From e6618f1cc0d9a93e12333bd1876bbff4d47e2e6c Mon Sep 17 00:00:00 2001
From: Joonas Koivunen <joonas@neon.tech>
Date: Tue, 14 Feb 2023 13:17:34 +0200
Subject: [PATCH] Update current logical size gauge (#3592)

Alternative to #3586.

Introduces usage of current_logical_size.current_size as a boundary after which we start to update the metric gauge on ingested wal. Previously any incremented value (ingested wal) would had updated the gauge, but this would had left the metric at zero for timelines which never receive any wal even if size had been calculated. Now the gauge is updated right away as the calculation completes, not requiring any wal to be received.
---
 pageserver/src/tenant/timeline.rs | 44 ++++++++++++++++++++-----------
 1 file changed, 29 insertions(+), 15 deletions(-)

diff --git a/pageserver/src/tenant/timeline.rs b/pageserver/src/tenant/timeline.rs
index bcbf8a12b4..5a829e42e5 100644
--- a/pageserver/src/tenant/timeline.rs
+++ b/pageserver/src/tenant/timeline.rs
@@ -292,18 +292,9 @@ impl LogicalSize {
         //                  we change the type.
         match self.initial_logical_size.get() {
             Some(initial_size) => {
-                let absolute_size_increment = u64::try_from(
-                    size_increment
-                        .checked_abs()
-                        .with_context(|| format!("Size added after initial {size_increment} is not expected to be i64::MIN"))?,
-                    ).expect("casting nonnegative i64 to u64 should not fail");
-
-                if size_increment < 0 {
-                    initial_size.checked_sub(absolute_size_increment)
-                } else {
-                    initial_size.checked_add(absolute_size_increment)
-                }.with_context(|| format!("Overflow during logical size calculation, initial_size: {initial_size}, size_increment: {size_increment}"))
-                .map(CurrentLogicalSize::Exact)
+                initial_size.checked_add_signed(size_increment)
+                    .with_context(|| format!("Overflow during logical size calculation, initial_size: {initial_size}, size_increment: {size_increment}"))
+                    .map(CurrentLogicalSize::Exact)
             }
             None => {
                 let non_negative_size_increment = u64::try_from(size_increment).unwrap_or(0);
@@ -1625,13 +1616,31 @@ impl Timeline {
                     }
                     x @ Err(_) => x.context("Failed to calculate logical size")?,
                 };
+
+                // we cannot query current_logical_size.current_size() to know the current
+                // *negative* value, only truncated to u64.
+                let added = self_clone
+                    .current_logical_size
+                    .size_added_after_initial
+                    .load(AtomicOrdering::Relaxed);
+
+                let sum = calculated_size.saturating_add_signed(added);
+
+                // set the gauge value before it can be set in `update_current_logical_size`.
+                self_clone.metrics.current_logical_size_gauge.set(sum);
+
                 match self_clone
                     .current_logical_size
                     .initial_logical_size
                     .set(calculated_size)
                 {
                     Ok(()) => (),
-                    Err(existing_size) => {
+                    Err(_what_we_just_attempted_to_set) => {
+                        let existing_size = self_clone
+                            .current_logical_size
+                            .initial_logical_size
+                            .get()
+                            .expect("once_cell set was lost, then get failed, impossible.");
                         // This shouldn't happen because the semaphore is initialized with 1.
                         // But if it happens, just complain & report success so there are no further retries.
                         error!("Tried to update initial timeline size value to {calculated_size}, but the size was already set to {existing_size}, not changing")
@@ -1814,10 +1823,15 @@ impl Timeline {
         // one value while current_logical_size is set to the
         // other.
         match logical_size.current_size() {
-            Ok(new_current_size) => self
+            Ok(CurrentLogicalSize::Exact(new_current_size)) => self
                 .metrics
                 .current_logical_size_gauge
-                .set(new_current_size.size()),
+                .set(new_current_size),
+            Ok(CurrentLogicalSize::Approximate(_)) => {
+                // don't update the gauge yet, this allows us not to update the gauge back and
+                // forth between the initial size calculation task.
+            }
+            // this is overflow
             Err(e) => error!("Failed to compute current logical size for metrics update: {e:?}"),
         }
     }