pageserver: add deletion queue

pageserver/src/control_plane_client.rs

@@ -83,15 +83,15 @@ impl ControlPlaneClient {
                     .json(&request)
                     .send()
                     .await
-                    .map_err(|e| RemoteAttemptError::Remote(e))?;
+                    .map_err(RemoteAttemptError::Remote)?;
 
                 response
                     .error_for_status_ref()
-                    .map_err(|e| RemoteAttemptError::Remote(e))?;
+                    .map_err(RemoteAttemptError::Remote)?;
                 response
                     .json::<T>()
                     .await
-                    .map_err(|e| RemoteAttemptError::Remote(e))
+                    .map_err(RemoteAttemptError::Remote)
             },
             |_| false,
             3,
@@ -128,11 +128,11 @@ impl ControlPlaneGenerationsApi for ControlPlaneClient {
             response.tenants.len()
         );
 
-        return Ok(response
+        Ok(response
             .tenants
             .into_iter()
             .map(|t| (t.id, Generation::new(t.generation)))
-            .collect::<HashMap<_, _>>());
+            .collect::<HashMap<_, _>>())
     }
 
     async fn validate(

pageserver/src/deletion_queue/backend.rs

@@ -0,0 +1,330 @@
+use std::collections::HashMap;
+use std::sync::Arc;
+use std::time::Duration;
+
+use tokio_util::sync::CancellationToken;
+use tracing::debug;
+use tracing::info;
+use tracing::warn;
+
+use crate::config::PageServerConf;
+use crate::control_plane_client::ControlPlaneGenerationsApi;
+use crate::metrics::DELETION_QUEUE_DROPPED;
+use crate::metrics::DELETION_QUEUE_ERRORS;
+
+use super::executor::ExecutorMessage;
+use super::DeletionHeader;
+use super::DeletionList;
+use super::DeletionQueueError;
+use super::FlushOp;
+use super::VisibleLsnUpdates;
+
+// After this length of time, do any validation work that is pending,
+// even if we haven't accumulated many keys to delete.
+//
+// This also causes updates to remote_consistent_lsn to be validated, even
+// if there were no deletions enqueued.
+const AUTOFLUSH_INTERVAL: Duration = Duration::from_secs(10);
+
+// If we have received this number of keys, proceed with attempting to execute
+const AUTOFLUSH_KEY_COUNT: usize = 16384;
+
+#[derive(Debug)]
+pub(super) enum BackendQueueMessage {
+    Delete(DeletionList),
+    Flush(FlushOp),
+}
+pub struct BackendQueueWorker {
+    conf: &'static PageServerConf,
+    rx: tokio::sync::mpsc::Receiver<BackendQueueMessage>,
+    tx: tokio::sync::mpsc::Sender<ExecutorMessage>,
+
+    // Client for calling into control plane API for validation of deletes
+    control_plane_client: Option<Arc<dyn ControlPlaneGenerationsApi + Send + Sync>>,
+
+    // DeletionLists which are waiting generation validation.  Not safe to
+    // execute until [`validate`] has processed them.
+    pending_lists: Vec<DeletionList>,
+
+    // DeletionLists which have passed validation and are ready to execute.
+    validated_lists: Vec<DeletionList>,
+
+    // Sum of all the lengths of lists in pending_lists
+    pending_key_count: usize,
+
+    // Lsn validation state: we read projected LSNs and write back visible LSNs
+    // after validation.  This is the LSN equivalent of `pending_validation_lists`:
+    // it is drained in [`validate`]
+    lsn_table: Arc<std::sync::RwLock<VisibleLsnUpdates>>,
+
+    cancel: CancellationToken,
+}
+
+impl BackendQueueWorker {
+    pub(super) fn new(
+        conf: &'static PageServerConf,
+        rx: tokio::sync::mpsc::Receiver<BackendQueueMessage>,
+        tx: tokio::sync::mpsc::Sender<ExecutorMessage>,
+        control_plane_client: Option<Arc<dyn ControlPlaneGenerationsApi + Send + Sync>>,
+        lsn_table: Arc<std::sync::RwLock<VisibleLsnUpdates>>,
+        cancel: CancellationToken,
+    ) -> Self {
+        Self {
+            conf,
+            rx,
+            tx,
+            control_plane_client,
+            lsn_table,
+            pending_lists: Vec::new(),
+            validated_lists: Vec::new(),
+            pending_key_count: 0,
+            cancel,
+        }
+    }
+
+    async fn cleanup_lists(&mut self, lists: Vec<DeletionList>) {
+        for list in lists {
+            let list_path = self.conf.deletion_list_path(list.sequence);
+            debug!("Removing deletion list {list} at {}", list_path.display());
+
+            if let Err(e) = tokio::fs::remove_file(&list_path).await {
+                // Unexpected: we should have permissions and nothing else should
+                // be touching these files.  We will leave the file behind.  Subsequent
+                // pageservers will try and load it again: hopefully whatever storage
+                // issue (probably permissions) has been fixed by then.
+                tracing::error!("Failed to delete {}: {e:#}", list_path.display());
+                break;
+            }
+        }
+    }
+
+    /// Process any outstanding validations of generations of pending LSN updates or pending
+    /// DeletionLists.
+    ///
+    /// Valid LSN updates propagate back to their result channel immediately, valid DeletionLists
+    /// go into the queue of ready-to-execute lists.
