Do not auto-create .taskrc when stdout is not a TTY (#3888)

* Do not auto-create .taskrc when stdout is not a TTY

This avoids prompting or automatically creating such a file, both of
which are unexpected when performing command-line completion.

Fixes #3751.

* Test case for taskrc creation no longer works

A taskrc is only created when stdout is a tty, which would require
allocating a pty, which is very platform-dependent and definitely not
worth the trouble for this test.

src/Context.cpp

@@ -1173,6 +1173,13 @@ void Context::staticInitialization() {
 void Context::createDefaultConfig() {
   // Do we need to create a default rc?
   if (rc_file._data != "" && !rc_file.exists()) {
+    // If stdout is not a file, we are probably executing in a completion context and should not
+    // prompt (as the user won't see it) or modify the config (as completion functions are typically
+    // read-only).
+    if (!isatty(STDOUT_FILENO)) {
+      throw std::string("Cannot proceed without rc file.");
+    }
+
     if (config.getBoolean("confirmation") &&
         !confirm(format("A configuration file could not be found in {1}\n\nWould you like a sample "
                         "{2} created, so Taskwarrior can proceed?",

test/taskrc.test.py

@@ -40,6 +40,7 @@ class TestTaskrc(TestCase):
         """Executed before each test in the class"""
         self.t = Task()
 
+    @unittest.skip("taskrc generation requires a tty - see #3751")
     def test_default_taskrc(self):
         """Verify that a default .taskrc is generated"""
         os.remove(self.t.taskrc)