fix: require at least 4 GB RAM on macOS for local commands

Adds a check in the 'local' command group that requires at least
4 GB of RAM to be allocated to Docker when running any of the
local subcommands on macOS. This addresses a common issue where
Docker's default setting (2 GB) causes startup to crash with
misleading error messages.

CHANGELOG.md

@@ -7,6 +7,7 @@ Note: Breaking changes between versions are indicated by "💥".
 ## v12.1.5 (2021-10-25)
 
 - 💥[Improvement] Change the `settheme` command such that, by default, a custom theme is assigned to the LMS and the CMS, both in production and development mode.
+- [Bugfix] Check Docker memory allocation on macOS when running tutor local commands (#463)
 
 ## v12.1.4 (2021-10-11)
 

tutor/commands/local.py

@@ -37,6 +37,7 @@ def docker_compose(root: str, config: Config, *command: str) -> int:
 @click.pass_obj
 def local(context: Context) -> None:
     context.docker_compose_func = docker_compose
+    utils.check_macos_memory()
 
 
 @click.command(help="Configure and run Open edX from scratch")

tutor/utils.py

@@ -244,3 +244,36 @@ def check_output(*command: str) -> bytes:
         raise exceptions.TutorError(
             "Command failed: {}".format(" ".join(command))
         ) from e
+
+
+def check_macos_memory() -> None:
+    """
+    Try to assert that the RAM allocated to the Docker VM on macOS is at least 4 GB.
+    """
+    if sys.platform != "darwin":
+        return
+
+    try:
+        settings_path = (
+            "{}/Library/Group Containers/group.com.docker/settings.json".format(
+                os.path.expanduser("~")
+            )
+        )
+        with open(settings_path) as fp:
+            data = json.load(fp)
+    except Exception as e:
+        # Ignore any IO and JSON parse errors
+        return
+
+    if "memoryMiB" not in data or not isinstance(data["memoryMiB"], int):
+        # Ignore absent / erratic values (Docker creates the file with the default setting of 2048 explicitly
+        # written in, so we shouldn't need to assume a default value here).
+        return
+
+    if data["memoryMiB"] < 4096:
+        raise exceptions.TutorError(
+            (
+                "Docker must be allocated at least 4 GB of memory on macOS (found {} MB). "
+                + "Please follow instructions from https://docs.tutor.overhang.io/install.html"
+            ).format(data["memoryMiB"])
+        )