io.string_reader: fix needs_fill_until check (#21005)

vlib/io/string_reader/string_reader.v

@@ -52,7 +52,7 @@ pub fn (r StringReader) needs_fill() bool {
 // needs_fill_until returns whether the buffer needs refilling in order to read
 // `n` bytes
 pub fn (r StringReader) needs_fill_until(n int) bool {
-	return r.offset + n >= r.builder.len
+	return r.offset + n > r.builder.len
 }
 
 // fill_bufer tries to read data into the buffer until either a 0 length read or if read_to_end_of_stream

vlib/io/string_reader/string_reader_test.v

@@ -89,6 +89,20 @@ fn test_from_string() {
 	}
 }
 
+fn test_from_string_read_byte_one_by_one() {
+	mut reader := StringReader.new(source: 'test')
+	assert reader.read_bytes(1)![0].ascii_str() == 't'
+	assert reader.read_bytes(1)![0].ascii_str() == 'e'
+	assert reader.read_bytes(1)![0].ascii_str() == 's'
+	assert reader.read_bytes(1)![0].ascii_str() == 't'
+
+	if _ := reader.read_all(false) {
+		assert false, 'should return Eof'
+	} else {
+		assert err is io.Eof
+	}
+}
+
 fn test_from_string_and_reader() {
 	buf := Buf{
 		bytes: 'buffer'.bytes()