[CS2] Support await in REPL without wrapper function (#4604)

* Support `await` in REPL without wrapper function

Rather than compiling top-level await expressions directly, which will
always throw a syntax error, the REPL wrapper will now wrap them in a
closure and handle the returned promise before calling back to the REPL.

Fixes #4603.

* Cancel REPL callback after a SIGINT during an awaited evaluation

This change allows users to 'cancel' awaited expressions that they don't
want to see the result of. Ultimately, this *does not* prevent the
expression from completing (e.g. for `await fetch(url)`, `url` will
still be downloaded, but the user won't have to wait for it to finish
and the result won't be printed).

lib/coffeescript/repl.js

@@ -1,6 +1,6 @@
 // Generated by CoffeeScript 2.0.0-beta3
 (function() {
-  var CoffeeScript, addHistory, addMultilineHandler, fs, getCommandId, merge, nodeREPL, path, replDefaults, runInContext, updateSyntaxError, vm;
+  var CoffeeScript, addHistory, addMultilineHandler, fs, getCommandId, merge, nodeREPL, path, replDefaults, runInContext, sawSIGINT, updateSyntaxError, vm;
 
   fs = require('fs');
 
@@ -14,6 +14,8 @@
 
   ({merge, updateSyntaxError} = require('./helpers'));
 
+  sawSIGINT = false;
+
   replDefaults = {
     prompt: 'coffee> ',
     historyFile: (function() {
@@ -24,12 +26,12 @@
       }
     })(),
     historyMaxInputSize: 10240,
-    eval: function(input, context, filename, cb) {
-      var Assign, Block, Literal, Value, ast, err, js, referencedVars, token, tokens;
+    eval: async function(input, context, filename, cb) {
+      var Assign, Block, Call, Code, Literal, Value, ast, err, isAsync, js, referencedVars, result, token, tokens;
       input = input.replace(/\uFF00/g, '\n');
       input = input.replace(/^\(([\s\S]*)\n\)$/m, '$1');
       input = input.replace(/^\s*try\s*{([\s\S]*)}\s*catch.*$/m, '$1');
-      ({Block, Assign, Value, Literal} = require('./nodes'));
+      ({Block, Assign, Value, Literal, Call, Code} = require('./nodes'));
       try {
         tokens = CoffeeScript.tokens(input);
         referencedVars = (function() {
@@ -45,12 +47,25 @@
         })();
         ast = CoffeeScript.nodes(tokens);
         ast = new Block([new Assign(new Value(new Literal('__')), ast, '=')]);
+        ast = new Code([], ast);
+        isAsync = ast.isAsync;
+        ast = new Block([new Call(ast)]);
         js = ast.compile({
           bare: true,
           locals: Object.keys(context),
-          referencedVars
+          referencedVars,
+          sharedScope: true
         });
-        return cb(null, runInContext(js, context, filename));
+        result = runInContext(js, context, filename);
+        if (isAsync) {
+          result = (await result);
+          if (!sawSIGINT) {
+            cb(null, result);
+          }
+          return sawSIGINT = false;
+        } else {
+          return cb(null, result);
+        }
       } catch (error) {
         err = error;
         updateSyntaxError(err, input);
@@ -150,6 +165,9 @@
         return lastLine = code;
       }
     });
+    repl.on('SIGINT', function() {
+      return sawSIGINT = true;
+    });
     repl.on('exit', function() {
       return fs.closeSync(fd);
     });

src/repl.coffee

@@ -5,6 +5,8 @@ nodeREPL = require 'repl'
 CoffeeScript = require './'
 {merge, updateSyntaxError} = require './helpers'
 
+sawSIGINT = no
+
 replDefaults =
   prompt: 'coffee> ',
   historyFile: do ->
@@ -22,23 +24,31 @@ replDefaults =
     input = input.replace /^\s*try\s*{([\s\S]*)}\s*catch.*$/m, '$1'
 
     # Require AST nodes to do some AST manipulation.
-    {Block, Assign, Value, Literal} = require './nodes'
+    {Block, Assign, Value, Literal, Call, Code} = require './nodes'
 
     try
       # Tokenize the clean input.
       tokens = CoffeeScript.tokens input
       # Collect referenced variable names just like in `CoffeeScript.compile`.
-      referencedVars = (
-        token[1] for token in tokens when token[0] is 'IDENTIFIER'
-      )
+      referencedVars = (token[1] for token in tokens when token[0] is 'IDENTIFIER')
       # Generate the AST of the tokens.
       ast = CoffeeScript.nodes tokens
-      # Add assignment to `_` variable to force the input to be an expression.
-      ast = new Block [
-        new Assign (new Value new Literal '__'), ast, '='
-      ]
-      js = ast.compile {bare: yes, locals: Object.keys(context), referencedVars}
-      cb null, runInContext js, context, filename
+      # Add assignment to `__` variable to force the input to be an expression.
+      ast = new Block [new Assign (new Value new Literal '__'), ast, '=']
+      # Wrap the expression in a closure to support top-level `await`
+      ast     = new Code [], ast
+      isAsync = ast.isAsync
+      # Invoke the wrapping closure
+      ast    = new Block [new Call ast]
+      js     = ast.compile {bare: yes, locals: Object.keys(context), referencedVars, sharedScope: yes}
+      result = runInContext js, context, filename
+      # Await an async result, if necessary
+      if isAsync
+        result = await result
+        cb null, result unless sawSIGINT
+        sawSIGINT = false
+      else
+        cb null, result
     catch err
       # AST's `compile` does not add source code information to syntax errors.
       updateSyntaxError err, input
@@ -131,6 +141,8 @@ addHistory = (repl, filename, maxSize) ->
       fs.writeSync fd, "#{code}\n"
       lastLine = code
 
+  # XXX: The SIGINT event from REPLServer is undocumented, so this is a bit fragile
+  repl.on 'SIGINT', -> sawSIGINT = yes
   repl.on 'exit', -> fs.closeSync fd
 
   # Add a command to show the history stack