initial sketch for process manager

src/main/java/JobServ/ProcessController.java

@@ -43,6 +43,14 @@ class ProcessController {
         this.outputScanner.useDelimieter("\\A");
     }
 
+    /*
+     * getPid()
+     * returns translated pid of this process
+     */
+    public int getPid() {
+        return this.pid;
+    }
+
     /*
      * getStatus()
      * returns whether or not the process is running
@@ -77,8 +85,7 @@ class ProcessController {
 
     /*
      * getOutput()
-     * gets new output from stream
-     * (TODO: investigate whether this would better be done by )
+     * gets output from process
      */
     public String getOutput() {
         String out = "";

src/main/java/JobServ/ProcessManager.java

@@ -0,0 +1,278 @@
+/*
+ * ProcessManager
+ *
+ * v1.0
+ *
+ * May 22, 2019
+ */
+
+package JobServ;
+
+import java.util.concurrent.Future;
+
+/*
+ * Holds a list of ProcessControllers and controls access to them via mutex
+ * Additionally, starts and manages a background thread that clears finished processes from the arraylist
+ */
+class ProcessManager {
+    // TODO: LOCK_TIMEOUT should be defined in a configuration management system
+    private final int LOCK_TIMEOUT = 5; // seconds
+    private ArrayList<ProcessController> processQueue;
+    private Boolean processQueueMutex = false; 
+    private Thread backgroundProcessCleaner;
+    private ExecutorService threadPool = Executors.newCachedThreadPool();
+
+    private Callable<void> getLockCallable = new Callable<void>() {
+            public void Object call() {
+                while(processQueueMutex){
+                    continue; // spin!
+                }
+
+                processQueueMutex = true;
+            }
+        }
+
+    /*
+     * Constructor
+     * initializes process queue and start the background process checking daemon
+     */
+    public ProcessManager() {
+        processQueue = new ArrayList<ProcessController>();
+        // TODO: In a long running server over a large period of time
+        // It is possible that the streams used to redirect IO in the
+        // Processes may become a significant use of resources.
+        // In this case a background thread should be called to periodically
+        // remove dead ProcessControllers after calling kill() on them.
+    }
+
+    /*
+     * newProcess()
+     * Takes a command and returns the translated pid of a new process
+     * Returns -1 if getLock fails
+     */
+    public int newProcess(String command) {
+        /*
+         * TRADEOFF: Could initialize new ProcessController out here
+         * Pro: would minimize time spent in critical section
+         * Con: what if initialization goes through but we dont get the lock
+         *      we would essentially have a dangling untrackable process
+         *      which likely changed system state before it was killed.
+         */
+
+        // Enter critical section
+        try {
+            this.getLock();
+
+        } catch (TimeoutException e) {
+            // (lock was not grabbed)
+            System.err.println("Timeout starting new job '%s': " + e.getMessage, command);
+            return -1
+        }
+
+        ProcessController newProc = ProcessController(command);
+        this.processQueue.add(newProc);
+
+        // Exit critical section
+        this.releaseLock();
+
+        return newProc.getPid();
+    }
+
+    /*
+     * getProcessStatus()
+     * returns whether or not a process is running.
+     * 0: running
+     * 1: not running
+     * 2: doesnt exist
+     * 3: couldnt grab lock
+     */
+    public int getProcessStatus(int pid) {
+        // Enter critical section
+        try {
+            this.getLock();
+
+        } catch (TimeoutException e) {
+            // lock could not be grabbed before timeout
+            System.err.println("Timeout getting process status for %s: " + e.getMessage(),
+                               Integer.toString(pid));
+            return 3;
+        }
+
+        for (ProcessController iter : this.processQueue) {
+            if (iter.getPid() == pid) {
+                this.releaseLock();
+                // release lock on finding process
+                return iter.getStatus();
+            }
+        }
+
+        // process must not exist
+        this.releaseLock();
+        return 2;
+    }
+
+    /*
+     * getProcessReturn()
+     * returns a code 0-255, or 256 if process still running
+     * additionally, returns 257 if lock not grabbable AND
+     * a 258 if process doesnt exist.
+     */
+    public int getProcessReturn(int pid) {
+        // Enter Critical section
