A recursive watch that hit the watch limit or an unreadable directory
threw after adding watches for part of the tree, which stayed in the
instance and kept counting against the user's limit. The call now
leaves the instance as it found it.
The test needs a second instance for the check, because a repeated
watch on the same instance only updates the existing one.
The stream's element is now the enum InotifyEvent, and the struct that
describes a change to a watched item is FileSystemEvent. A queue
overflow was an event with descriptor -1 and an empty path that every
consumer had to know about; as a case, the compiler makes them handle
it. The enum is also where failed watches of a growing tree will be
reported, since no call site can catch them.
`IN_Q_OVERFLOW` arrives with watch descriptor -1, so the path lookup
failed and the event was silently discarded. It is now delivered
with an empty path so consumers can rescan after the kernel dropped
events.
inotify exposes a /proc interface to limit kernel memory usage. If those
limits are set too low, inotify cannot add all watches. The integration
test verifies, that Inotify yields an error in that case.
T. R. Bernstein