 200\r\n"
+                        bEnd = nonControlIndex + 1;
+                    }
+                }
+                if (bEnd == bStart) {   // Happens when there are two SP next to each other
+                    throw new DecoderException("Invalid start-line: incorrect number of components, cannot find the " +
+                            (isDecodingRequest() ? "request-target" : "status-code") + ", expected: " +
+                            (isDecodingRequest() ? "method SP request-target SP HTTP-version" :
+                                    "HTTP-version SP status-code SP reason-phrase"));
                 }
 
                 final int cStart = bEnd + 1;    // Expect a single WS
@@ -908,7 +918,7 @@ private static int findLF(final ByteBuf buffer, final int fromIndex, final int t
     }
 
     private DecoderException newStartLineError(final String place) {
-        throw new DecoderException("Invalid start-line: incorrect number of components, cannot find the " + place +
+        return new DecoderException("Invalid start-line: incorrect number of components, cannot find the " + place +
                 " SP, expected: " + (isDecodingRequest() ? "method SP request-target SP HTTP-version" :
                 "HTTP-version SP status-code SP reason-phrase"));
     }

servicetalk-http-netty/src/test/java/io/servicetalk/http/netty/HttpObjectDecoderTest.java

@@ -86,6 +86,8 @@ void tearDown() throws Exception {
 
     abstract EmbeddedChannel channelSpecException();
 
+    abstract boolean isDecodingRequest();
+
     abstract String startLine();
 
     abstract HttpMetaData assertStartLine(EmbeddedChannel channel);
@@ -255,11 +257,11 @@ private static ByteBuf content(int contentLength) {
         return wrappedBuffer(content);
     }
 
-    private EmbeddedChannel channel(boolean crlf) {
+    EmbeddedChannel channel(boolean crlf) {
         return crlf ? channel() : channelSpecException();
     }
 
-    private static String br(boolean crlf) {
+    static String br(boolean crlf) {
         return crlf ? "\r\n" : "\n";
     }
 
@@ -822,7 +824,7 @@ private void unexpectedTrailersAfterContentLength(boolean crlf) {
         // https://tools.ietf.org/html/rfc7230#section-3.3
         DecoderException e = assertThrows(DecoderException.class,
                 () -> writeMsg("TrailerStatus: good" + br + br, channel));
-        assertThat(e.getMessage(), startsWith("Invalid start-line"));
+        assertThat(e.getMessage(), startsWith(isDecodingRequest() ? "Invalid start-line" : "Invalid HTTP version"));
         assertThat(channel.inboundMessages(), is(not(empty())));
     }
 
@@ -851,7 +853,7 @@ private void smuggleZeroContentLength(boolean smuggleBeforeContentLength, boolea
                         "Smuggled: " + startLine() + br + br + "Content-Length: 0" + br :
                         "Content-Length: 0" + br + "Smuggled: " + startLine() + br + br) +
                 "Connection: keep-alive" + br + br, channel));
-        assertThat(e.getMessage(), startsWith("Invalid start-line"));
+        assertThat(e.getMessage(), startsWith(isDecodingRequest() ? "Invalid start-line" : "Invalid HTTP version"));
 
         HttpMetaData metaData = assertStartLine(channel);
         assertSingleHeaderValue(metaData.headers(), HOST, "servicetalk.io");

servicetalk-http-netty/src/test/java/io/servicetalk/http/netty/HttpRequestDecoderTest.java

@@ -68,6 +68,11 @@ EmbeddedChannel channelSpecException() {
         return channelSpecException;
     }
 
+    @Override
+    boolean isDecodingRequest() {
+        return true;
+    }
+
     @Override
     String startLine() {
         return "GET / HTTP/1.1";

servicetalk-http-netty/src/test/java/io/servicetalk/http/netty/HttpResponseDecoderTest.java

@@ -42,6 +42,8 @@
 
 import io.netty.channel.embedded.EmbeddedChannel;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 
 import java.util.ArrayDeque;
 import java.util.ArrayList;
@@ -104,6 +106,11 @@ EmbeddedChannel channelSpecException() {
         return channelSpecException;
     }
 
+    @Override
+    boolean isDecodingRequest() {
+        return false;
+    }
+
     @Override
     String startLine() {
         return "HTTP/1.1 204 No Content";
@@ -131,19 +138,14 @@ void illegalPrefaceCharacter() {
 
     @Test
     void noVersion() {
-        assertDecoderException("200 OK" + "\r\n", "Invalid start-line");
+        assertDecoderException("200 OK" + "\r\n", "Invalid HTTP version");
     }
 
     @Test
     void noStatusCode() {
         assertDecoderException("HTTP/1.1 OK" + "\r\n", "Invalid start-line");
     }
 
-    @Test
-    void noSpAfterStatusCode() {
-        assertDecoderException("HTTP/1.1 200" + "\r\n", "Invalid start-line");
-    }
-
     @Test
     void invalidStartLineOrder() {
         assertDecoderException("HTTP/1.1 OK 200" + "\r\n", "Invalid start-line");
@@ -224,6 +226,18 @@ void validStartLineWithCustomHttpVersion() {
         assertFalse(channel.finishAndReleaseAll());
     }
 
+    @ParameterizedTest(name = "statusCode={0}, crlf={1}")
+    @CsvSource({"200,true", "200,false", "299,true", "299,false"})
+    void noSpAfterStatusCodeNoReasonPhrase(int statusCode, boolean crlf) {
+        HttpResponseStatus status = HttpResponseStatus.of(statusCode, "");
+        EmbeddedChannel channel = channel(crlf);
+        String br = br(crlf);
+        writeMsg("HTTP/1.1 " + status.code() /* no SP */ + br + br, channel);
+        assertResponseLine(HTTP_1_1, status, channel);
+        assertEmptyTrailers(channel);
+        assertFalse(channel.finishAndReleaseAll());
+    }
+
     @Test
     void emptyReasonPhrase() {
         testReasonPhrase("");