logging: Fix easylogging++ init with blank config

The upstream version of el::base::TypedConfigurations::unsafeGetConfigByRef
accesses uninitialized memory if a key doesn't exist.
Commit b2c59af patched the library to
throw in this case, avoiding the invalid access, but the more suitable
pattern, both logically, and as evidenced by the behavior of
unsafeGetConfigByVal, would be to return a const reference to a
default-initialized value with static storage duration.

external/easylogging++/easylogging++.h

@@ -1993,7 +1993,7 @@ class TypedConfigurations : public base::threading::ThreadSafe {
   }
 
   template <typename Conf_T>
-  inline Conf_T& getConfigByRef(Level level, std::unordered_map<Level, Conf_T>* confMap, const char* confName) {
+  inline const Conf_T& getConfigByRef(Level level, std::unordered_map<Level, Conf_T>* confMap, const char* confName) {
     base::threading::ScopedLock scopedLock(lock());
     return unsafeGetConfigByRef(level, confMap, confName);  // This is not unsafe anymore - mutex locked in scope
   }
@@ -2016,8 +2016,9 @@ class TypedConfigurations : public base::threading::ThreadSafe {
   }
 
   template <typename Conf_T>
-  Conf_T& unsafeGetConfigByRef(Level level, std::unordered_map<Level, Conf_T>* confMap, const char* confName) {
+  const Conf_T& unsafeGetConfigByRef(Level level, std::unordered_map<Level, Conf_T>* confMap, const char* confName) {
     ELPP_UNUSED(confName);
+    static const Conf_T empty;
     typename std::unordered_map<Level, Conf_T>::iterator it = confMap->find(level);
     if (it == confMap->end()) {
       try {
@@ -2026,7 +2027,7 @@ class TypedConfigurations : public base::threading::ThreadSafe {
         ELPP_INTERNAL_ERROR("Unable to get configuration [" << confName << "] for level ["
                             << LevelHelper::convertToString(level) << "]"
                             << std::endl << "Please ensure you have properly configured logger.", false);
-        throw; // The exception has to be rethrown, to abort a branch leading to UB.
+        return empty;
       }
     }
     return it->second;

tests/unit_tests/logging.cpp

@@ -209,9 +209,10 @@ TEST(logging, operator_equals_segfault)
     log2 = log1;
 }
 
-TEST(logging, empty_configurations_throws)
+TEST(logging, empty_configuration)
 {
     el::Logger log1("id1", nullptr);
     const el::Configurations cfg;
-    EXPECT_ANY_THROW(log1.configure(cfg));
+    EXPECT_NO_THROW(log1.configure(cfg));
+    EXPECT_EQ(log1.typedConfigurations()->filename(el::Level::Info), "");
 }