+    pub async fn validate(&mut self) -> Result<(), DeletionQueueError> {
+        let mut tenant_generations = HashMap::new();
+        for list in &self.pending_lists {
+            for (tenant_id, tenant_list) in &list.tenants {
+                // Note: DeletionLists are in logical time order, so generation always
+                // goes up.  By doing a simple insert() we will always end up with
+                // the latest generation seen for a tenant.
+                tenant_generations.insert(*tenant_id, tenant_list.generation);
+            }
+        }
+
+        let pending_lsn_updates = {
+            let mut lsn_table = self.lsn_table.write().expect("Lock should not be poisoned");
+            let mut pending_updates = VisibleLsnUpdates::new();
+            std::mem::swap(&mut pending_updates, &mut lsn_table);
+            pending_updates
+        };
+        for (tenant_id, update) in &pending_lsn_updates.tenants {
+            let entry = tenant_generations
+                .entry(*tenant_id)
+                .or_insert(update.generation);
+            if update.generation > *entry {
+                *entry = update.generation;
+            }
+        }
+
+        if tenant_generations.is_empty() {
+            // No work to do
+            return Ok(());
+        }
+
+        let tenants_valid = if let Some(control_plane_client) = &self.control_plane_client {
+            control_plane_client
+                .validate(tenant_generations.iter().map(|(k, v)| (*k, *v)).collect())
+                .await
+                // The only wait a validation call returns an error is when the cancellation token fires
+                .map_err(|_| DeletionQueueError::ShuttingDown)?
+        } else {
+            // Control plane API disabled.  In legacy mode we consider everything valid.
+            tenant_generations.keys().map(|k| (*k, true)).collect()
+        };
+
+        let mut validated_sequence: Option<u64> = None;
+
+        // Apply the validation results to the pending LSN updates
+        for (tenant_id, tenant_lsn_state) in pending_lsn_updates.tenants {
+            let validated_generation = tenant_generations
+                .get(&tenant_id)
+                .expect("Map was built from the same keys we're reading");
+
+            // If the tenant was missing from the validation response, it has been deleted.  We may treat
+            // deletions as valid as the tenant's remote storage is all to be wiped anyway.
+            let valid = tenants_valid.get(&tenant_id).copied().unwrap_or(true);
+
+            if valid && *validated_generation == tenant_lsn_state.generation {
+                for (_timeline_id, pending_lsn) in tenant_lsn_state.timelines {
+                    // Drop result of send: it is legal for the Timeline to have been dropped along
+                    // with its queue receiver while we were doing validation.
+                    drop(pending_lsn.result_tx.send(pending_lsn.projected).await);
+                }
+            } else {
+                // If we failed validation, then do not apply any of the projected updates
+                warn!("Dropped remote consistent LSN updates for tenant {tenant_id} in stale generation {0:?}", tenant_lsn_state.generation);
+            }
+        }
+
+        // Apply the validation results to the pending deletion lists
+        for list in &mut self.pending_lists {
+            // Filter the list based on whether the server responded valid: true.
+            // If a tenant is omitted in the response, it has been deleted, and we should
+            // proceed with deletion.
+            let mut mutated = false;
+            list.tenants.retain(|tenant_id, tenant| {
+                let validated_generation = tenant_generations
+                    .get(tenant_id)
+                    .expect("Map was built from the same keys we're reading");
+
+                // If the tenant was missing from the validation response, it has been deleted.  We may treat
+                // deletions as valid as the tenant's remote storage is all to be wiped anyway.
+                let valid = tenants_valid.get(tenant_id).copied().unwrap_or(true);
+
+                // A list is valid if it comes from the current _or previous_ generation.
+                // The previous generation case is due to how we store deletion lists locally:
+                // if we see the immediately previous generation in a locally stored deletion list,
+                // it proves that this node's disk was used for both current & previous generations,
+                // and therefore no other node was involved in between: the two generations may be
+                // logically treated as the same.
+                let this_list_valid = valid
+                    && (tenant.generation == *validated_generation);
+
+                if !this_list_valid {
+                    warn!("Dropping stale deletions for tenant {tenant_id} in generation {:?}, objects may be leaked", tenant.generation);
+                    DELETION_QUEUE_DROPPED.inc_by(tenant.len() as u64);
+                    mutated = true;
+                }
