Create rule S6901: setDaemon, setPriority and getThreadGroup should n…

…ot be invoked on virtual threads (#3592)

rules/S6901/java/metadata.json

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+{
+  "title": "\"setDaemon\", \"setPriority\" and \"getThreadGroup\" should not be invoked on virtual threads",
+  "type": "BUG",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "java21", "bug"
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-6901",
+  "sqKey": "S6901",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "MEDIUM"
+    },
+    "attribute": "CONVENTIONAL"
+  }
+}

rules/S6901/java/rule.adoc

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+== Why is this an issue?
+
+The `Thread` class has some methods that are used to monitor and manage its execution.
+With the introduction of virtual threads in Java 21, there are three of these methods that behave differently
+between the standard platform threads and the virtual ones.
+
+For virtual threads:
+
+* `Thread.setDaemon(boolean)` will throw an `IllegalArgumentException` if `false` is passed as an argument as a virtual thread daemon status is always true.
+* `Thread.setPriority(int priority)` will never change the actual priority of a virtual thread, which is always equal to `Thread.NORM_PRIORITY`
+* `Thread.getThreadGroup()` will return a dummy "VirtualThreads" group that is empty and should not be used
+
+This rule reports an issue when one of these methods is invoked on a virtual thread.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,java]
+----
+Thread t = Thread.ofVirtual().unstarted(()->{/* some task */});
+t.setPriority(1); // Noncompliant; virtual threads' priority cannot be changed
+t.setDaemon(false); // Noncompliant; will throw IllegalArgumentException
+t.setDaemon(true); // Noncompliant; redundant
+t.start();
+var threadGroup = t.getThreadGroup(); // Noncompliant; virtual thread groups should not be used
+----
+
+== Resources
+
+=== Documentation
+
+* Java Documentation - https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Thread.html#setDaemon(boolean)[Thread.setDaemon]
+* Java Documentation - https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Thread.html#setPriority(int)[Thread.setPriority]
+* Java Documentation - https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Thread.html#getThreadGroup()[Thread.getThreadGroup]
+* Java Documentation - https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/ThreadGroup.html#virtualthreadgroup[Virtual threads group]

rules/S6901/metadata.json

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+{
+}