Fix tls certificate generation in cron jobs

tls renewal and generation was failing in cron jobs because of "The
input device is not a TTY" errors. This is because the "-it" docker
option does not work when a tty is not available.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 Note: Breaking changes between versions are indicated by "💥".
 
+## Unreleased
+
+- [Bugfix] Fix tls certificate generation in cron jobs
+
 ## v3.11.6 (2020-03-13)
 
 - [Bugfix] Fix "Unable to resolve dependency" error during forum initialisation

tutor/commands/compose.py

@@ -3,6 +3,7 @@ import click
 from .. import config as tutor_config
 from .. import fmt
 from .. import scripts
+from .. import utils
 
 
 class ScriptRunner(scripts.BaseRunner):
@@ -90,7 +91,10 @@ def restart(context, services):
 @click.pass_obj
 def run(context, args):
     config = tutor_config.load(context.root)
-    context.docker_compose(context.root, config, "run", "--rm", *args)
+    command = ["run", "--rm"]
+    if not utils.is_a_tty():
+        command.append("-T")
+    context.docker_compose(context.root, config, *command, *args)
 
 
 @click.command(

tutor/utils.py

@@ -4,6 +4,7 @@ import random
 import shutil
 import string
 import subprocess
+import sys
 
 import click
 
@@ -66,7 +67,10 @@ def walk_files(path):
 
 
 def docker_run(*command):
-    return docker("run", "--rm", "-it", *command)
+    args = ["run", "--rm"]
+    if is_a_tty():
+        args.append("-it")
+    return docker(*args, *command)
 
 
 def docker(*command):
@@ -92,6 +96,12 @@ def kubectl(*command):
         )
     return execute("kubectl", *command)
 
+def is_a_tty():
+    """
+    Return True if stdin is able to allocate a tty. Tty allocation sometimes cannot be
+    enabled, for instance in cron jobs
+    """
+    return os.isatty(sys.stdin.fileno())
 
 def execute(*command):
     click.echo(fmt.command(" ".join(command)))