+                this_list_valid
+            });
+            list.validated = true;
+
+            if mutated {
+                // Save the deletion list if we had to make changes due to stale generations.  The
+                // saved list is valid for execution.
+                if let Err(e) = list.save(self.conf).await {
+                    // Highly unexpected.  Could happen if e.g. disk full.
+                    // If we didn't save the trimmed list, it is _not_ valid to execute.
+                    warn!("Failed to save modified deletion list {list}: {e:#}");
+
+                    // Rather than have a complex retry process, just drop it and leak the objects,
+                    // scrubber will clean up eventually.
+                    list.tenants.clear(); // Result is a valid-but-empty list, which is a no-op for execution.
+                }
+            }
+
+            validated_sequence = Some(list.sequence);
+        }
+
+        if let Some(validated_sequence) = validated_sequence {
+            // Write the queue header to record how far validation progressed.  This avoids having
+            // to rewrite each DeletionList to set validated=true in it.
+            let header = DeletionHeader::new(validated_sequence);
+
+            // Drop result because the validated_sequence is an optimization.  If we fail to save it,
+            // then restart, we will drop some deletion lists, creating work for scrubber.
+            // The save() function logs a warning on error.
+            if let Err(e) = header.save(self.conf).await {
+                warn!("Failed to write deletion queue header: {e:#}");
+                DELETION_QUEUE_ERRORS
+                    .with_label_values(&["put_header"])
+                    .inc();
+            }
+        }
+
+        // Transfer the validated lists to the validated queue, for eventual execution
+        self.validated_lists.append(&mut self.pending_lists);
+
+        Ok(())
+    }
+
+    pub async fn flush(&mut self) {
+        // Issue any required generation validation calls to the control plane
+        if let Err(DeletionQueueError::ShuttingDown) = self.validate().await {
+            warn!("Shutting down");
+            return;
+        }
+
+        // After successful validation, nothing is pending: any lists that
+        // made it through validation will be in validated_lists.
+        assert!(self.pending_lists.is_empty());
+        self.pending_key_count = 0;
+
+        // Return quickly if we have no validated lists to execute.
+        if self.validated_lists.is_empty() {
+            return;
+        }
+
+        // Drain `validated_lists` into the executor
+        let mut executing_lists = Vec::new();
+        for mut list in self.validated_lists.drain(..) {
+            let objects = list.drain_paths();
+            if let Err(_e) = self.tx.send(ExecutorMessage::Delete(objects)).await {
+                warn!("Shutting down");
+                return;
+            };
+
+            executing_lists.push(list);
+        }
+
+        // Flush the executor, so that all the keys referenced by these deletion lists
+        // are actually removed from remote storage.  This is a precondition to deleting
+        // the deletion lists themselves.
+        let (tx, rx) = tokio::sync::oneshot::channel::<()>();
+        let flush_op = FlushOp { tx };
+        if let Err(_e) = self.tx.send(ExecutorMessage::Flush(flush_op)).await {
+            warn!("Shutting down");
+            return;
+        };
+        if rx.await.is_err() {
+            warn!("Shutting down");
+            return;
+        }
+
+        // Erase the deletion lists whose keys have all be deleted from remote storage
+        self.cleanup_lists(executing_lists).await;
+    }
+
+    pub async fn background(&mut self) {
+        while !self.cancel.is_cancelled() {
+            let msg = match tokio::time::timeout(AUTOFLUSH_INTERVAL, self.rx.recv()).await {
+                Ok(Some(m)) => m,
+                Ok(None) => {
+                    // All queue senders closed
+                    info!("Shutting down");
+                    break;
+                }
+                Err(_) => {
+                    // Timeout, we hit deadline to execute whatever we have in hand.  These functions will
+                    // return immediately if no work is pending
+                    self.flush().await;
+
+                    continue;
+                }
+            };
+
+            match msg {
+                BackendQueueMessage::Delete(list) => {
+                    if list.validated {
+                        self.validated_lists.push(list)
+                    } else {
+                        self.pending_key_count += list.len();
+                        self.pending_lists.push(list);
+                    }
+
+                    if self.pending_key_count > AUTOFLUSH_KEY_COUNT {
+                        self.flush().await;
+                    }
+                }
+                BackendQueueMessage::Flush(op) => {
+                    self.flush().await;
+
+                    op.fire();
+                }
+            }
+        }
+    }
+}

