Refactor hard linking into one spawn_blocking

pageserver/src/tenant/mgr.rs

@@ -1577,33 +1577,39 @@ impl TenantManager {
             }
             (parent_timelines, parent_layers)
         };
+
+        let mut child_prefixes = Vec::new();
+        let mut create_dirs = Vec::new();
+
         for child in child_shards {
             let child_prefix = self.conf.tenant_path(&child);
-            let mut create_dirs = vec![child_prefix.clone()];
+            create_dirs.push(child_prefix.clone());
             create_dirs.extend(
                 parent_timelines
                     .iter()
                     .map(|t| self.conf.timeline_path(&child, t)),
             );
-            let layers_clone = parent_layers.clone();
-            let parent_path = parent_path.clone();
 
-            // Since we will do a large number of small filesystem metadata operations, batch them into
-            // spawn_blocking calls rather than doing each one as a tokio::fs round-trip.
-            let jh = tokio::task::spawn_blocking(move || -> anyhow::Result<()> {
-                for dir in create_dirs {
-                    if let Err(e) = std::fs::create_dir_all(&dir) {
-                        // Ignore AlreadyExists errors, drop out on all other errors
-                        match e.kind() {
-                            std::io::ErrorKind::AlreadyExists => {}
-                            _ => {
-                                return Err(anyhow::anyhow!(e).context(format!("Creating {dir}")));
-                            }
+            child_prefixes.push(child_prefix);
+        }
+
+        // Since we will do a large number of small filesystem metadata operations, batch them into
+        // spawn_blocking calls rather than doing each one as a tokio::fs round-trip.
+        let jh = tokio::task::spawn_blocking(move || -> anyhow::Result<usize> {
+            for dir in create_dirs {
+                if let Err(e) = std::fs::create_dir_all(&dir) {
+                    // Ignore AlreadyExists errors, drop out on all other errors
+                    match e.kind() {
+                        std::io::ErrorKind::AlreadyExists => {}
+                        _ => {
+                            return Err(anyhow::anyhow!(e).context(format!("Creating {dir}")));
                         }
                     }
                 }
+            }
 
-                for relative_layer in layers_clone {
+            for child_prefix in child_prefixes {
+                for relative_layer in &parent_layers {
                     let parent_path = parent_path.join(&relative_layer);
                     let child_path = child_prefix.join(&relative_layer);
                     if let Err(e) = std::fs::hard_link(&parent_path, &child_path) {
@@ -1617,32 +1623,28 @@ impl TenantManager {
                             }
                             _ => {
                                 return Err(anyhow::anyhow!(e).context(format!(
-                                    "Hard linking {relative_layer} into {child}"
+                                    "Hard linking {relative_layer} into {child_prefix}"
                                 )))
                             }
                         }
                     }
                 }
+            }
 
-                Ok(())
-            });
+            Ok(parent_layers.len())
+        });
 
-            match jh.await {
-                Ok(Ok(())) => {
-                    tracing::info!(
-                        "Linked {} layers into child shard {}",
-                        parent_layers.len(),
-                        child
-                    );
-                }
-                Ok(Err(e)) => {
-                    // This is an optimization, so we tolerate failure.
-                    tracing::warn!("Error hard-linking layers, proceeding anyway: {e}")
-                }
-                Err(e) => {
-                    // This is something totally unexpected like a panic, so bail out.
-                    anyhow::bail!("Error joining hard linking task: {e}");
-                }
+        match jh.await {
+            Ok(Ok(layer_count)) => {
+                tracing::info!("Linked {layer_count} layers into child shards");
+            }
+            Ok(Err(e)) => {
+                // This is an optimization, so we tolerate failure.
+                tracing::warn!("Error hard-linking layers, proceeding anyway: {e}")
+            }
+            Err(e) => {
+                // This is something totally unexpected like a panic, so bail out.
+                anyhow::bail!("Error joining hard linking task: {e}");
             }
         }