add allow commands list

src/nexus_sync/client/__init__.py

@@ -0,0 +1,17 @@
+from nexus_sync.client.config import (
+    COMMAND_ACCESS_ENV,
+    FULL_ACCESS_VALUE,
+    load_command_access_policy,
+)
+from nexus_sync.client.execute import (
+    CommandAccessPolicy,
+    execute_command,
+)
+
+__all__ = [
+    "COMMAND_ACCESS_ENV",
+    "CommandAccessPolicy",
+    "FULL_ACCESS_VALUE",
+    "execute_command",
+    "load_command_access_policy",
+]

src/nexus_sync/client/config.py

@@ -0,0 +1,25 @@
+import os
+from collections.abc import Mapping
+
+from nexus_sync.client.execute import CommandAccessPolicy
+
+COMMAND_ACCESS_ENV = "NEXUS_SYNC_ALLOWED_COMMANDS"
+FULL_ACCESS_VALUE = "full_access"
+
+
+def load_command_access_policy(
+    env: Mapping[str, str] = os.environ,
+    *,
+    default: CommandAccessPolicy | None = None,
+) -> CommandAccessPolicy:
+    raw_value = env.get(COMMAND_ACCESS_ENV)
+    if raw_value is None or not raw_value.strip():
+        return default or CommandAccessPolicy.deny_all()
+
+    command_names = [item.strip() for item in raw_value.split(",") if item.strip()]
+    if len(command_names) == 1 and command_names[0].lower() == FULL_ACCESS_VALUE:
+        return CommandAccessPolicy.allow_all()
+    if any(command_name.lower() == FULL_ACCESS_VALUE for command_name in command_names):
+        raise ValueError(f"{FULL_ACCESS_VALUE} cannot be mixed with explicit command names")
+
+    return CommandAccessPolicy.allow(command_names)

src/nexus_sync/client/execute.py

@@ -1,5 +1,7 @@
 import platform
 import subprocess
+from collections.abc import Iterable
+from dataclasses import dataclass, field
 from datetime import UTC, datetime
 from typing import Any, Callable, Mapping, Sequence
 
@@ -10,6 +12,27 @@ PresetBuilder = Callable[[Mapping[str, Any]], Sequence[str]]
 DEFAULT_OUTPUT_LIMIT_BYTES = 64 * 1024
 
 
+@dataclass(frozen=True)
+class CommandAccessPolicy:
+    allowed_commands: frozenset[str] = field(default_factory=frozenset)
+    full_access: bool = False
+
+    @classmethod
+    def allow(cls, command_names: Iterable[str]) -> "CommandAccessPolicy":
+        return cls(allowed_commands=frozenset(command_names))
+
+    @classmethod
+    def allow_all(cls) -> "CommandAccessPolicy":
+        return cls(full_access=True)
+
+    @classmethod
+    def deny_all(cls) -> "CommandAccessPolicy":
+        return cls()
+
+    def allows(self, command_name: str) -> bool:
+        return self.full_access or command_name in self.allowed_commands
+
+
 def _reject(command: Command, message: str) -> CommandResult:
     now = datetime.now(UTC)
     return CommandResult(
@@ -45,6 +68,7 @@ DEFAULT_PRESETS: dict[str, PresetBuilder] = {
     "hostname": _hostname,
     "network_interfaces": _network_interfaces,
 }
+DEFAULT_COMMAND_ACCESS_POLICY = CommandAccessPolicy.allow_all()
 
 
 def execute_command(
@@ -52,6 +76,7 @@ def execute_command(
     *,
     stdin: str | None = None,
     presets: Mapping[str, PresetBuilder] = DEFAULT_PRESETS,
+    access_policy: CommandAccessPolicy = DEFAULT_COMMAND_ACCESS_POLICY,
     output_limit_bytes: int = DEFAULT_OUTPUT_LIMIT_BYTES,
 ) -> CommandResult:
     if command.kind != CommandKind.EXEC:
@@ -60,6 +85,8 @@ def execute_command(
     builder = presets.get(command.name)
     if builder is None:
         return _reject(command, f"unknown command preset: {command.name}")
+    if not access_policy.allows(command.name):
+        return _reject(command, f"command preset is not allowed: {command.name}")
 
     try:
         argv = list(builder(command.args))