Don't unarchive a timeline if its ancestor is archived (#8853)

If a timeline unarchival request comes in, give an error if the parent
timeline is archived. This prevents us from the situation of having an
archived timeline with children that are not archived.

Follow up of #8824

Part of #8088

---------

Co-authored-by: Joonas Koivunen <joonas@neon.tech>

pageserver/src/http/routes.rs

@@ -324,6 +324,9 @@ impl From<crate::tenant::TimelineArchivalError> for ApiError {
         match value {
             NotFound => ApiError::NotFound(anyhow::anyhow!("timeline not found").into()),
             Timeout => ApiError::Timeout("hit pageserver internal timeout".into()),
+            e @ HasArchivedParent(_) => {
+                ApiError::PreconditionFailed(e.to_string().into_boxed_str())
+            }
             HasUnarchivedChildren(children) => ApiError::PreconditionFailed(
                 format!(
                     "Cannot archive timeline which has non-archived child timelines: {children:?}"

pageserver/src/tenant.rs

@@ -509,6 +509,9 @@ pub enum TimelineArchivalError {
     #[error("Timeout")]
     Timeout,
 
+    #[error("ancestor is archived: {}", .0)]
+    HasArchivedParent(TimelineId),
+
     #[error("HasUnarchivedChildren")]
     HasUnarchivedChildren(Vec<TimelineId>),
 
@@ -524,6 +527,7 @@ impl Debug for TimelineArchivalError {
         match self {
             Self::NotFound => write!(f, "NotFound"),
             Self::Timeout => write!(f, "Timeout"),
+            Self::HasArchivedParent(p) => f.debug_tuple("HasArchivedParent").field(p).finish(),
             Self::HasUnarchivedChildren(c) => {
                 f.debug_tuple("HasUnarchivedChildren").field(c).finish()
             }
@@ -1369,11 +1373,20 @@ impl Tenant {
         let timeline = {
             let timelines = self.timelines.lock().unwrap();
 
-            let timeline = match timelines.get(&timeline_id) {
-                Some(t) => t,
-                None => return Err(TimelineArchivalError::NotFound),
+            let Some(timeline) = timelines.get(&timeline_id) else {
+                return Err(TimelineArchivalError::NotFound);
             };
 
+            if state == TimelineArchivalState::Unarchived {
+                if let Some(ancestor_timeline) = timeline.ancestor_timeline() {
+                    if ancestor_timeline.is_archived() == Some(true) {
+                        return Err(TimelineArchivalError::HasArchivedParent(
+                            ancestor_timeline.timeline_id,
+                        ));
+                    }
+                }
+            }
+
             // Ensure that there are no non-archived child timelines
             let children: Vec<TimelineId> = timelines
                 .iter()

pageserver/src/tenant/timeline.rs

@@ -867,6 +867,11 @@ impl Timeline {
             .map(|ancestor| ancestor.timeline_id)
     }
 
+    /// Get the ancestor timeline
+    pub(crate) fn ancestor_timeline(&self) -> Option<&Arc<Timeline>> {
+        self.ancestor_timeline.as_ref()
+    }
+
     /// Get the bytes written since the PITR cutoff on this branch, and
     /// whether this branch's ancestor_lsn is within its parent's PITR.
     pub(crate) fn get_pitr_history_stats(&self) -> (u64, bool) {

test_runner/regress/test_timeline_archive.py

@@ -94,3 +94,29 @@ def test_timeline_archive(neon_simple_env: NeonEnv):
         timeline_id=parent_timeline_id,
         state=TimelineArchivalState.ARCHIVED,
     )
+
+    # Test that the leaf can't be unarchived
+    with pytest.raises(
+        PageserverApiException,
+        match="ancestor is archived",
+    ) as exc:
+        assert timeline_path.exists()
+
+        ps_http.timeline_archival_config(
+            tenant_id=env.initial_tenant,
+            timeline_id=leaf_timeline_id,
+            state=TimelineArchivalState.UNARCHIVED,
+        )
+
+    # Unarchive works for the leaf if the parent gets unarchived first
+    ps_http.timeline_archival_config(
+        tenant_id=env.initial_tenant,
+        timeline_id=parent_timeline_id,
+        state=TimelineArchivalState.UNARCHIVED,
+    )
+
+    ps_http.timeline_archival_config(
+        tenant_id=env.initial_tenant,
+        timeline_id=leaf_timeline_id,
+        state=TimelineArchivalState.UNARCHIVED,
+    )