+        try {
+            this.getLock();
+
+        } catch (TimeoutException e) {
+            System.err.println("Timeout getting process return for %s: " + e.getMessage(),
+                               Integer.toString(pid));
+            return 257;
+        }
+
+        for (ProcessController iter : this.processQueue) {
+            if (iter.getPid() == pid) {
+                this.releaseLock();
+                return iter.getReturn();
+            }
+        }
+
+        this.releaseLock();
+        return 258;
+    }
+
+    /*
+     * getProcessOutput()
+     * returns output of process 'pid'
+     * or returns description of error
+     */
+    public String getProcessOutput(int pid) {
+        try {
+            this.getLock();
+
+        } catch (TimeoutException e) {
+            System.err.println("Timeout getting process output for %s: " + e.getMessage(),
+                               Integer.toString());
+            return "[-] ERROR: Timeout grabbing lock to access process information";
+        }
+
+        for (ProcessController iter : this.processQueue) {
+            if (iter.getPid() == pid) {
+                output = iter.getOutput();
+                this.releaseLock();
+                return output;
+            }
+        }
+
+        this.releaseLock();
+        return "[-] ERROR: Process not found"
+    }
+
+    /*
+     * killProcess()
+     * returns false if couldnt grab lock
+     * ALSO RETURNS TRUE IF PROCESS DOESNT EXIST
+     */
+    public Boolean killProcess(int pid) {
+        try {
+            this.getLock();
+
+        } catch (TimeoutException e) {
+            System.err.println("Timeout killing process: " + e.getMessage);
+            return false;
+        }
+
+        for (ProcessController iter : this.processQueue) {
+            if (iter.getPid() == pid) {
+                iter.kill();
+                break;
+            }
+        }
+
+        this.releaseLock();
+        return true;
+    }
+
+    /*
+     * cleanProcessQueue()
+     * represents a background thread that sits and cleans finished processes
+     */
+    private void cleanProcessQueue() {
+        while(true){
+            try {
+                this.getLock();
+
+            } catch (TimeoutException e) {
+                continue;
+            }
+
+            for (ProcessController iter : this.processQueue) {
+                if(!iter.getStatus()) {
+                    iter.kill();
+                    this.processQueue.remove(iter);
+                }
+            }
+
+            this.releaseLock();
+            Thread.sleep(5000);
+        }
+    }
+
+    /*
+     * getLock()
+     * Locks access to this.processQueue
+     * Waits for a predefined timeout period and then grabs the mutex
+     * Throws TimeoutException when it fails to get the lock.
+     */
+    private synchronized void getLock() throws TimeoutException {
+        try {
+            Future<void> future = executor.submit(task);
+            void result = future.get(this.LOCK_TIMEOUT, TimeUnit.SECONDS);
+
+        } catch (InterruptedException e) {
+            System.err.println("[!] ERROR: " + e.getMessage());
+            throw new TimeoutException();
+            // rethrowing a timeout exception tells the calling process that they dont have the lock
+
+        } catch (ExecutionException e) {
+            System.err.println("[!] ERROR: " + e.getMessage());
+            throw new TimeoutException();
+
+        // cancel the attempt to grab the lock
+        } finally {
+            future.cancel(true);
+        }
+
+        /*
+         * TODO: touch of tech debt here
+         * There should honestly be an
+         * operation retry queue for ops
+         * That dont get the lock in time.
+         *
+         * This would require a scheduler
+         * that manages a queue of callbacks
+         * This scheduler would also likely
+         * mediate access to the ProcessManager
+         * object for fresh calls as well.
+         */
+    }
+
+    /*
+     * releaseLock()
+     * releases mutex so other threads can operate on processqueue
+     */
+    private void releaseLock() {
+        this.processQueueMutex = false;
+    }
+
+    /*
+     * shutdown()
+     * called (eventually) by the grpc shutdown hook
+     * (AKA when user hits control c in the shell)
+     * releases resources held in the processController objects
+     */
+    private void shutdown() {
+        this.getLock();
+        for (ProcessController p : this.processQueue) {
+            p.kill(); // exit threads, release IO streams, etc.
+        }
+    }
+}