pageserver/src/deletion_queue/executor.rs

@@ -0,0 +1,143 @@
+use remote_storage::GenericRemoteStorage;
+use remote_storage::RemotePath;
+use remote_storage::MAX_KEYS_PER_DELETE;
+use std::time::Duration;
+use tokio_util::sync::CancellationToken;
+use tracing::info;
+use tracing::warn;
+
+use crate::metrics::DELETION_QUEUE_ERRORS;
+use crate::metrics::DELETION_QUEUE_EXECUTED;
+
+use super::DeletionQueueError;
+use super::FlushOp;
+
+const AUTOFLUSH_INTERVAL: Duration = Duration::from_secs(10);
+
+pub(super) enum ExecutorMessage {
+    Delete(Vec<RemotePath>),
+    Flush(FlushOp),
+}
+
+/// Non-persistent deletion queue, for coalescing multiple object deletes into
+/// larger DeleteObjects requests.
+pub struct ExecutorWorker {
+    // Accumulate up to 1000 keys for the next deletion operation
+    accumulator: Vec<RemotePath>,
+
+    rx: tokio::sync::mpsc::Receiver<ExecutorMessage>,
+
+    cancel: CancellationToken,
+    remote_storage: GenericRemoteStorage,
+}
+
+impl ExecutorWorker {
+    pub(super) fn new(
+        remote_storage: GenericRemoteStorage,
+        rx: tokio::sync::mpsc::Receiver<ExecutorMessage>,
+        cancel: CancellationToken,
+    ) -> Self {
+        Self {
+            remote_storage,
+            rx,
+            cancel,
+            accumulator: Vec::new(),
+        }
+    }
+
+    /// Wrap the remote `delete_objects` with a failpoint
+    pub async fn remote_delete(&self) -> Result<(), anyhow::Error> {
+        fail::fail_point!("deletion-queue-before-execute", |_| {
+            info!("Skipping execution, failpoint set");
+            DELETION_QUEUE_ERRORS
+                .with_label_values(&["failpoint"])
+                .inc();
+            Err(anyhow::anyhow!("failpoint hit"))
+        });
+
+        self.remote_storage.delete_objects(&self.accumulator).await
+    }
+
+    /// Block until everything in accumulator has been executed
+    pub async fn flush(&mut self) -> Result<(), DeletionQueueError> {
+        while !self.accumulator.is_empty() && !self.cancel.is_cancelled() {
+            match self.remote_delete().await {
+                Ok(()) => {
+                    // Note: we assume that the remote storage layer returns Ok(()) if some
+                    // or all of the deleted objects were already gone.
+                    DELETION_QUEUE_EXECUTED.inc_by(self.accumulator.len() as u64);
+                    info!(
+                        "Executed deletion batch {}..{}",
+                        self.accumulator
+                            .first()
+                            .expect("accumulator should be non-empty"),
+                        self.accumulator
+                            .last()
+                            .expect("accumulator should be non-empty"),
+                    );
+                    self.accumulator.clear();
+                }
+                Err(e) => {
+                    warn!("DeleteObjects request failed: {e:#}, will retry");
+                    DELETION_QUEUE_ERRORS.with_label_values(&["execute"]).inc();
+                }
+            };
+        }
+        if self.cancel.is_cancelled() {
+            // Expose an error because we may not have actually flushed everything
+            Err(DeletionQueueError::ShuttingDown)
+        } else {
+            Ok(())
+        }
+    }
+
+    pub async fn background(&mut self) -> Result<(), DeletionQueueError> {
+        self.accumulator.reserve(MAX_KEYS_PER_DELETE);
+
+        loop {
+            if self.cancel.is_cancelled() {
+                return Err(DeletionQueueError::ShuttingDown);
+            }
+
+            let msg = match tokio::time::timeout(AUTOFLUSH_INTERVAL, self.rx.recv()).await {
+                Ok(Some(m)) => m,
+                Ok(None) => {
+                    // All queue senders closed
+                    info!("Shutting down");
+                    return Err(DeletionQueueError::ShuttingDown);
+                }
+                Err(_) => {
+                    // Timeout, we hit deadline to execute whatever we have in hand.  These functions will
+                    // return immediately if no work is pending
+                    self.flush().await?;
+
+                    continue;
+                }
+            };
+
+            match msg {
+                ExecutorMessage::Delete(mut list) => {
+                    while !list.is_empty() || self.accumulator.len() == MAX_KEYS_PER_DELETE {
+                        if self.accumulator.len() == MAX_KEYS_PER_DELETE {
+                            self.flush().await?;
+                            // If we have received this number of keys, proceed with attempting to execute
+                            assert_eq!(self.accumulator.len(), 0);
+                        }
+
+                        let available_slots = MAX_KEYS_PER_DELETE - self.accumulator.len();
+                        let take_count = std::cmp::min(available_slots, list.len());
+                        for path in list.drain(list.len() - take_count..) {
+                            self.accumulator.push(path);
+                        }
+                    }
+                }
+                ExecutorMessage::Flush(flush_op) => {
+                    // If flush() errors, we drop the flush_op and the caller will get
+                    // an error recv()'ing their oneshot channel.
+                    self.flush().await?;
+                    flush_op.fire();
+                }
+            }
+        }
+    }
+}

