std.fs.File: fix streaming seekBy ignoring buffer

streaming seekBy completely ignored the buffer. the new logic checks
if the seek offset is smaller than the buffer and adds the offset to
r.interface.seek. otherwise the old logic is invoked.

Author: rhizoome

lib/std/fs/File.zig

@@ -1206,14 +1206,20 @@ pub const Reader = struct {
                     }
                 };
                 var remaining = std.math.cast(u64, offset) orelse return seek_err;
-                while (remaining > 0) {
-                    remaining -= discard(&r.interface, .limited64(remaining)) catch |err| {
-                        r.seek_err = err;
-                        return err;
-                    };
+                const bufferedLen = r.interface.bufferedLen();
+                if (remaining < bufferedLen) {
+                    r.interface.seek += @intCast(remaining);
+                } else {
+                    remaining -= bufferedLen;
+                    while (remaining > 0) {
+                        remaining -= discard(&r.interface, .limited64(remaining)) catch |err| {
+                            r.seek_err = err;
+                            return err;
+                        };
+                    }
+                    r.interface.seek = 0;
+                    r.interface.end = 0;
                 }
-                r.interface.seek = 0;
-                r.interface.end = 0;
             },
             .failure => return r.seek_err.?,
         }

lib/std/fs/test.zig

@@ -2139,21 +2139,26 @@ test "seek keeping partial buffer" {
 
     try testing.expectEqualStrings("6789", &buf);
 }
-
 test "seekBy streaming" {
     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 read_buf: [10]u8 = undefined;
+    var buffer: [10]u8 = undefined;
 
-    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]);
+    var reader = f.readerStreaming(&read_buf);
+    const n1 = try reader.interface.readSliceShort(buffer[0..2]);
+    try testing.expectEqual(2, n1);
+    try testing.expectEqualStrings("le", buffer[0..2]);
+
+    // seek past bufferedLen: causing discard
+    try reader.seekBy(10);
+    const n2 = try reader.interface.readSliceShort(&buffer);
+    try testing.expectEqual(5, n2);
+    try testing.expectEqualStrings("eekBy", buffer[0..5]);
 }
 
 test "seekBy positional" {