From 0c8ed229397af4137dfab6a8d5968709bd76c183 Mon Sep 17 00:00:00 2001
From: Anastasia Lubennikova <anastasia@neon.tech>
Date: Tue, 28 Feb 2023 15:41:46 +0200
Subject: [PATCH] Use saturating_sub for resident_physical_size_gauge to avoid
 metric overflow, if the gauge is not initialized yet.

---
 pageserver/src/tenant/timeline.rs | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/pageserver/src/tenant/timeline.rs b/pageserver/src/tenant/timeline.rs
index d46ac26e7d..cd2c9b5b2c 100644
--- a/pageserver/src/tenant/timeline.rs
+++ b/pageserver/src/tenant/timeline.rs
@@ -1082,7 +1082,10 @@ impl Timeline {
                 }
 
                 if let Some(layer_size) = layer_size {
-                    self.metrics.resident_physical_size_gauge.sub(layer_size);
+                    let current_size = self.metrics.resident_physical_size_gauge.get();
+                    self.metrics
+                        .resident_physical_size_gauge
+                        .set(current_size.saturating_sub(layer_size));
                 }
 
                 true
@@ -1504,7 +1507,11 @@ impl Timeline {
                             assert!(local_layer_path.exists(), "we would leave the local_layer without a file if this does not hold: {}", local_layer_path.display());
                             anyhow::bail!("could not rename file {local_layer_path:?}: {err:?}");
                         } else {
-                            self.metrics.resident_physical_size_gauge.sub(local_size);
+                            let current_size = self.metrics.resident_physical_size_gauge.get();
+                            self.metrics
+                                .resident_physical_size_gauge
+                                .set(current_size.saturating_sub(local_size));
+
                             updates.remove_historic(local_layer);
                             // fall-through to adding the remote layer
                         }
@@ -1946,7 +1953,10 @@ impl Timeline {
 
         layer.delete()?;
         if let Some(layer_size) = layer_size {
-            self.metrics.resident_physical_size_gauge.sub(layer_size);
+            let current_size = self.metrics.resident_physical_size_gauge.get();
+            self.metrics
+                .resident_physical_size_gauge
+                .set(current_size.saturating_sub(layer_size));
         }
 
         // TODO Removing from the bottom of the layer map is expensive.