pageserver/src/deletion_queue/frontend.rs

@@ -0,0 +1,430 @@
+use super::BackendQueueMessage;
+use super::DeletionHeader;
+use super::DeletionList;
+use super::FlushOp;
+
+use std::collections::HashMap;
+use std::fs::create_dir_all;
+use std::time::Duration;
+
+use regex::Regex;
+use remote_storage::RemotePath;
+use tokio_util::sync::CancellationToken;
+use tracing::debug;
+use tracing::info;
+use tracing::warn;
+use utils::generation::Generation;
+use utils::id::TenantId;
+use utils::id::TimelineId;
+
+use crate::config::PageServerConf;
+use crate::metrics::DELETION_QUEUE_ERRORS;
+use crate::metrics::DELETION_QUEUE_SUBMITTED;
+use crate::tenant::remote_timeline_client::remote_layer_path;
+use crate::tenant::storage_layer::LayerFileName;
+
+// The number of keys in a DeletionList before we will proactively persist it
+// (without reaching a flush deadline).  This aims to deliver objects of the order
+// of magnitude 1MB when we are under heavy delete load.
+const DELETION_LIST_TARGET_SIZE: usize = 16384;
+
+// Ordinarily, we only flush to DeletionList periodically, to bound the window during
+// which we might leak objects from not flushing a DeletionList after
+// the objects are already unlinked from timeline metadata.
+const FRONTEND_DEFAULT_TIMEOUT: Duration = Duration::from_millis(10000);
+
+// If someone is waiting for a flush to DeletionList, only delay a little to accumulate
+// more objects before doing the flush.
+const FRONTEND_FLUSHING_TIMEOUT: Duration = Duration::from_millis(100);
+
+#[derive(Debug)]
+pub(super) struct DeletionOp {
+    pub(super) tenant_id: TenantId,
+    pub(super) timeline_id: TimelineId,
+    // `layers` and `objects` are both just lists of objects.  `layers` is used if you do not
+    // have a config object handy to project it to a remote key, and need the consuming worker
+    // to do it for you.
+    pub(super) layers: Vec<(LayerFileName, Generation)>,
+    pub(super) objects: Vec<RemotePath>,
+
+    /// The _current_ generation of the Tenant attachment in which we are enqueuing
+    /// this deletion.
+    pub(super) generation: Generation,
+}
+
+#[derive(Debug)]
+pub(super) struct RecoverOp {
+    pub(super) attached_tenants: HashMap<TenantId, Generation>,
+}
+
+#[derive(Debug)]
+pub(super) enum FrontendQueueMessage {
+    Delete(DeletionOp),
+    // Wait until all prior deletions make it into a persistent DeletionList
+    Flush(FlushOp),
+    // Wait until all prior deletions have been executed (i.e. objects are actually deleted)
+    FlushExecute(FlushOp),
+    // Call once after re-attaching to control plane, to notify the deletion queue about
+    // latest attached generations & load any saved deletion lists from disk.
+    Recover(RecoverOp),
+}
+
+pub struct FrontendQueueWorker {
+    conf: &'static PageServerConf,
+
+    // Incoming frontend requests to delete some keys
+    rx: tokio::sync::mpsc::Receiver<FrontendQueueMessage>,
+
+    // Outbound requests to the backend to execute deletion lists we have composed.
+    tx: tokio::sync::mpsc::Sender<BackendQueueMessage>,
+
+    // The list we are currently building, contains a buffer of keys to delete
+    // and our next sequence number
+    pending: DeletionList,
+
+    // These FlushOps should fire the next time we flush
+    pending_flushes: Vec<FlushOp>,
+
+    // Worker loop is torn down when this fires.
+    cancel: CancellationToken,
+}
+
+impl FrontendQueueWorker {
+    // Initially DeletionHeader.validated_sequence is zero.  The place we start our
+    // sequence numbers must be higher than that.
+    const BASE_SEQUENCE: u64 = 1;
+
+    pub(super) fn new(
+        conf: &'static PageServerConf,
+        rx: tokio::sync::mpsc::Receiver<FrontendQueueMessage>,
+        tx: tokio::sync::mpsc::Sender<BackendQueueMessage>,
+        cancel: CancellationToken,
+    ) -> Self {
+        Self {
+            pending: DeletionList::new(Self::BASE_SEQUENCE),
+            conf,
+            rx,
+            tx,
+            pending_flushes: Vec::new(),
+            cancel,
+        }
+    }
+
+    /// Try to flush `list` to persistent storage
+    ///
+    /// This does not return errors, because on failure to flush we do not lose
+    /// any state: flushing will be retried implicitly on the next deadline
+    async fn flush(&mut self) {
+        if self.pending.is_empty() {
+            for f in self.pending_flushes.drain(..) {
+                f.fire();
+            }
+            return;
+        }
+
+        match self.pending.save(self.conf).await {
+            Ok(_) => {
