Purge the query cache to prevent GC livelocks (rerun-io#7370)

When the GC needs to reclaim memory but has already exhausted all the
data from the store, then purge the query cache directly.
The complete rationale for this patch is there:
rerun-io#7369 (comment).

Before:

![image](https://github.com/user-attachments/assets/0ee825d1-11a5-43ad-b4ac-4e9ee6134108)

After:

![image](https://github.com/user-attachments/assets/6bb83281-94e7-42ca-8cd6-5502bdefe47b)


- Fixes rerun-io#7369

### Checklist
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the [Code of
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* [x] I've included a screenshot or gif (if applicable)
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* Using examples from latest `main` build:
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* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7370?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
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checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
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- [PR Build Summary](https://build.rerun.io/pr/7370)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

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full-check`.

crates/store/re_entity_db/src/entity_db.rs

@@ -386,14 +386,21 @@ impl EntityDb {
         re_tracing::profile_function!();
 
         assert!((0.0..=1.0).contains(&fraction_to_purge));
-        self.gc(&GarbageCollectionOptions {
+        if !self.gc(&GarbageCollectionOptions {
             target: GarbageCollectionTarget::DropAtLeastFraction(fraction_to_purge as _),
             protect_latest: 1,
             time_budget: DEFAULT_GC_TIME_BUDGET,
-        });
+        }) {
+            // If we weren't able to collect any data, then we need to GC the cache itself in order
+            // to regain some space.
+            // See <https://github.com/rerun-io/rerun/issues/7369#issuecomment-2335164098> for the
+            // complete rationale.
+            self.query_caches.purge_fraction_of_ram(fraction_to_purge);
+        }
     }
 
-    pub fn gc(&mut self, gc_options: &GarbageCollectionOptions) {
+    /// Returns `true` if anything at all was actually GC'd.
+    pub fn gc(&mut self, gc_options: &GarbageCollectionOptions) -> bool {
         re_tracing::profile_function!();
 
         let (store_events, stats_diff) = self.data_store.gc(gc_options);
@@ -405,6 +412,8 @@ impl EntityDb {
         );
 
         self.on_store_deletions(&store_events);
+
+        !store_events.is_empty()
     }
 
     /// Unconditionally drops all the data for a given [`EntityPath`] .

crates/store/re_query/src/latest_at.rs

@@ -171,6 +171,28 @@ impl Caches {
 
         results
     }
+
+    /// Free up some RAM by forgetting the older parts of all timelines.
+    pub fn purge_fraction_of_ram(&mut self, fraction_to_purge: f32) {
+        re_tracing::profile_function!();
+
+        let mut caches = self.latest_at_per_cache_key.write();
+        for (_key, cache) in caches.iter_mut() {
+            let mut cache = cache.write();
+
+            let split_point =
+                (cache.per_query_time.len().saturating_sub(1) as f32 * fraction_to_purge) as usize;
+
+            if let Some(split_time) = cache.per_query_time.keys().nth(split_point).copied() {
+                // NOTE: By not clearing the pending invalidations set, we risk invalidating a
+                // future result that need not be invalidated.
+                // That is a much better outcome that the opposite though: not invalidating a
+                // future result that in fact should have been.
+                // See `handle_pending_invalidation` for more information.
+                cache.per_query_time = cache.per_query_time.split_off(&split_time);
+            }
+        }
+    }
 }
 
 // --- Results ---