From a2bb060967315c4fcd3ce9c71640e89460b22dae Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Thu, 26 Feb 2026 10:16:58 +0300
Subject: [PATCH] io: fix stream_reader read function does not work (fixes
 #21768)

---
 vlib/io/string_reader/string_reader.v      | 31 +++++++++++++++++++--
 vlib/io/string_reader/string_reader_test.v | 32 ++++++++++++++++++++++
 2 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/vlib/io/string_reader/string_reader.v b/vlib/io/string_reader/string_reader.v
index 24beeea9c..edfce10b1 100644
--- a/vlib/io/string_reader/string_reader.v
+++ b/vlib/io/string_reader/string_reader.v
@@ -40,6 +40,10 @@ pub fn StringReader.new(params StringReaderParams) StringReader {
 	if source := params.source {
 		r.builder = strings.new_builder(source.len)
 		r.builder.write_string(source)
+		if params.reader == none {
+			// There is no upstream reader; only the preloaded buffer can be consumed.
+			r.end_of_stream = true
+		}
 	} else {
 		r.builder = strings.new_builder(params.initial_size)
 	}
@@ -190,9 +194,32 @@ pub fn (mut r StringReader) read_string(n int) !string {
 
 // read implements the Reader interface
 pub fn (mut r StringReader) read(mut buf []u8) !int {
-	start := r.offset
+	if buf.len == 0 {
+		return 0
+	}
+
+	available := r.builder.len - r.offset
+	if available < buf.len && !r.end_of_stream {
+		if r.reader != none {
+			missing := buf.len - available
+			r.fill_buffer_until(missing) or {
+				if err !is io.Eof {
+					return err
+				}
+			}
+		}
+	}
 
-	read := r.fill_buffer_until(buf.len)!
+	available_after_fill := r.builder.len - r.offset
+	if available_after_fill == 0 {
+		if r.reader == none && !r.end_of_stream {
+			return error('reader is not set')
+		}
+		return io.Eof{}
+	}
+
+	read := if available_after_fill < buf.len { available_after_fill } else { buf.len }
+	start := r.offset
 	r.offset += read
 
 	copy(mut buf, r.builder[start..start + read])
diff --git a/vlib/io/string_reader/string_reader_test.v b/vlib/io/string_reader/string_reader_test.v
index 2807ace2a..bb1d7f037 100644
--- a/vlib/io/string_reader/string_reader_test.v
+++ b/vlib/io/string_reader/string_reader_test.v
@@ -36,6 +36,10 @@ fn (mut r TwoByteReader) read(mut buf []u8) !int {
 	return min
 }
 
+fn read_from_reader_interface(mut r io.Reader, mut buf []u8) !int {
+	return r.read(mut buf)
+}
+
 fn test_read_all() {
 	mut reader := StringReader.new()
 
@@ -107,6 +111,34 @@ fn test_from_string() {
 	}
 }
 
+fn test_read_from_source_read() {
+	source := 'line 1\nline 2\n'
+	mut reader := StringReader.new(source: source)
+	mut buf := []u8{len: 100}
+	read := reader.read(mut buf)!
+	assert read == source.len
+	assert buf[..read].bytestr() == source
+	if _ := reader.read(mut buf) {
+		assert false, 'should return io.Eof'
+	} else {
+		assert err is io.Eof
+	}
+}
+
+fn test_read_from_source_read_via_reader_interface() {
+	source := 'line 1\nline 2\n'
+	mut reader := StringReader.new(source: source)
+	mut buf := []u8{len: 100}
+	read := read_from_reader_interface(mut reader, mut buf)!
+	assert read == source.len
+	assert buf[..read].bytestr() == source
+	if _ := read_from_reader_interface(mut reader, mut buf) {
+		assert false, 'should return io.Eof'
+	} else {
+		assert err is io.Eof
+	}
+}
+
 fn test_from_string_read_byte_one_by_one() {
 	mut reader := StringReader.new(source: 'test')
 	assert reader.read_bytes(1)![0].ascii_str() == 't'
-- 
2.39.5