MAINT-5232: Introduce LL_VLOGS() macro: log call with variable level.

In some places we want to log the same information but with different severity
depending on specifics. In other cases we need to test the availability of the
logging subsystem before engaging it. LL_VLOGS() accepts an LLError::ELevel
argument that can differ with each call, while retaining the desirable feature
of deciding only once for each level.

indra/llcommon/llerror.h

@@ -174,7 +174,8 @@ namespace LLError
 			// not really a level
 			// used to indicate that no messages should be logged
 	};
-	
+	// If you change ELevel, please update llvlog() macro below.
+
 	/*	Macro support
 		The classes CallSite and Log are used by the logging macros below.
 		They are not intended for general use.
@@ -314,24 +315,29 @@ typedef LLError::NoClassInfo _LL_CLASS_TO_LOG;
 // through lllog(), it initializes a local static LLError::CallSite with that
 // *first* ELevel value. All subsequent visits will decide whether or not to
 // emit output based on the *first* ELevel value bound into that static
-// CallSite instance. lllog() assumes its ELevel argument never varies.
+// CallSite instance. Use LL_VLOGS() instead. lllog() assumes its ELevel
+// argument never varies.
 
 // this macro uses a one-shot do statement to avoid parsing errors when
 // writing control flow statements without braces:
 // if (condition) LL_INFOS() << "True" << LL_ENDL; else LL_INFOS()() << "False" << LL_ENDL;
 
-#define lllog(level, once, ...)                                     \
-	do {                                                            \
-		const char* tags[] = {"", ##__VA_ARGS__};                   \
-		::size_t tag_count = LL_ARRAY_SIZE(tags) - 1;               \
-		static LLError::CallSite _site(                             \
-			level, __FILE__, __LINE__, typeid(_LL_CLASS_TO_LOG),    \
-			__FUNCTION__, once, &tags[1], tag_count);               \
-		if (LL_UNLIKELY(_site.shouldLog()))                         \
-		{                                                           \
-			std::ostringstream* _out = LLError::Log::out();         \
+#define lllog(level, once, ...)                                         \
+	do {                                                                \
+		const char* tags[] = {"", ##__VA_ARGS__};                       \
+		static LLError::CallSite _site(lllog_site_args_(level, once, tags)); \
+		lllog_test_()
+
+#define lllog_test_()                                       \
+		if (LL_UNLIKELY(_site.shouldLog()))                 \
+		{                                                   \
+			std::ostringstream* _out = LLError::Log::out(); \
 			(*_out)
 
+#define lllog_site_args_(level, once, tags)                 \
+	level, __FILE__, __LINE__, typeid(_LL_CLASS_TO_LOG),    \
+	__FUNCTION__, once, &tags[1], LL_ARRAY_SIZE(tags)-1
+
 //Use this construct if you need to do computation in the middle of a
 //message:
 //	
@@ -372,4 +378,46 @@ typedef LLError::NoClassInfo _LL_CLASS_TO_LOG;
 #define LL_INFOS_ONCE(...)	lllog(LLError::LEVEL_INFO, true, ##__VA_ARGS__)
 #define LL_WARNS_ONCE(...)	lllog(LLError::LEVEL_WARN, true, ##__VA_ARGS__)
 
+// Use this if you need to pass LLError::ELevel as a variable.
+#define LL_VLOGS(level, ...)      llvlog(level, false, ##__VA_ARGS__)
+#define LL_VLOGS_ONCE(level, ...) llvlog(level, true,  ##__VA_ARGS__)
+
+// The problem with using lllog() with a variable level is that the first time
+// through, it initializes a static CallSite instance with whatever level you
+// pass. That first level is bound into the CallSite; the level parameter is
+// never again examined. One approach to variable level would be to
+// dynamically construct a CallSite instance every call -- which could get
+// expensive, depending on context. So instead, initialize a static CallSite
+// for each level value we support, then dynamically select the CallSite
+// instance for the passed level value.
+// Compare implementation to lllog() above.
+#define llvlog(level, once, ...)                                        \
+	do {                                                                \
+		const char* tags[] = {"", ##__VA_ARGS__};                       \
+		/* Need a static CallSite instance per expected ELevel value. */ \
+		/* Since we intend to index this array with the ELevel, */      \
+		/* _sites[0] should be ELevel(0), and so on -- avoid using */   \
+		/* ELevel symbolic names when initializing -- except for */     \
+		/* the last entry, which handles anything beyond the end. */    \
+		/* (Commented ELevel value names are from 2016-09-01.) */       \
+		/* Passing an ELevel past the end of this array is itself */    \
+		/* a fatal error, so ensure the last is LEVEL_ERROR. */         \
+		static LLError::CallSite _sites[] =                             \
+		{                                                               \
+			/* LEVEL_DEBUG */                                           \
+			LLError::CallSite(lllog_site_args_(LLError::ELevel(0), once, tags)), \
+			/* LEVEL_INFO */                                            \
+			LLError::CallSite(lllog_site_args_(LLError::ELevel(1), once, tags)), \
+			/* LEVEL_WARN */                                            \
+			LLError::CallSite(lllog_site_args_(LLError::ELevel(2), once, tags)), \
+			/* LEVEL_ERROR */                                           \
+			LLError::CallSite(lllog_site_args_(LLError::LEVEL_ERROR, once, tags)) \
+		};                                                              \
+		/* Clamp the passed 'level' to at most last entry */            \
+		std::size_t which((std::size_t(level) >= LL_ARRAY_SIZE(_sites)) ? \
+						  (LL_ARRAY_SIZE(_sites) - 1) : std::size_t(level)); \
+		/* selected CallSite *must* be named _site for LL_ENDL */       \
+		LLError::CallSite& _site(_sites[which]);                        \
+		lllog_test_()
+
 #endif // LL_LLERROR_H