From 17b5426813997b3aca12515e580ba09f392333a6 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 15 Apr 2026 05:01:22 +0300
Subject: [PATCH] io: fix BufferedReader.read_line undocumented behavior on new
 line characters (fixes #18751)

---
 vlib/io/buffered_reader.v             | 22 +++++++++++++---------
 vlib/io/reader_test.v                 | 14 ++++++++++++++
 vlib/io/string_reader/string_reader.v |  6 ++++--
 3 files changed, 31 insertions(+), 11 deletions(-)

diff --git a/vlib/io/buffered_reader.v b/vlib/io/buffered_reader.v
index b72f467d4..bc35f155f 100644
--- a/vlib/io/buffered_reader.v
+++ b/vlib/io/buffered_reader.v
@@ -26,7 +26,7 @@ pub:
 @[params]
 pub struct BufferedReadLineConfig {
 pub:
-	delim u8 = `\n` // line delimiter
+	delim u8 = `\n` // line delimiter; `\n` also strips a preceding `\r`
 }
 
 // new_buffered_reader creates a new BufferedReader.
@@ -120,8 +120,10 @@ pub fn (r BufferedReader) end_of_stream() bool {
 }
 
 // read_line attempts to read a line from the buffered reader.
-// It will read until it finds the specified line delimiter
-// such as (\n, the default or \0) or the end of stream.
+// It reads until it finds the specified delimiter or the end of stream.
+// The returned string does not include the delimiter.
+// When the delimiter is `\n`, a preceding `\r` is treated as part of CRLF
+// and is also omitted from the returned string.
 pub fn (mut r BufferedReader) read_line(config BufferedReadLineConfig) !string {
 	if r.end_of_stream {
 		return Eof{}
@@ -146,13 +148,15 @@ pub fn (mut r BufferedReader) read_line(config BufferedReadLineConfig) !string {
 			c := r.buf[i]
 			if c == config.delim {
 				// great, we hit something
-				// do some checking for whether we hit \r\n or just \n
-				if i != 0 && config.delim == `\n` && r.buf[i - 1] == `\r` {
-					x := i - 1
-					line << r.buf[r.offset..x]
-				} else {
-					line << r.buf[r.offset..i]
+				mut end := i
+				if config.delim == `\n` {
+					if i > r.offset && r.buf[i - 1] == `\r` {
+						end--
+					} else if i == r.offset && line.len > 0 && line[line.len - 1] == `\r` {
+						line.delete_last()
+					}
 				}
+				line << r.buf[r.offset..end]
 				r.offset = i + 1
 				return line.bytestr()
 			}
diff --git a/vlib/io/reader_test.v b/vlib/io/reader_test.v
index 528bae796..5639bc13d 100644
--- a/vlib/io/reader_test.v
+++ b/vlib/io/reader_test.v
@@ -182,6 +182,20 @@ fn test_leftover() {
 	assert r.end_of_stream()
 }
 
+fn test_read_line_strips_crlf_across_buffer_fills() {
+	text := '12345\r\n67890\r\n'
+	mut s := StringReaderTest{
+		text: text
+	}
+	mut r := new_buffered_reader(reader: s, cap: 2)
+	assert r.read_line()! == '12345'
+	assert r.read_line()! == '67890'
+	if _ := r.read_line() {
+		assert false
+	}
+	assert r.end_of_stream()
+}
+
 fn test_totalread_read() {
 	text := 'Some testing text'
 	mut s := StringReaderTest{
diff --git a/vlib/io/string_reader/string_reader.v b/vlib/io/string_reader/string_reader.v
index edfce10b1..c8c399d2a 100644
--- a/vlib/io/string_reader/string_reader.v
+++ b/vlib/io/string_reader/string_reader.v
@@ -227,8 +227,10 @@ pub fn (mut r StringReader) read(mut buf []u8) !int {
 }
 
 // read_line attempts to read a line from the reader.
-// It will read until it finds the specified line delimiter
-// such as (\n, the default or \0) or the end of stream.
+// It reads until it finds the specified delimiter or the end of stream.
+// The returned string does not include the delimiter.
+// When the delimiter is `\n`, a preceding `\r` is treated as part of CRLF
+// and is also omitted from the returned string.
 @[direct_array_access]
 pub fn (mut r StringReader) read_line(config io.BufferedReadLineConfig) !string {
 	if r.end_of_stream && r.needs_fill() {
-- 
2.39.5