Add test on conditional evaluation of the message

Summary: Avoiding evaluation of the message expression is an important feature, since it may be expensive while being almost always disabled, however this guarantee is not specified anywhere and the way that the code ensures this is a bit subtle, so add a test to make sure we don't regress on this.

Reviewed By: skrueger

Differential Revision: D90598684

fbshipit-source-id: 11f41246c1ab313aebaca2a8b2e047899ce5ec04

Author: ot

third-party/folly/src/folly/logging/test/XlogTest.cpp

@@ -151,6 +151,27 @@ TEST_F(XlogTest, xlogIf) {
   XLOGF_IF(DBG1, true, "plain format string");
   ASSERT_EQ(1, messages.size());
   messages.clear();
+
+  // If the log is not enabled (false condition or below current level) the
+  // message expression should not be evaluated.
+  bool called = false;
+  const auto call = [&] {
+    called = true;
+    return "called";
+  };
+
+  XLOG_IF(INFO, false) << call();
+  EXPECT_FALSE(called);
+  XLOGF_IF(INFO, false, "{}", call());
+  EXPECT_FALSE(called);
+
+  XLOG(DBG2) << call();
+  EXPECT_FALSE(called);
+  XLOGF(DBG2, "{}", call());
+  EXPECT_FALSE(called);
+
+  XLOG(INFO) << call();
+  EXPECT_TRUE(called);
 }
 
 TEST_F(XlogTest, xlog) {