Debian – How to create a fake inotify event on debian

From what I see in the kernel source, inotify does only fire up after a write is completed (i.e. your guess is wrong). After the notification is triggered, only two more things happen in sys_write, the function that implements the write syscall: setting some scheduler parameters, and updating the position on the file descriptor. This code has been similar as far back as 2.6.14. By the time the notification fires, the file already has its new size.

Check for things that may go wrong:

• Maybe the reader is getting old notifications, from the previous write.
• If the reader calls stat and then calls read or vice versa, something might happen in between. If you keep appending to the file, calling stat first guarantees that you'll be able to read that far, but it's possible that more data has been written by the time the reader calls read, even if it hasn't yet received the inotify notification.
• Just because the writer calls write doesn't mean that the kernel will write the requested number of characters. There are very few circumstances where atomic writes are guaranteed up to any size. Each write call is guaranteed atomic, however: at some point the data isn't written yet, and then suddenly n bytes have been written, where n is the return value of the write call. If you observe a partially-written file, it means that write returned less than its size argument.

Useful tools to investigate what's going on include:

• strace -tt
• the auditd subsystem

I think your approach is correct, and tracking the cookie is a robust way of doing this. However, the only place in the source of inotify-tools (3.14) that cookie is referenced is in the header defining the struct to match the kernel API.

If you like living on the edge, this patch (issue #72) applies cleanly to 3.14 and adds a %c format specifier for the event cookie in hex:

--- libinotifytools/src/inotifytools.c.orig     2014-10-23 18:05:24.000000000 +0100
+++ libinotifytools/src/inotifytools.c  2014-10-23 18:15:47.000000000 +0100
@@ -1881,6 +1881,12 @@
                        continue;
                }

+               if ( ch1 == 'c' ) {
+                       ind += snprintf( &out[ind], size-ind, "%x", event->cookie);
+                       ++i;
+                       continue;
+               }
+
                if ( ch1 == 'e' ) {
                        eventstr = inotifytools_event_to_str( event->mask );
                        strncpy( &out[ind], eventstr, size - ind );

This change modifies libinotifytools.so, not the inotifywait binary. To test before installation:

LD_PRELOAD=./libinotifytools/src/.libs/libinotifytools.so.0.4.1 \
  inotifywait  --format="%c %e %f" -m -e move /tmp/test
Setting up watches.
Watches established.
40ff8 MOVED_FROM b
40ff8 MOVED_TO a

Assuming that MOVED_FROM always occurs before MOVED_TO (it does, see fsnotify_move(), and it's an ordered queue, though independent moves might get interleaved) in your script you cache the details when you see a MOVED_FROM line (perhaps in an associative array indexed by ID), and run your processing when you see a MOVED_TO with the matching half of the information.

declare -A cache
inotifywait  --format="%c %e %f" -m -e move /tmp/test |
while read id event file; do
    if [ "$event" = "MOVED_FROM" ]; then
        cache[$id]=$file
    fi
    if [ "$event" = "MOVED_TO" ]; then
        if [ "${cache[$id]}" ]; then
            echo "processing ..."
            unset cache[$id]
        else
            echo "mismatch for $id"
        fi
    fi
done

(With three threads running to shuffle a pair of files each 10,000 times, I never saw a single out of order event, or event interleaving. It may depend on filesystem and other conditions of course.)