Proper error reporting of missing javac

Author: johncf

README.md

@@ -12,6 +12,7 @@ A web-based Java compiler frontend to play with single-source console programs.
 - A modern browser
   - Supporting [`localStorage`][], [`history.replaceState`][], [`const`][], [`box-sizing`][], [`flex`][]
   - Supposedly Firefox 29+, Chrome 29+, Edge, Safari 9+, IE 11, Opera 17+
+- `javac` and `java` in `PATH` when executing server ([example batch file](https://gist.github.com/johncf/a98cd3aa49491c5f58372566883e7810) which does this).
 
 [`localStorage`]: https://caniuse.com/#feat=const
 [`history.replaceState`]: https://caniuse.com/#feat=history

compiler.py

@@ -33,10 +33,18 @@ def __init__(self, source, dirpath, callbacks):
         self._cbs = callbacks
 
     def _compile(self):
-        return Popen(["javac", "-Xlint", self._name + ".java"], cwd=self._dir, stdout=PIPE, stderr=STDOUT)
+        try:
+            return Popen(["javac", "-Xlint", self._name + ".java"], cwd=self._dir, stdout=PIPE, stderr=STDOUT)
+        except FileNotFoundError as e:
+            print("Error:", e.strerror)
+            return None
 
     def _execute(self):
-        return Popen(["java", self._name], cwd=self._dir, stdin=PIPE, stdout=PIPE, stderr=PIPE)
+        try:
+            return Popen(["java", self._name], cwd=self._dir, stdin=PIPE, stdout=PIPE, stderr=PIPE)
+        except FileNotFoundError as e:
+            print("Error:", e.strerror)
+            return None
 
     def spawn_bg(self):
         t = _spawn(_main, args=(self,))
@@ -74,6 +82,9 @@ def read2Q(key, stream, notifq, limit_size=4096, limit_lines=256):
 
 def _main(program):
     proc = program._compile()
+    if proc is None:
+        program._cbs.error("Backend did not find 'javac' in PATH.")
+        return
     outt = _spawn(read2Q, args=('stdout', proc.stdout, program._queue))
     outt.start()
     done = False

server.py

@@ -46,6 +46,9 @@ def stdin_ack(self, data):
     def done(self, ecode):
         self._emit('done', {'ecode': ecode})
 
+    def error(self, msg):
+        self._emit('backend_error', {'description': msg})
+
 def reset_dir(path):
     if os.path.isdir(path):
         for root, dirs, files in os.walk(path, topdown=False):

static/js/main.js

@@ -69,6 +69,14 @@ const compile_button = document.getElementById("compile");
 const kill_button = document.getElementById("kill");
 const stdin_input = document.getElementById("stdin");
 
+function fatalError(msg) {
+  logClear();
+  logAppend(msg, true);
+  compile_button.disabled = true;
+  kill_button.disabled = true;
+  stdin_input.disabled = true;
+}
+
 const socket = io("/compiler");
 socket.on('connect', function() {
   logAppend("Click \"Compile & Execute\" when ready.", true);
@@ -77,11 +85,10 @@ socket.on('connect', function() {
   stdin_input.disabled = true;
 });
 socket.on('disconnect', function() {
-  logClear();
-  logAppend("Connection with compiler lost. Please rerun the backend.", true);
-  compile_button.disabled = true;
-  kill_button.disabled = true;
-  stdin_input.disabled = true;
+  fatalError("Connection with compiler lost. Please rerun the backend.");
+});
+socket.on('backend_error', function(e) {
+  fatalError(e.description);
 });
 socket.on('started', function(msg) {
   logAppend("Compiling...", true);