Add checker for static final buffers (#12)

Add validation for static final byte buffers to make sure they are unreleasable and read-only as a follow up for netty/netty#11792. Also enabled support for checkstyle warning suppression using `SuppressWarnings` annotation.

Verified by installing it locally and running it against local Netty code. Example:

```
→ mvn validate
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Detecting the operating system and CPU architecture
[INFO] ------------------------------------------------------------------------
[INFO] os.detected.name: osx
[INFO] os.detected.arch: x86_64
[INFO] os.detected.bitness: 64
[INFO] os.detected.version: 10.16
[INFO] os.detected.version.major: 10
[INFO] os.detected.version.minor: 16
[INFO] os.detected.classifier: osx-x86_64
[INFO]
[INFO] ---------------------< io.netty:netty-codec-http >----------------------
[INFO] Building Netty/Codec/HTTP 4.1.70.Final-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-enforcer-plugin:1.4.1:enforce (enforce-maven) @ netty-codec-http ---
[INFO]
[INFO] --- maven-enforcer-plugin:1.4.1:enforce (enforce-tools) @ netty-codec-http ---
[INFO]
[INFO] --- maven-checkstyle-plugin:3.1.0:check (check-style) @ netty-codec-http ---
[INFO] Starting audit...
[ERROR] /Users/alex.chekunkov/Dev/github.com/alexc-db/netty/codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectEncoder.java:53: static final buffer assignment should match pattern (Unpooled\s*\.)?unreleasableBuffer\(.*?\)\s*\.asReadOnly\(\) [StaticFinalBuffer]
[ERROR] /Users/alex.chekunkov/Dev/github.com/alexc-db/netty/codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectEncoder.java:54: static final buffer assignment should match pattern (Unpooled\s*\.)?unreleasableBuffer\(.*?\)\s*\.asReadOnly\(\) [StaticFinalBuffer]
[ERROR] /Users/alex.chekunkov/Dev/github.com/alexc-db/netty/codec-http/src/main/java/io/netty/handler/codec/http/websocketx/WebSocket00FrameEncoder.java:34: static final buffer assignment should match pattern (Unpooled\s*\.)?unreleasableBuffer\(.*?\)\s*\.asReadOnly\(\) [StaticFinalBuffer]
[ERROR] /Users/alex.chekunkov/Dev/github.com/alexc-db/netty/codec-http/src/main/java/io/netty/handler/codec/http/websocketx/WebSocket00FrameEncoder.java:36: static final buffer assignment should match pattern (Unpooled\s*\.)?unreleasableBuffer\(.*?\)\s*\.asReadOnly\(\) [StaticFinalBuffer]
[ERROR] /Users/alex.chekunkov/Dev/github.com/alexc-db/netty/codec-http/src/main/java/io/netty/handler/codec/http/websocketx/WebSocket00FrameEncoder.java:38: static final buffer assignment should match pattern (Unpooled\s*\.)?unreleasableBuffer\(.*?\)\s*\.asReadOnly\(\) [StaticFinalBuffer]
Audit done.
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  3.755 s
[INFO] Finished at: 2021-10-26T20:00:56-07:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-checkstyle-plugin:3.1.0:check (check-style) on project netty-codec-http: Failed during checkstyle execution: There are 5 errors reported by Checkstyle 8.29 with io/netty/checkstyle.xml ruleset. -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
```

Findings from the check are fixed in netty/netty#11802.

