update_agent: add hidden knob to override interactive session check

An issue as old as ~time~Fedora CoreOS is that when you're testing
something related to updates, you want Zincati to reboot but because
`cosa run` gives you an interactive session and you can't *not* have a
session without losing the VM, you're stuck waiting for the 10 minutes
grace period to expire.

Just add a hidden knob for this to make that scenario less painful.

The API is pretty much: when you're ready to have Zincati reboot, just
`touch /run/zincati/override-interactive-check`.

src/update_agent/mod.rs

@@ -16,6 +16,7 @@ use serde::{Deserialize, Deserializer};
 use std::cell::Cell;
 use std::collections::BTreeSet;
 use std::fs;
+use std::path::Path;
 use std::rc::Rc;
 use std::time::Duration;
 use tokio::sync::RwLock;
@@ -37,6 +38,10 @@ const DEFAULT_POSTPONEMENT_TIME_SECS: u64 = 60; // 1 minute.
 /// before abandoning a target update.
 const MAX_DEPLOY_ATTEMPTS: u8 = 12;
 
+/// This is undocumented; it's for testing Zincati in e.g. cosa where you have
+/// an interactive session but you don't want to wait for the full timeout.
+const INTERACTIVE_SESSION_OVERRIDE: &str = "/run/zincati/override-interactive-check";
+
 /// Maximum number of postponements to finalizing an update in the
 /// `UpdateStaged` state before forcing an update finalization and reboot.
 pub(crate) const MAX_FINALIZE_POSTPONEMENTS: u8 = 10;
@@ -284,6 +289,15 @@ impl UpdateAgentMachineState {
             return true;
         }
 
+        // Allow even with interactive sessions if we're overriding this check
+        if Path::new(INTERACTIVE_SESSION_OVERRIDE).exists() {
+            log::debug!(
+                "ignoring interactive sessions due to {}",
+                INTERACTIVE_SESSION_OVERRIDE
+            );
+            return true;
+        }
+
         let (release, postponements_remaining) = match self {
             UpdateAgentMachineState::UpdateStaged((r, p)) => (r, *p),
             _ => unreachable!(