From a1afa99e636d2f4c5e339e6747b67ee6329b334e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=B6nke=20Ludwig?= <sludwig@outerproduct.org>
Date: Tue, 16 Apr 2024 20:51:49 +0200
Subject: [PATCH] Fix lazy evaluation of log arguments.

Fixes https://github.com/vibe-d/vibe-http/pull/36
---
 source/vibe/core/log.d | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/source/vibe/core/log.d b/source/vibe/core/log.d
index 74baca60..9f7b180e 100644
--- a/source/vibe/core/log.d
+++ b/source/vibe/core/log.d
@@ -941,8 +941,12 @@ private void doLog(S, T...)(LogLevel level, string mod, string func, string file
     nothrow
 {
 	try {
-		static if(T.length != 0)
-			auto args_copy = args;
+		static if(T.length != 0) {
+			if (!getLoggers().any!(l => l.minLevel <= level))
+				return;
+
+			T args_copy = args;
+		}
 
 		foreach (l; getLoggers())
 			if (l.minLevel <= level) { // WARNING: TYPE SYSTEM HOLE: accessing field of shared class!
@@ -957,6 +961,13 @@ private void doLog(S, T...)(LogLevel level, string mod, string func, string file
 	} catch(Exception e) debug assert(false, e.msg);
 }
 
+unittest { // ensure arguments are evaluated lazily
+	int i = 0;
+	setLogLevel(LogLevel.info);
+	logDebug("not visible: %s", i++);
+	assert(i == 0);
+}
+
 private struct LogOutputRange {
 	LogLine info;
 	ScopedLock!Logger* logger;