+                info!(sequence = self.pending.sequence, "Stored deletion list");
+
+                for f in self.pending_flushes.drain(..) {
+                    f.fire();
+                }
+
+                let onward_list = self.pending.drain();
+
+                // We have consumed out of pending: reset it for the next incoming deletions to accumulate there
+                self.pending = DeletionList::new(self.pending.sequence + 1);
+
+                if let Err(e) = self.tx.send(BackendQueueMessage::Delete(onward_list)).await {
+                    // This is allowed to fail: it will only happen if the backend worker is shut down,
+                    // so we can just drop this on the floor.
+                    info!("Deletion list dropped, this is normal during shutdown ({e:#})");
+                }
+            }
+            Err(e) => {
+                DELETION_QUEUE_ERRORS.with_label_values(&["put_list"]).inc();
+                warn!(
+                    sequence = self.pending.sequence,
+                    "Failed to write deletion list, will retry later ({e:#})"
+                );
+            }
+        }
+    }
+
+    /// Load the header, to learn the sequence number up to which deletions
+    /// have been validated.  We will apply validated=true to DeletionLists
+    /// <= this sequence when loading them.
+    ///
+    /// It is not an error for the header to not exist: we return None, and
+    /// the caller should act as if validated_sequence is 0
+    async fn load_validated_sequence(&self) -> Result<Option<u64>, anyhow::Error> {
+        let header_path = self.conf.deletion_header_path();
+        match tokio::fs::read(&header_path).await {
+            Ok(header_bytes) => {
+                match serde_json::from_slice::<DeletionHeader>(&header_bytes) {
+                    Ok(h) => Ok(Some(h.validated_sequence)),
+                    Err(e) => {
+                        warn!(
+                            "Failed to deserialize deletion header, ignoring {}: {e:#}",
+                            header_path.display()
+                        );
+                        // This should never happen unless we make a mistake with our serialization.
+                        // Ignoring a deletion header is not consequential for correctnes because all deletions
+                        // are ultimately allowed to fail: worst case we leak some objects for the scrubber to clean up.
+                        Ok(None)
+                    }
+                }
+            }
+            Err(e) => {
+                if e.kind() == std::io::ErrorKind::NotFound {
+                    debug!(
+                        "Deletion header {} not found, first start?",
+                        header_path.display()
+                    );
+                    Ok(None)
+                } else {
+                    Err(anyhow::anyhow!(e))
+                }
+            }
+        }
+    }
+
+    async fn recover(
+        &mut self,
+        attached_tenants: HashMap<TenantId, Generation>,
+    ) -> Result<(), anyhow::Error> {
+        debug!(
+            "recovering with {} attached tenants",
+            attached_tenants.len()
+        );
+
+        // Load the header
+        let validated_sequence = self.load_validated_sequence().await?.unwrap_or(0);
+
+        // Start our next deletion list from after the last location validated by
+        // previous process lifetime, or after the last location found (it is updated
+        // below after enumerating the deletion lists)
+        self.pending.sequence = std::cmp::max(self.pending.sequence, validated_sequence + 1);
+
+        let deletion_directory = self.conf.deletion_prefix();
+        let mut dir = match tokio::fs::read_dir(&deletion_directory).await {
+            Ok(d) => d,
+            Err(e) => {
+                warn!(
+                    "Failed to open deletion list directory {}: {e:#}",
+                    deletion_directory.display(),
+                );
+
+                // Give up: if we can't read the deletion list directory, we probably can't
+                // write lists into it later, so the queue won't work.
+                return Err(e.into());
+            }
+        };
+
+        let list_name_pattern = Regex::new("([a-zA-Z0-9]{16})-([a-zA-Z0-9]{2}).list").unwrap();
+
+        let header_path = self.conf.deletion_header_path();
+        let mut seqs: Vec<u64> = Vec::new();
+        while let Some(dentry) = dir.next_entry().await? {
+            if Some(dentry.file_name().as_os_str()) == header_path.file_name() {
