From 642f5df0abbcfda47df94ba657b24a913d6903d4 Mon Sep 17 00:00:00 2001
From: Jonathan Marler <johnnymarler@gmail.com>
Date: Thu, 24 Jun 2021 04:11:28 -0600
Subject: [PATCH] Expose default std.log implementation

This allows root.log implementations to capture log messages, process them, and forward them to the default implementation if desired.
---
 lib/std/log.zig | 51 +++++++++++++++++++++++++++++++------------------
 1 file changed, 32 insertions(+), 19 deletions(-)

diff --git a/lib/std/log.zig b/lib/std/log.zig
index a20648626e..5cff132e44 100644
--- a/lib/std/log.zig
+++ b/lib/std/log.zig
@@ -145,30 +145,43 @@ fn log(
             if (@typeInfo(@TypeOf(root.log)) != .Fn)
                 @compileError("Expected root.log to be a function");
             root.log(message_level, scope, format, args);
-        } else if (std.Target.current.os.tag == .freestanding) {
-            // On freestanding one must provide a log function; we do not have
-            // any I/O configured.
-            return;
         } else {
-            const level_txt = switch (message_level) {
-                .emerg => "emergency",
-                .alert => "alert",
-                .crit => "critical",
-                .err => "error",
-                .warn => "warning",
-                .notice => "notice",
-                .info => "info",
-                .debug => "debug",
-            };
-            const prefix2 = if (scope == .default) ": " else "(" ++ @tagName(scope) ++ "): ";
-            const stderr = std.io.getStdErr().writer();
-            const held = std.debug.getStderrMutex().acquire();
-            defer held.release();
-            nosuspend stderr.print(level_txt ++ prefix2 ++ format ++ "\n", args) catch return;
+            defaultLog(message_level, scope, format, args);
         }
     }
 }
 
+/// The default implementation for root.log.  root.log may forward log messages
+/// to this function.
+pub fn defaultLog(
+    comptime message_level: Level,
+    comptime scope: @Type(.EnumLiteral),
+    comptime format: []const u8,
+    args: anytype,
+) void {
+    if (std.Target.current.os.tag == .freestanding) {
+        // On freestanding one must provide a log function; we do not have
+        // any I/O configured.
+        return;
+    }
+
+    const level_txt = switch (message_level) {
+        .emerg => "emergency",
+        .alert => "alert",
+        .crit => "critical",
+        .err => "error",
+        .warn => "warning",
+        .notice => "notice",
+        .info => "info",
+        .debug => "debug",
+    };
+    const prefix2 = if (scope == .default) ": " else "(" ++ @tagName(scope) ++ "): ";
+    const stderr = std.io.getStdErr().writer();
+    const held = std.debug.getStderrMutex().acquire();
+    defer held.release();
+    nosuspend stderr.print(level_txt ++ prefix2 ++ format ++ "\n", args) catch return;
+}
+
 /// Returns a scoped logging namespace that logs all messages using the scope
 /// provided here.
 pub fn scoped(comptime scope: @Type(.EnumLiteral)) type {