The thumbnail endpoint GET
/api/v1/message/{ID}/part/{PartID}/thumb parses EXIF
metadata of image attachments with github.com/rwcarlsen/goexif
to apply orientation. goexif's TIFF IFD walker only detects
a self-referencing IFD (next == prev). An attachment whose
IFD chain forms a cycle of length 2 (IFD0 → IFD1 → IFD0)
sends the request handler into an infinite loop that
appends a new *tiff.Dir on every iteration. The goroutine
never terminates, pins one CPU core permanently, and its
live heap grows without bound. A 651-byte JPEG is
sufficient.