Merge #20495: sync: Use decltype(auto) return type for WITH_LOCK

3eb94ec81b72b14f72a1f6ce5c9aa24476df755a sync: Use decltype(auto) return type for WITH_LOCK (Carl Dong)

Pull request description:

  > Now that we're using C++17, we can use the decltype(auto) return type
  > for functions and lambda expressions.
  >
  > As demonstrated in this commit, this can simplify cases where previously
  > the compiler failed to deduce the correct return type.
  >
  > Just for reference, for the "assign to ref" cases fixed here, there are
  > 3 possible solutions:
  >
  > - Return a pointer and immediately deref as used before this commit
  > - Make sure the function/lambda returns declspec(auto) as used after
  >   this commit
  > - Class& i = WITH_LOCK(..., return std::ref(...));
  >
  > -----
  >
  > References:
  > 1. https://en.cppreference.com/w/cpp/language/function#Return_type_deduction
  > 2. https://en.cppreference.com/w/cpp/language/template_argument_deduction#Other_contexts
  > 3. https://en.cppreference.com/w/cpp/language/auto
  > 4. https://en.cppreference.com/w/cpp/language/decltype
  >
  > Explanations:
  > 1. https://stackoverflow.com/a/21369192
  > 2. https://stackoverflow.com/a/21369170

  Thanks to sipa and ryanofsky for helping me understand this

ACKs for top commit:
  jnewbery:
    utACK 3eb94ec81b72b14f72a1f6ce5c9aa24476df755a
  hebasto:
    ACK 3eb94ec81b72b14f72a1f6ce5c9aa24476df755a, I have reviewed the code and it looks OK, I agree it can be merged. I have verified possible warnings:
  ryanofsky:
    Code review ACK 3eb94ec81b72b14f72a1f6ce5c9aa24476df755a

Tree-SHA512: 5f55c7722aeca8ea70e5c1a8db93e93ba0e356e8967e7f607ada38003df4b153d73c29bd2cea8d7ec1344720d37d857ea7dbfd2a88da1d92e0e9cbb9abd287df

src/sync.h

@@ -257,7 +257,22 @@ using DebugLock = UniqueLock<typename std::remove_reference<typename std::remove
 //!
 //!   int val = WITH_LOCK(cs, return shared_val);
 //!
-#define WITH_LOCK(cs, code) [&] { LOCK(cs); code; }()
+//! Note:
+//!
+//! Since the return type deduction follows that of decltype(auto), while the
+//! deduced type of:
+//!
+//!   WITH_LOCK(cs, return {int i = 1; return i;});
+//!
+//! is int, the deduced type of:
+//!
+//!   WITH_LOCK(cs, return {int j = 1; return (j);});
+//!
+//! is &int, a reference to a local variable
+//!
+//! The above is detectable at compile-time with the -Wreturn-local-addr flag in
+//! gcc and the -Wreturn-stack-address flag in clang, both enabled by default.
+#define WITH_LOCK(cs, code) [&]() -> decltype(auto) { LOCK(cs); code; }()
 
 class CSemaphore
 {

src/test/validation_chainstate_tests.cpp

@@ -40,7 +40,7 @@ BOOST_AUTO_TEST_CASE(validation_chainstate_resize_caches)
         return outp;
     };
 
-    CChainState& c1 = *WITH_LOCK(cs_main, return &manager.InitializeChainstate(&mempool, *m_node.mnhf_manager, *m_node.evodb, llmq::chainLocksHandler, llmq::quorumInstantSendManager, llmq::quorumBlockProcessor));
+    CChainState& c1 = WITH_LOCK(cs_main, return manager.InitializeChainstate(&mempool, *m_node.mnhf_manager, *m_node.evodb, llmq::chainLocksHandler, llmq::quorumInstantSendManager, llmq::quorumBlockProcessor));
     c1.InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
     WITH_LOCK(::cs_main, c1.InitCoinsCache(1 << 23));

src/test/validation_chainstatemanager_tests.cpp

@@ -42,7 +42,7 @@ BOOST_AUTO_TEST_CASE(chainstatemanager)
 
     // Create a legacy (IBD) chainstate.
     //
-    CChainState& c1 = *WITH_LOCK(::cs_main, return &manager.InitializeChainstate(&mempool, *m_node.mnhf_manager, evodb, llmq::chainLocksHandler, llmq::quorumInstantSendManager, llmq::quorumBlockProcessor));
+    CChainState& c1 = WITH_LOCK(::cs_main, return manager.InitializeChainstate(&mempool, *m_node.mnhf_manager, evodb, llmq::chainLocksHandler, llmq::quorumInstantSendManager, llmq::quorumBlockProcessor));
     chainstates.push_back(&c1);
     c1.InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
@@ -75,7 +75,7 @@ BOOST_AUTO_TEST_CASE(chainstatemanager)
     // Create a snapshot-based chainstate.
     //
     const uint256 snapshot_blockhash = GetRandHash();
-    CChainState& c2 = *WITH_LOCK(::cs_main, return &manager.InitializeChainstate(
+    CChainState& c2 = WITH_LOCK(::cs_main, return manager.InitializeChainstate(
         &mempool, *m_node.mnhf_manager, evodb, llmq::chainLocksHandler, llmq::quorumInstantSendManager, llmq::quorumBlockProcessor,
         snapshot_blockhash)
     );
@@ -145,7 +145,7 @@ BOOST_AUTO_TEST_CASE(chainstatemanager_rebalance_caches)
 
     // Create a legacy (IBD) chainstate.
     //
-    CChainState& c1 = *WITH_LOCK(cs_main, return &manager.InitializeChainstate(&mempool, *m_node.mnhf_manager, evodb, llmq::chainLocksHandler, llmq::quorumInstantSendManager, llmq::quorumBlockProcessor));
+    CChainState& c1 = WITH_LOCK(cs_main, return manager.InitializeChainstate(&mempool, *m_node.mnhf_manager, evodb, llmq::chainLocksHandler, llmq::quorumInstantSendManager, llmq::quorumBlockProcessor));
     chainstates.push_back(&c1);
     c1.InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
@@ -163,7 +163,7 @@ BOOST_AUTO_TEST_CASE(chainstatemanager_rebalance_caches)
 
     // Create a snapshot-based chainstate.
     //
-    CChainState& c2 = *WITH_LOCK(cs_main, return &manager.InitializeChainstate(&mempool, *m_node.mnhf_manager, evodb, llmq::chainLocksHandler, llmq::quorumInstantSendManager, llmq::quorumBlockProcessor, GetRandHash()));
+    CChainState& c2 = WITH_LOCK(cs_main, return manager.InitializeChainstate(&mempool, *m_node.mnhf_manager, evodb, llmq::chainLocksHandler, llmq::quorumInstantSendManager, llmq::quorumBlockProcessor, GetRandHash()));
     chainstates.push_back(&c2);
     c2.InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);

src/validation.cpp

@@ -5797,7 +5797,7 @@ bool ChainstateManager::PopulateAndValidateSnapshot(
 {
     // It's okay to release cs_main before we're done using `coins_cache` because we know
     // that nothing else will be referencing the newly created snapshot_chainstate yet.
-    CCoinsViewCache& coins_cache = *WITH_LOCK(::cs_main, return &snapshot_chainstate.CoinsTip());
+    CCoinsViewCache& coins_cache = WITH_LOCK(::cs_main, return snapshot_chainstate.CoinsTip());
 
     uint256 base_blockhash = metadata.m_base_blockhash;