Create rule S6810: Async methods should return void or Future (#3243)

rules/S6810/java/metadata.json

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+{
+  "title": "Async methods should return void or Future",
+  "type": "BUG",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-6810",
+  "sqKey": "S6810",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "MEDIUM"
+    },
+    "attribute": "CONVENTIONAL"
+  }
+}

rules/S6810/java/rule.adoc

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+== Why is this an issue?
+
+The Spring framework provides the annotation `Async` to mark a method (or all methods of a type) as a candidate for asynchronous execution.
+
+Asynchronous methods do not necessarily, by their nature, return the result of their calculation immediately.
+Hence, it is unexpected and in clear breach of the `Async` contract for such methods to have a return type that is neither `void` nor a `Future` type.
+
+== How to fix it
+
+Use `void` as the return type if the method is not expected to return a result.
+Otherwise, a `Future` should be returned, allowing the caller to retrieve the result once it is ready.
+It is permitted to return more specific subtypes that inherit from `Future`.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,java,diff-id=1,diff-type=noncompliant]
+----
+@Async
+public String asyncMethod() {
+  ...
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=1,diff-type=compliant]
+----
+@Async
+public Future<String> asyncMethod() {
+  ...
+}
+----
+
+Alternatively, if the method does not need to return a result:
+
+[source,java]
+----
+@Async
+public void asyncMethod() {
+  ...
+}
+----
+
+== Resources
+=== Documentation
+* Spring Framework Documentation - https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/scheduling/annotation/Async.html[Annotation Interface Async]

rules/S6810/metadata.json

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+{
+}