Read a stopped container instead of claiming there is nothing to read

A project that was merely stopped reported "This project has no container
or snapshot image yet, so there is nothing to compare against the base
image" — with its container sitting right there — and Update stayed
disabled. Start it and the checks passed, which is the tell: the staleness
probe had only two sources, a *running* container via `docker exec` or the
project's snapshot image.

The snapshot is not a checkpoint. `commit_container_snapshot` runs only
before a container is destroyed (a config-change recreate) or inside a
migration, never on stop, so a project in daily use for a year can have no
snapshot at all — and five of the six projects on the box that reported
this had none. Absence of a snapshot was being read as absence of anything
to inspect.

So probe the stopped container directly: commit its writable layer to a
throwaway image, probe that, drop it. A stopped container now also outranks
the snapshot, for the same reason a running one already did — the snapshot
lags it by everything installed since the last commit. `pick_probe_source`
is the whole decision and is unit-tested; the message it used to emit now
describes only the case it is true of, no container and no snapshot.

Two things found on the way, both documented in CLAUDE.md:

`bollard` never hands back the image id from a commit — its `Commit` model
deserialises "ID" while the daemon sends "Id" — so the probe image has to be
tagged, and a tagged image is dangling-proof and therefore invisible to
`sweep_orphaned_snapshots`, `reap_stale_migration_pins` and
`scrub_secrets_from_snapshots` alike. Without a reaper of its own a crashed
probe would leak a multi-gigabyte image that nothing could ever reclaim, so
`reap_probe_images` runs at startup beside `reap_probe_containers`, age-gated
for the same reason that one is: `reference=` is daemon-wide and a second
instance's live probe matches the glob.

It removes by tag, never by image id: a force removal by id untags an image
everywhere, which is how a first draft of the reaper test deleted an
unrelated `alpine:latest`. Names are unique per call rather than stable per
container, because container ids do not survive a recreate and two
overlapping probes would otherwise fight over one tag.

Verified against the container that reported the bug: 13,365 paths and an
apt delta of cmake, ffmpeg, libobs-dev, qt6-base-dev and nine more — the
migration payload the Update flow could not see. 546 unit tests plus three
live-Docker tests pass; no new clippy warnings.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019RSaoDLovVV2wmH4H8VVxz

app/src-tauri/src/commands/migration_commands.rs

@@ -92,8 +92,72 @@ fn pick_recorded_lineage(
         .or_else(|| from_snapshot.filter(|v| !v.is_empty()))
 }
 
-/// Read-only. Runs two filesystem probes (~3 s each) and is therefore meant to
-/// be called on demand, not polled.
+/// Reported as `probe_error` when there is genuinely nothing to read: no
+/// container, stopped or otherwise, and no snapshot image.
+///
+/// It used to be reported for a *stopped* container too, which was simply
+/// untrue — the container was sitting right there — and it disabled Update on
+/// exactly the long-lived projects that had never been recreated and so had no
+/// snapshot to fall back on.
+const NOTHING_TO_PROBE: &str = "This project has no container or snapshot image yet, so there is nothing to compare against the base image.";
+
+/// Where [`get_container_staleness`] reads the project's *current* filesystem
+/// from, in descending order of how current the answer is.
+#[derive(Debug, PartialEq, Eq)]
+enum ProbeSource {
+    /// `docker exec` into the live container. The only source that includes
+    /// everything installed since the last commit *in this session*.
+    RunningContainer,
+    /// Commit the stopped container's writable layer to a throwaway image and
+    /// probe that. Exactly as current as the container, which is what makes it
+    /// preferable to the snapshot — see below.
+    StoppedContainer,
+    /// A throwaway container from `triple-c-snapshot-<id>:latest`.
+    Snapshot,
+    /// Nothing to read: no container, no snapshot.
+    Nothing,
+}
+
+/// Pick the probe source. `container_running` is `None` when the project has no
+/// container at all, `Some(false)` when it has a stopped one.
+///
+/// **A stopped container outranks the snapshot.** The snapshot image is not a
+/// checkpoint — `commit_container_snapshot` runs only before a removal (a
+/// config-change recreate) or inside a migration, so a project that has never
+/// hit either has *no snapshot at all*, however long it has been in use, and
+/// one that has is stale by everything installed since. The container's
+/// writable layer is the truth in both cases. This is the same argument
+/// [`mig::manifest_from_container`] already makes for the running case; it does
+/// not stop applying when the container is stopped.
+///
+/// Getting this wrong is what made a stopped, never-recreated project report
+/// "no container or snapshot image yet" — with its container sitting right
+/// there — and left Update disabled on the projects that most needed it.
+fn pick_probe_source(container_running: Option<bool>, snapshot_exists: bool) -> ProbeSource {
+    match (container_running, snapshot_exists) {
+        (Some(true), _) => ProbeSource::RunningContainer,
+        (Some(false), _) => ProbeSource::StoppedContainer,
+        (None, true) => ProbeSource::Snapshot,
+        (None, false) => ProbeSource::Nothing,
+    }
+}
+
+/// Runs two filesystem probes (~3 s each) and is therefore meant to be called
+/// on demand, not polled.
+///
+/// **Not read-only, despite only reporting.** The stopped-container path commits
+/// a throwaway image and force-removes it, which makes this a writer of a
+/// `triple-c-probe-*` image and puts it in the class of thing
+/// [`crate::project_lock`] exists for — and it takes no claim. That is
+/// deliberate: this is what the migration banner calls to decide whether to
+/// offer an update, including while a migration is in flight, so refusing it
+/// under a claim would blank the banner exactly when it has the most to say.
+/// The exposure is bounded to a surfaced error — a concurrent Recreate, Reset or
+/// migration can remove the container out from under the commit, and the result
+/// is a `probe_error` the user can retry, never a damaged container or a
+/// mislabelled image. Two overlapping probes cannot collide either, because
+/// probe image names are unique per call; see
+/// [`crate::docker::container::get_probe_image_name`].
 #[tauri::command]
 pub async fn get_container_staleness(
     project_id: String,
@@ -145,16 +209,25 @@ pub async fn get_container_staleness(
     };
 
