Add a sketch of a try_lock that takes multiple locks.

Author: tewalds

mutex_protected.h

@@ -3,6 +3,7 @@
 
 #include <chrono>
 #include <concepts>
+#include <expected>
 #include <mutex>
 #include <shared_mutex>
 #include <thread>
@@ -253,6 +254,16 @@ auto lock(MutexProtected &...mps) {
   return std::make_tuple(mps.adopt_lock()...);
 }
 
+template <typename... MutexProtected>
+std::expected<auto, int> try_lock(MutexProtected &...mps) {
+  int r = std::try_lock(mps.mutex...);
+  if (r >= 0) {
+    return r;
+  } else {
+    return std::make_tuple(mps.adopt_lock()...);
+  }
+}
+
 }  // namespace xyz
 
 #endif  // XYZ_MUTEX_PROTECTED_H

mutex_protected_test.cc

@@ -185,6 +185,37 @@ TYPED_TEST(MutexProtectedTest, LockMultiple) {
   }
 }
 
+TYPED_TEST(MutexProtectedTest, TryLockMultiple) {
+  mutex_protected<int, TypeParam> a(1);
+  mutex_protected<int, TypeParam> b(2);
+  {
+    auto r = xyz::try_lock(a, b);
+    ASSERT_TRUE(r.has_value());
+    auto [la, lb] = r.value();
+    EXPECT_EQ(*la, 1);
+    EXPECT_EQ(*lb, 2);
+    *la += 10;
+    *lb += 10;
+  }
+  {
+    auto la = a.lock();
+    auto r = xyz::try_lock(a, b);
+    ASSERT_FALSE(r.has_value());
+    EXPECT_EQ(r.error(), 0);
+  }
+  {
+    auto lb = b.lock();
+    auto r = xyz::try_lock(a, b);
+    ASSERT_FALSE(r.has_value());
+    EXPECT_EQ(r.error(), 1);
+  }
+  {
+    auto [lb, la] = xyz::lock(b, a);
+    EXPECT_EQ(*la, 11);
+    EXPECT_EQ(*lb, 12);
+  }
+}
+
 template <typename T>
 class SharedMutexProtectedTest : public testing::Test {};