+                // Don't try and parse the header's name like a list
+                continue;
+            }
+
+            let file_name = dentry.file_name().to_owned();
+            let basename = file_name.to_string_lossy();
+            let seq_part = if let Some(m) = list_name_pattern.captures(&basename) {
+                m.get(1)
+                    .expect("Non optional group should be present")
+                    .as_str()
+            } else {
+                warn!("Unexpected key in deletion queue: {basename}");
+                continue;
+            };
+
+            let seq: u64 = match u64::from_str_radix(seq_part, 16) {
+                Ok(s) => s,
+                Err(e) => {
+                    warn!("Malformed key '{basename}': {e}");
+                    continue;
+                }
+            };
+            seqs.push(seq);
+        }
+        seqs.sort();
+
+        // Initialize the next sequence number in the frontend based on the maximum of the highest list we see,
+        // and the last list that was deleted according to the header.  Combined with writing out the header
+        // prior to deletions, this guarnatees no re-use of sequence numbers.
+        if let Some(max_list_seq) = seqs.last() {
+            self.pending.sequence = std::cmp::max(self.pending.sequence, max_list_seq + 1);
+        }
+
+        for s in seqs {
+            let list_path = self.conf.deletion_list_path(s);
+
+            let list_bytes = tokio::fs::read(&list_path).await?;
+
+            let mut deletion_list = match serde_json::from_slice::<DeletionList>(&list_bytes) {
+                Ok(l) => l,
+                Err(e) => {
+                    // Drop the list on the floor: any objects it referenced will be left behind
+                    // for scrubbing to clean up.  This should never happen unless we have a serialization bug.
+                    warn!(sequence = s, "Failed to deserialize deletion list: {e}");
+                    continue;
+                }
+            };
+
+            if deletion_list.sequence <= validated_sequence {
+                // If the deletion list falls below valid_seq, we may assume that it was
+                // already validated the last time this pageserver ran.  Otherwise, we still
+                // load it, as it may still contain content valid in this generation.
+                deletion_list.validated = true;
+            } else {
+                // Special case optimization: if a tenant is still attached, and no other
+                // generation was issued to another node in the interval while we restarted,
+                // then we may treat deletion lists from the previous generation as if they
+                // belong to our currently attached generation, and proceed to validate & execute.
+                for (tenant_id, tenant_list) in &mut deletion_list.tenants {
+                    if let Some(attached_gen) = attached_tenants.get(tenant_id) {
+                        if attached_gen.previous() == tenant_list.generation {
+                            tenant_list.generation = *attached_gen;
+                        }
+                    }
+                }
+            }
+
+            info!(
+                validated = deletion_list.validated,
+                sequence = deletion_list.sequence,
+                "Recovered deletion list"
+            );
+
+            // We will drop out of recovery if this fails: it indicates that we are shutting down
+            // or the backend has panicked
+            DELETION_QUEUE_SUBMITTED.inc_by(deletion_list.len() as u64);
+            self.tx
+                .send(BackendQueueMessage::Delete(deletion_list))
+                .await?;
+        }
+
+        info!(next_sequence = self.pending.sequence, "Replay complete");
+
+        Ok(())
+    }
+
+    /// This is the front-end ingest, where we bundle up deletion requests into DeletionList
+    /// and write them out, for later validation by the backend and execution by the executor.
+    pub async fn background(&mut self) {
+        info!("Started deletion frontend worker");
+
+        // Synchronous, but we only do it once per process lifetime so it's tolerable
+        if let Err(e) = create_dir_all(&self.conf.deletion_prefix()) {
+            tracing::error!(
+                "Failed to create deletion list directory {}, deletions will not be executed ({e})",
+                self.conf.deletion_prefix().display()
+            );
+            return;
+        }
+
+        while !self.cancel.is_cancelled() {
+            let timeout = if self.pending_flushes.is_empty() {
