ChildProcess can handle commands that return large amounts of data.

When a command returned a large amount of data, it was blocking on the stderr  callback when `[fileHandle availableData]` was called. From what I can tell, this is because stderr was being called with a zero-length string.

This was fixed when `[fileHandle availableData]` was moved to run inside the NSTask thread (instead of on the main thread). It now returns a zero-length string rather than blocking forever.

An unresolved question is why stderr is being called with zero-length strings.

native/v8_extensions/native.mm

@@ -406,15 +406,12 @@ bool Native::Execute(const CefString& name,
     [task setStandardError:stderr];
 
     CefRefPtr<CefV8Context> context = CefV8Context::GetCurrentContext();
-    void (^outputHandle)(NSFileHandle *fileHandle, CefRefPtr<CefV8Value> function) = nil;
+    void (^outputHandle)(NSString *contents, CefRefPtr<CefV8Value> function) = nil;
     void (^taskTerminatedHandle)() = nil;
 
-    outputHandle = ^(NSFileHandle *fileHandle, CefRefPtr<CefV8Value> function) {
+    outputHandle = ^(NSString *contents, CefRefPtr<CefV8Value> function) {
       context->Enter();
 
-      NSData *data = [fileHandle availableData];
-      NSString *contents = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
-
       CefV8ValueList args;
       args.push_back(CefV8Value::CreateString(std::string([contents UTF8String], [contents lengthOfBytesUsingEncoding:NSUTF8StringEncoding])));
       CefRefPtr<CefV8Value> retval = function->ExecuteFunction(function, args);
@@ -423,7 +420,6 @@ bool Native::Execute(const CefString& name,
         throwException(context->GetGlobal(), function->GetException(), @"Error thrown in OutputHandle");
       }
 
-      [contents release];
       context->Exit();
     };
 
@@ -457,18 +453,24 @@ bool Native::Execute(const CefString& name,
     CefRefPtr<CefV8Value> stdoutFunction = options->GetValue("stdout");
     if (stdoutFunction->IsFunction()) {
       stdout.fileHandleForReading.writeabilityHandler = ^(NSFileHandle *fileHandle) {
+        NSData *data = [fileHandle availableData];
+        NSString *contents = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
         dispatch_sync(dispatch_get_main_queue(), ^() {
-          outputHandle(fileHandle, stdoutFunction);
+          outputHandle(contents, stdoutFunction);
         });
+        [contents release];
       };
     }
 
     CefRefPtr<CefV8Value> stderrFunction = options->GetValue("stderr");
     if (stderrFunction->IsFunction()) {
       stderr.fileHandleForReading.writeabilityHandler = ^(NSFileHandle *fileHandle) {
+        NSData *data = [fileHandle availableData];
+        NSString *contents = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
         dispatch_sync(dispatch_get_main_queue(), ^() {
-          outputHandle(fileHandle, stderrFunction);
+          outputHandle(contents, stderrFunction);
         });
+        [contents release];
       };
     }
 

spec/stdlib/child-process-spec.coffee

@@ -104,3 +104,26 @@ describe 'Child Processes', ->
           ChildProcess.exec(cmd, options).fail (error) ->
             expect(error.exitStatus).toBe 2
             expect(errorOutput).toBe "bad\n"
+
+    describe "when a command returns a large amount of data (over 10k)", ->
+      originalTimeout = null
+      beforeEach ->
+        originalTimeout = jasmine.getEnv().defaultTimeoutInterval
+        jasmine.getEnv().defaultTimeoutInterval = 1000
+
+      afterEach ->
+        jasmine.getEnv().defaultTimeoutInterval = originalTimeout
+
+      it "does not block indefinitally on stdout or stderr callbacks (regression)", ->
+        output = []
+
+        waitsForPromise ->
+          cmd = "for i in {1..20000}; do echo $RANDOM; done"
+          options =
+            stdout: (data) -> output.push(data)
+            stderr: (data) -> console.log data.length
+
+          ChildProcess.exec(cmd, options)
+
+        runs ->
+          expect(output.length).toBeGreaterThan 1