std.fs.File.Reader: fix seekBy

Tested locally:

stage3/bin/zig build test -Dskip-release -Dskip-non-native

closes #24495
closes #24498
closes #24713

2 files changed, 19 insertions, 6 deletions

diff --git a/lib/std/fs/File.zig b/lib/std/fs/File.zig
index ace7cb8292..c698840f24 100644
--- a/lib/std/fs/File.zig
+++ b/lib/std/fs/File.zig
@@ -1247,12 +1247,10 @@ pub const Reader = struct {
                 };
                 var remaining = std.math.cast(u64, offset) orelse return seek_err;
                 while (remaining > 0) {
-                    const n = discard(&r.interface, .limited64(remaining)) catch |err| {
+                    remaining -= discard(&r.interface, .limited64(remaining)) catch |err| {
                         r.seek_err = err;
                         return err;
                     };
-                    r.pos += n;
-                    remaining -= n;
                 }
                 r.interface.seek = 0;
                 r.interface.end = 0;
@@ -1436,9 +1434,8 @@ pub const Reader = struct {
                 fallback: {
                     if (r.size_err == null and r.seek_err == null) break :fallback;
                     var trash_buffer: [128]u8 = undefined;
-                    const trash = &trash_buffer;
                     if (is_windows) {
-                        const n = windows.ReadFile(file.handle, trash, null) catch |err| {
+                        const n = windows.ReadFile(file.handle, limit.slice(&trash_buffer), null) catch |err| {
                             r.err = err;
                             return error.ReadFailed;
                         };
@@ -1453,7 +1450,7 @@ pub const Reader = struct {
                     var iovecs_i: usize = 0;
                     var remaining = @intFromEnum(limit);
                     while (remaining > 0 and iovecs_i < iovecs.len) {
-                        iovecs[iovecs_i] = .{ .base = trash, .len = @min(trash.len, remaining) };
+                        iovecs[iovecs_i] = .{ .base = &trash_buffer, .len = @min(trash_buffer.len, remaining) };
                         remaining -= iovecs[iovecs_i].len;
                         iovecs_i += 1;
                     }
diff --git a/lib/std/fs/test.zig b/lib/std/fs/test.zig
index d51fe3db0e..60879a5ead 100644
--- a/lib/std/fs/test.zig
+++ b/lib/std/fs/test.zig
@@ -2129,3 +2129,19 @@ test "seek keeping partial buffer" {
 
     try testing.expectEqualStrings("6789", &buf);
 }
+
+test "seekBy" {
+    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 reader = f.readerStreaming(&.{});
+    try reader.seekBy(2);
+
+    var buffer: [20]u8 = undefined;
+    const n = try reader.interface.readSliceShort(&buffer);
+    try testing.expectEqual(15, n);
+    try testing.expectEqualStrings("t's test seekBy", buffer[0..15]);
+}