From 2f39caf1806a595eb5e24f89bb9c38681c138360 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 4 Jan 2024 15:05:46 +0100
Subject: [PATCH] Sync: Add support for shared locks

---
 src/libutil/sync.hh | 53 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 40 insertions(+), 13 deletions(-)

diff --git a/src/libutil/sync.hh b/src/libutil/sync.hh
index 47e4512b1..20dd6ee52 100644
--- a/src/libutil/sync.hh
+++ b/src/libutil/sync.hh
@@ -3,6 +3,7 @@
 
 #include <cstdlib>
 #include <mutex>
+#include <shared_mutex>
 #include <condition_variable>
 #include <cassert>
 
@@ -24,8 +25,8 @@ namespace nix {
  * Here, "data" is automatically unlocked when "data_" goes out of
  * scope.
  */
-template<class T, class M = std::mutex>
-class Sync
+template<class T, class M, class WL, class RL>
+class SyncBase
 {
 private:
     M mutex;
@@ -33,23 +34,22 @@ private:
 
 public:
 
-    Sync() { }
-    Sync(const T & data) : data(data) { }
-    Sync(T && data) noexcept : data(std::move(data)) { }
+    SyncBase() { }
+    SyncBase(const T & data) : data(data) { }
+    SyncBase(T && data) noexcept : data(std::move(data)) { }
 
+    template<class L>
     class Lock
     {
-    private:
-        Sync * s;
-        std::unique_lock<M> lk;
-        friend Sync;
-        Lock(Sync * s) : s(s), lk(s->mutex) { }
+    protected:
+        SyncBase * s;
+        L lk;
+        friend SyncBase;
+        Lock(SyncBase * s) : s(s), lk(s->mutex) { }
     public:
         Lock(Lock && l) : s(l.s) { abort(); }
         Lock(const Lock & l) = delete;
         ~Lock() { }
-        T * operator -> () { return &s->data; }
-        T & operator * () { return s->data; }
 
         void wait(std::condition_variable & cv)
         {
@@ -83,7 +83,34 @@ public:
         }
     };
 
-    Lock lock() { return Lock(this); }
+    struct WriteLock : Lock<WL>
+    {
+        T * operator -> () { return &WriteLock::s->data; }
+        T & operator * () { return WriteLock::s->data; }
+    };
+
+    /**
+     * Acquire write (exclusive) access to the inner value.
+     */
+    WriteLock lock() { return WriteLock(this); }
+
+    struct ReadLock : Lock<RL>
+    {
+        const T * operator -> () { return &ReadLock::s->data; }
+        const T & operator * () { return ReadLock::s->data; }
+    };
+
+    /**
+     * Acquire read access to the inner value. When using
+     * `std::shared_mutex`, this will use a shared lock.
+     */
+    ReadLock read() const { return ReadLock(const_cast<SyncBase *>(this)); }
 };
 
+template<class T>
+using Sync = SyncBase<T, std::mutex, std::unique_lock<std::mutex>, std::unique_lock<std::mutex>>;
+
+template<class T>
+using SharedSync = SyncBase<T, std::shared_mutex, std::unique_lock<std::shared_mutex>, std::shared_lock<std::shared_mutex>>;
+
 }