common/src/main/java/io/netty/build/checkstyle/StaticFinalBufferCheck.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2021 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package io.netty.build.checkstyle;
+
+import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
+import com.puppycrawl.tools.checkstyle.api.DetailAST;
+import com.puppycrawl.tools.checkstyle.api.FileContents;
+import com.puppycrawl.tools.checkstyle.api.FullIdent;
+import com.puppycrawl.tools.checkstyle.api.TokenTypes;
+
+import java.util.regex.Pattern;
+
+/**
+ * This check verifies that static final buffers are unreleasable and read-only.
+ *
+ * It aims to prevent corruption bugs like https://github.com/netty/netty/issues/11792
+ * from happening in the future.
+ */
+public class StaticFinalBufferCheck extends AbstractCheck {
+
+    // Pattern is not multiline because variable definition is flattened and trimmed before the match.
+    private static final Pattern pattern = Pattern.compile(
+            "(Unpooled\\s*\\.)?unreleasableBuffer\\(.*?\\)\\s*\\.asReadOnly\\(\\)");
+
+    @Override
+    public int[] getRequiredTokens() {
+        return new int[]{TokenTypes.VARIABLE_DEF};
+    }
+
+    @Override
+    public int[] getDefaultTokens() {
+        return getRequiredTokens();
+    }
+
+    @Override
+    public int[] getAcceptableTokens() {
+        return getRequiredTokens();
+    }
+
+    @Override
+    public void visitToken(DetailAST ast) {
+        DetailAST modifiersAST = ast.findFirstToken(TokenTypes.MODIFIERS);
+        boolean isStatic = modifiersAST.findFirstToken(TokenTypes.LITERAL_STATIC) != null;
+        boolean isFinal = modifiersAST.findFirstToken(TokenTypes.FINAL) != null;
+        FullIdent typeIdent = FullIdent.createFullIdentBelow(ast.findFirstToken(TokenTypes.TYPE));
+        if (!isStatic || !isFinal || !typeIdent.getText().endsWith("Buf")) {
+            return;
+        }
+        DetailAST assignAST = ast.findFirstToken(TokenTypes.ASSIGN);
+        DetailAST semiAST = ast.findFirstToken(TokenTypes.SEMI);
+        if (assignAST == null || semiAST == null) {
+            log(ast.getLineNo(), "Missing assignment for static final buffer");
+            return;
+        }
+        FileContents fc = getFileContents();
+        StringBuilder sb = new StringBuilder();
+        for (int i = assignAST.getLineNo(); i <= semiAST.getLineNo(); i++) {
+            // getLineNo returns 1-based line number, getLine expects 0-based.
+            sb.append(fc.getLine(i - 1).trim());
+        }
+        if (!pattern.matcher(sb.toString()).find()) {
+            log(ast.getLineNo(), "static final buffer assignment should match pattern " + pattern);
+        }
+    }
+}

common/src/main/resources/io/netty/checkstyle.xml

@@ -55,7 +55,7 @@
   <module name="RegexpSingleline">
     <property name="format" value="\s+$"/>
     <property name="message" value="trailing whitespace"/>
-  </module> 
+  </module>
   <!-- Prohibit consecutive empty lines (except the lines after package/import) -->
   <module name="RegexpMultiline">
     <property name="format" value="\n *(?!package )(?!import )[^\n]+\n{3,}"/>
@@ -75,8 +75,12 @@
     <property name="max" value="120"/>
     <property name="fileExtensions" value="java"/>
   </module>
+  <module name="SuppressWarningsFilter" />
 
   <module name="TreeWalker">
+    <module name="SuppressWarningsHolder" />
+    <!-- Make sure final static buffers are unreleasable and read-only. -->
+    <module name="io.netty.build.checkstyle.StaticFinalBufferCheck"/>
     <module name="WhitespaceAfter"/>
     <module name="WhitespaceAround">
       <property name="tokens" value="ASSIGN, BAND, BAND_ASSIGN, BOR, BOR_ASSIGN, BSR, BSR_ASSIGN, BXOR, BXOR_ASSIGN, DIV, DIV_ASSIGN, EQUAL, GE, GT, LAND, LCURLY, LE, LITERAL_ASSERT, LITERAL_CATCH, LITERAL_DO, LITERAL_ELSE, LITERAL_FINALLY, LITERAL_FOR, LITERAL_IF, LITERAL_RETURN, LITERAL_SYNCHRONIZED, LITERAL_TRY, LITERAL_WHILE, LOR, LT, MINUS, MINUS_ASSIGN, MOD, MOD_ASSIGN, NOT_EQUAL, PLUS, PLUS_ASSIGN, RCURLY, SL, SLIST, SL_ASSIGN, SR, SR_ASSIGN, STAR, STAR_ASSIGN, TYPE_EXTENSION_AND"/>