log: set stderr as default log output, add .set_output_stream() (#23296)

vlib/log/common.v

@@ -19,8 +19,8 @@ pub enum LogTarget {
 	both
 }
 
-// tag_to_cli returns the tag for log level `l` as a colored string.
-fn tag_to_cli(l Level, short_tag bool) string {
+// tag_to_console returns the tag for log level `l` as a colored string.
+fn tag_to_console(l Level, short_tag bool) string {
 	if short_tag {
 		return match l {
 			.disabled { ' ' }

vlib/log/log.v

@@ -5,6 +5,7 @@ module log
 
 import os
 import time
+import io
 
 // TimeFormat define the log time string format, come from time/format.v
 pub enum TimeFormat {
@@ -36,6 +37,7 @@ mut:
 	custom_time_format string     = 'MMMM Do YY N kk:mm:ss A' // timestamp with custom format
 	short_tag          bool
 	always_flush       bool // flush after every single .fatal(), .error(), .warn(), .info(), .debug() call
+	output_stream      io.Writer = os.stderr()
 pub mut:
 	output_file_name string // log output to this file
 }
@@ -77,6 +79,11 @@ pub fn (mut l Log) set_output_path(output_file_path string) {
 	l.ofile = ofile
 }
 
+// set_output_stream sets the output stream to write log e.g. stderr, stdout, etc.
+pub fn (mut l Log) set_output_stream(stream io.Writer) {
+	l.output_stream = stream
+}
+
 // log_to_console_too turns on logging to the console too, in addition to logging to a file.
 // You have to call it *after* calling .set_output_path(output_file_path).
 pub fn (mut l Log) log_to_console_too() {
@@ -131,13 +138,19 @@ fn (mut l Log) log_file(s string, level Level) {
 	}
 }
 
-// log_cli writes log line `s` with `level` to stdout.
-fn (l &Log) log_cli(s string, level Level) {
+// log_stream writes log line `s` with `level` to stderr or stderr depending on set output stream.
+fn (mut l Log) log_stream(s string, level Level) {
 	timestamp := l.time_format(time.utc())
-	e := tag_to_cli(level, l.short_tag)
-	println('${timestamp} [${e}] ${s}')
+	tag := tag_to_console(level, l.short_tag)
+	msg := '${timestamp} [${tag}] ${s}\n'
+	arr := msg.bytes()
+	l.output_stream.write(arr) or {}
 	if l.always_flush {
-		flush_stdout()
+		match (l.output_stream as os.File).fd {
+			1 { flush_stdout() }
+			2 { flush_stderr() }
+			else {}
+		}
 	}
 }
 
@@ -148,7 +161,7 @@ pub fn (mut l Log) send_output(s &string, level Level) {
 		l.log_file(s, level)
 	}
 	if l.output_target == .console || l.output_target == .both {
-		l.log_cli(s, level)
+		l.log_stream(s, level)
 	}
 }
 
@@ -202,6 +215,7 @@ pub fn (mut f Log) free() {
 		f.output_label.free()
 		f.ofile.close()
 		f.output_file_name.free()
+		f.output_stream.free()
 	}
 }