io.string_reader: fix fill_buffer(), add tests (#25682)

Author: tankf33der

vlib/io/string_reader/string_reader.v

@@ -76,13 +76,13 @@ pub fn (mut r StringReader) fill_buffer(read_till_end_of_stream bool) !int {
 	}
 
 	for {
-		read := reader.read(mut r.builder[start..]) or {
+		read := reader.read(mut r.builder[end..]) or {
 			r.end_of_stream = true
 			break
 		}
 		end += read
 
-		if !read_till_end_of_stream && read == 0 {
+		if !read_till_end_of_stream || read == 0 {
 			break
 		} else if r.builder.len == end {
 			unsafe { r.builder.grow_len(io.read_all_grow_len) }

vlib/io/string_reader/string_reader_test.v

@@ -9,6 +9,12 @@ mut:
 	i int
 }
 
+struct TwoByteReader {
+mut:
+	data string
+	pos  int
+}
+
 fn (mut b Buf) read(mut buf []u8) !int {
 	if !(b.i < b.bytes.len) {
 		return io.Eof{}
@@ -18,6 +24,18 @@ fn (mut b Buf) read(mut buf []u8) !int {
 	return n
 }
 
+fn (mut r TwoByteReader) read(mut buf []u8) !int {
+	if r.pos >= r.data.len {
+		return io.Eof{}
+	}
+	min := int_min(r.data.len - r.pos, 2)
+	for i in 0 .. min {
+		buf[i] = r.data[r.pos]
+		r.pos++
+	}
+	return min
+}
+
 fn test_read_all() {
 	mut reader := StringReader.new()
 
@@ -122,3 +140,21 @@ fn test_flush() {
 	assert reader.offset == 0
 	assert reader.builder.len == 0
 }
+
+fn test_fill_buffer_true() {
+	mut two := TwoByteReader{
+		data: '12345'
+	}
+	mut reader := StringReader.new(reader: two)
+	assert reader.fill_buffer(true)! == 5
+	assert reader.get_string() == '12345'
+}
+
+fn test_fill_buffer_false() {
+	mut two := TwoByteReader{
+		data: '12345'
+	}
+	mut reader := StringReader.new(reader: two)
+	assert reader.fill_buffer(false)! == 2
+	assert reader.get_string() == '12'
+}