+                FRONTEND_DEFAULT_TIMEOUT
+            } else {
+                FRONTEND_FLUSHING_TIMEOUT
+            };
+
+            let msg = match tokio::time::timeout(timeout, self.rx.recv()).await {
+                Ok(Some(msg)) => msg,
+                Ok(None) => {
+                    // Queue sender destroyed, shutting down
+                    break;
+                }
+                Err(_) => {
+                    // Hit deadline, flush.
+                    self.flush().await;
+                    continue;
+                }
+            };
+
+            match msg {
+                FrontendQueueMessage::Delete(op) => {
+                    debug!(
+                        "Delete: ingesting {} layers, {} other objects",
+                        op.layers.len(),
+                        op.objects.len()
+                    );
+
+                    let mut layer_paths = Vec::new();
+                    for (layer, generation) in op.layers {
+                        layer_paths.push(remote_layer_path(
+                            &op.tenant_id,
+                            &op.timeline_id,
+                            &layer,
+                            generation,
+                        ));
+                    }
+                    layer_paths.extend(op.objects);
+
+                    if !self.pending.push(
+                        &op.tenant_id,
+                        &op.timeline_id,
+                        op.generation,
+                        &mut layer_paths,
+                    ) {
+                        self.flush().await;
+                        let retry_succeeded = self.pending.push(
+                            &op.tenant_id,
+                            &op.timeline_id,
+                            op.generation,
+                            &mut layer_paths,
+                        );
+                        if !retry_succeeded {
+                            // Unexpected: after we flush, we should have
+                            // drained self.pending, so a conflict on
+                            // generation numbers should be impossible.
+                            tracing::error!(
+                                "Failed to enqueue deletions, leaking objects.  This is a bug."
+                            );
+                        }
+                    }
+                }
+                FrontendQueueMessage::Flush(op) => {
+                    if self.pending.is_empty() {
+                        // Execute immediately
+                        debug!("Flush: No pending objects, flushing immediately");
+                        op.fire()
+                    } else {
+                        // Execute next time we flush
+                        debug!("Flush: adding to pending flush list for next deadline flush");
+                        self.pending_flushes.push(op);
+                    }
+                }
+                FrontendQueueMessage::FlushExecute(op) => {
+                    debug!("FlushExecute: passing through to backend");
+                    // We do not flush to a deletion list here: the client sends a Flush before the FlushExecute
+                    if let Err(e) = self.tx.send(BackendQueueMessage::Flush(op)).await {
+                        info!("Can't flush, shutting down ({e})");
+                        // Caller will get error when their oneshot sender was dropped.
+                    }
+                }
+                FrontendQueueMessage::Recover(op) => {
+                    if let Err(e) = self.recover(op.attached_tenants).await {
+                        // This should only happen in truly unrecoverable cases, like the recovery finding that the backend
+                        // queue receiver has been dropped, or something is critically broken with
+                        // the local filesystem holding deletion lists.
+                        info!(
+                            "Deletion queue recover aborted, deletion queue will not proceed ({e})"
+                        );
+                        return;
+                    }
+                }
+            }
+
+            if self.pending.len() > DELETION_LIST_TARGET_SIZE || !self.pending_flushes.is_empty() {
+                self.flush().await;
+            }
+        }
+        info!("Deletion queue shut down.");
+    }
+}

pageserver/src/tenant.rs

@@ -117,7 +117,7 @@ mod span;
 
 pub mod metadata;
 mod par_fsync;
-mod remote_timeline_client;
+pub mod remote_timeline_client;
 pub mod storage_layer;
 
 pub mod config;