     // ── Probes ───────────────────────────────────────────────────────────
-    let running = match &container_id {
-        Some(id) => docker::is_container_running(id).await.unwrap_or(false),
-        None => false,
+    let container_running = match &container_id {
+        Some(id) => Some(docker::is_container_running(id).await.unwrap_or(false)),
+        None => None,
     };
-    let from_manifest = if running {
-        mig::manifest_from_container(container_id.as_ref().unwrap()).await
-    } else if docker::image_exists(&snapshot_image).await.unwrap_or(false) {
-        mig::manifest_from_image(&snapshot_image).await
-    } else {
-        Err("This project has no container or snapshot image yet, so there is nothing to compare against the base image.".to_string())
+    let snapshot_exists = docker::image_exists(&snapshot_image).await.unwrap_or(false);
+    let from_manifest = match (
+        pick_probe_source(container_running, snapshot_exists),
+        &container_id,
+    ) {
+        (ProbeSource::RunningContainer, Some(id)) => mig::manifest_from_container(id).await,
+        (ProbeSource::StoppedContainer, Some(id)) => {
+            mig::manifest_from_stopped_container(id).await
+        }
+        (ProbeSource::Snapshot, _) => mig::manifest_from_image(&snapshot_image).await,
+        // `container_running` is `Some` exactly when `container_id` is, so the
+        // two arms above are the only ones those variants can reach. This arm
+        // is `ProbeSource::Nothing` — and now *only* that: it used to also
+        // swallow every stopped container, which is the bug.
+        (_, _) => Err(NOTHING_TO_PROBE.to_string()),
     };
 
     let (from_manifest, base_manifest) = match from_manifest {
@@ -1964,6 +2037,42 @@ mod tests {
         assert_eq!(pick_recorded_lineage(some(""), None), None);
     }
 
+    #[test]
+    fn a_stopped_container_is_probed_rather_than_reported_missing() {
+        // The regression: a container that exists but is stopped, with no
+        // snapshot ever taken, read as "nothing to compare against".
+        assert_eq!(
+            pick_probe_source(Some(false), false),
+            ProbeSource::StoppedContainer
+        );
+    }
+
+    #[test]
+    fn the_container_outranks_the_snapshot_whether_or_not_it_is_running() {
+        // The snapshot lags the container by everything installed since the
+        // last commit, in both states.
+        assert_eq!(
+            pick_probe_source(Some(true), true),
+            ProbeSource::RunningContainer
+        );
+        assert_eq!(
+            pick_probe_source(Some(false), true),
+            ProbeSource::StoppedContainer
+        );
+    }
+
+    #[test]
+    fn the_snapshot_is_the_fallback_only_once_the_container_is_gone() {
+        assert_eq!(pick_probe_source(None, true), ProbeSource::Snapshot);
+    }
+
+    #[test]
+    fn nothing_to_probe_is_reserved_for_no_container_and_no_snapshot() {
+        // The one case the "no container or snapshot image yet" message may
+        // still describe.
+        assert_eq!(pick_probe_source(None, false), ProbeSource::Nothing);
+    }
+
     #[test]
     fn byte_sizes_read_the_way_a_disk_warning_should() {
         assert_eq!(human_bytes(512), "512 B");