Port the flags of nix-daemon to nix daemon (#8788)

The new `nix daemon` command didn't accept the same flags that `nix-daemon` did.

* docs(daemon): clarify the daemon trust override flags
* fix: change declaration order
* docs: add examples of nix daemon usage
* Apply suggestions from code review

---------

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Co-authored-by: John Ericson <git@JohnEricson.me>
Co-authored-by: tomberek <tomberek@users.noreply.github.com>

src/nix/daemon.cc

@@ -500,6 +500,45 @@ static RegisterLegacyCommand r_nix_daemon("nix-daemon", main_nix_daemon);
 
 struct CmdDaemon : StoreCommand
 {
+    bool stdio = false;
+    std::optional<TrustedFlag> isTrustedOpt = std::nullopt;
+
+    CmdDaemon()
+    {
+        addFlag({
+            .longName = "stdio",
+            .description = "Attach to standard I/O, instead of trying to bind to a UNIX socket.",
+            .handler = {&stdio, true},
+        });
+
+        addFlag({
+            .longName = "force-trusted",
+            .description = "Force the daemon to trust connecting clients.",
+            .handler = {[&]() {
+                isTrustedOpt = Trusted;
+            }},
+            .experimentalFeature = Xp::DaemonTrustOverride,
+        });
+
+        addFlag({
+            .longName = "force-untrusted",
+            .description = "Force the daemon to not trust connecting clients. The connection will be processed by the receiving daemon before forwarding commands.",
+            .handler = {[&]() {
+                isTrustedOpt = NotTrusted;
+            }},
+            .experimentalFeature = Xp::DaemonTrustOverride,
+        });
+
+        addFlag({
+            .longName = "default-trust",
+            .description = "Use Nix's default trust.",
+            .handler = {[&]() {
+                isTrustedOpt = std::nullopt;
+            }},
+            .experimentalFeature = Xp::DaemonTrustOverride,
+        });
+    }
+
     std::string description() override
     {
         return "daemon to perform store operations on behalf of non-root clients";
@@ -516,7 +555,7 @@ struct CmdDaemon : StoreCommand
 
     void run(ref<Store> store) override
     {
-        runDaemon(false, std::nullopt);
+        runDaemon(stdio, isTrustedOpt);
     }
 };
 

src/nix/daemon.md

@@ -1,20 +1,44 @@
 R""(
 
-# Example
+# Examples
 
-* Run the daemon in the foreground:
+* Run the daemon:
 
   ```console
   # nix daemon
   ```
 
+* Run the daemon and listen on standard I/O instead of binding to a UNIX socket:
+
+  ```console
+  # nix daemon --stdio
+  ```
+
+* Run the daemon and force all connections to be trusted:
+
+  ```console
+  # nix daemon --force-trusted
+  ```
+
+* Run the daemon and force all connections to be untrusted:
+
+  ```console
+  # nix daemon --force-untrusted
+  ```
+
+* Run the daemon, listen on standard I/O, and force all connections to use Nix's default trust:
+
+  ```console
+  # nix daemon --stdio --default-trust
+  ```
+
 # Description
 
 This command runs the Nix daemon, which is a required component in
 multi-user Nix installations. It runs build tasks and other
 operations on the Nix store on behalf of non-root users. Usually you
 don't run the daemon directly; instead it's managed by a service
-management framework such as `systemd`.
+management framework such as `systemd` on Linux, or `launchctl` on Darwin.
 
 Note that this daemon does not fork into the background.