From 8494288c425e470d12b5817a45a752e9aa8d78bc Mon Sep 17 00:00:00 2001
From: Anthony Ramine <nox@nox.paris>
Date: Thu, 13 Oct 2022 10:37:05 +0200
Subject: [PATCH 1/2] metrics/family: Fix race condition in
 `Family::get_or_create`

The method should not overwrite an existing key after it obtains the write lock.

Consider the following scenario with `Family<LabelSet, Gauge>` used by threads A and B:

1. A and B both call `family.get_or_insert(&label_set)`
2. A and B both acquire read lock and finds no key
3. A gets write lock and inserts a new gauge with value 0
4. A increments returned gauge to 1
5. B gets write lock and inserts a new gauge with value 0
6. B increments returned gauge to 1
7. A decrements gauge back to 0
8. B decrements gauge which now overflows to `u64::MAX`

Signed-off-by: Anthony Ramine <nox@nox.paris>
---
 CHANGELOG.md          |  8 ++++++++
 Cargo.toml            |  2 +-
 src/metrics/family.rs | 11 +++++++----
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 18229832..85ce2589 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,14 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.18.1] - unreleased
+
+### Fixed
+
+- Fix race condition in `Family::get_or_create`. See [PR 102].
+
+[PR 102]: https://github.com/prometheus/client_rust/pull/102
+
 ## [0.18.0]
 
 ### Changed
diff --git a/Cargo.toml b/Cargo.toml
index a114a050..41ba9dd8 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "prometheus-client"
-version = "0.18.0"
+version = "0.18.1"
 authors = ["Max Inden <mail@max-inden.de>"]
 edition = "2021"
 description = "Open Metrics client library allowing users to natively instrument applications."
diff --git a/src/metrics/family.rs b/src/metrics/family.rs
index fb5dbeb6..63d13f36 100644
--- a/src/metrics/family.rs
+++ b/src/metrics/family.rs
@@ -3,7 +3,7 @@
 //! See [`Family`] for details.
 
 use super::{MetricType, TypedMetric};
-use parking_lot::{MappedRwLockReadGuard, RwLock, RwLockReadGuard};
+use parking_lot::{MappedRwLockReadGuard, RwLock, RwLockReadGuard, RwLockWriteGuard};
 use std::collections::HashMap;
 use std::sync::Arc;
 
@@ -222,11 +222,14 @@ impl<S: Clone + std::hash::Hash + Eq, M, C: MetricConstructor<M>> Family<S, M, C
         }
 
         let mut write_guard = self.metrics.write();
-        write_guard.insert(label_set.clone(), self.constructor.new_metric());
 
-        drop(write_guard);
+        write_guard
+            .entry(label_set.clone())
+            .or_insert_with(|| self.constructor.new_metric());
 
-        RwLockReadGuard::map(self.metrics.read(), |metrics| {
+        let read_guard = RwLockWriteGuard::downgrade(write_guard);
+
+        RwLockReadGuard::map(read_guard, |metrics| {
             metrics
                 .get(label_set)
                 .expect("Metric to exist after creating it.")

From ffd3c3e0a447b9ced8795bb701f94e89c6bdeb08 Mon Sep 17 00:00:00 2001
From: Max Inden <mail@max-inden.de>
Date: Fri, 14 Oct 2022 10:25:32 +0100
Subject: [PATCH 2/2] CHANGELOG: Prepare v0.18.1

Signed-off-by: Max Inden <mail@max-inden.de>
---
 CHANGELOG.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 85ce2589..a17c7105 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [0.18.1] - unreleased
+## [0.18.1]
 
 ### Fixed