std.fs.File: fix positional seekBy using wrong position

positional seekBy should use the logical position, but it used the file
position which includes the buffered data. the test did not catch this
because it used an empty buffer. fixes #25087 #25020

Author: rhizoome

lib/std/fs/File.zig

@@ -1189,7 +1189,7 @@ pub const Reader = struct {
     pub fn seekBy(r: *Reader, offset: i64) Reader.SeekError!void {
         switch (r.mode) {
             .positional, .positional_reading => {
-                setPosAdjustingBuffer(r, @intCast(@as(i64, @intCast(r.pos)) + offset));
+                setPosAdjustingBuffer(r, @intCast(@as(i64, @intCast(r.logicalPos())) + offset));
             },
             .streaming, .streaming_reading => {
                 if (posix.SEEK == void) {

lib/std/fs/test.zig

@@ -2140,7 +2140,7 @@ test "seek keeping partial buffer" {
     try testing.expectEqualStrings("6789", &buf);
 }
 
-test "seekBy" {
+test "seekBy streaming" {
     var tmp_dir = testing.tmpDir(.{});
     defer tmp_dir.cleanup();
 
@@ -2156,6 +2156,26 @@ test "seekBy" {
     try testing.expectEqualStrings("t's test seekBy", buffer[0..15]);
 }
 
+test "seekBy positional" {
+    var tmp_dir = testing.tmpDir(.{});
+    defer tmp_dir.cleanup();
+
+    try tmp_dir.dir.writeFile(.{ .sub_path = "blah.txt", .data = "let's test seekBy" });
+    const f = try tmp_dir.dir.openFile("blah.txt", .{ .mode = .read_only });
+    defer f.close();
+    var read_buf: [32]u8 = undefined;
+    var reader = f.reader(&read_buf);
+    var buffer: [20]u8 = undefined;
+    const n1 = try reader.interface.readSliceShort(buffer[0..2]);
+    try testing.expectEqual(2, n1);
+    try testing.expectEqualStrings("le", buffer[0..2]);
+    try reader.seekBy(2);
+
+    const n2 = try reader.interface.readSliceShort(&buffer);
+    try testing.expectEqual(13, n2);
+    try testing.expectEqualStrings("s test seekBy", buffer[0..13]);
+}
+
 test "seekTo flushes buffered data" {
     var tmp = std.testing.tmpDir(.{});
     defer tmp.cleanup();