Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Duncan Coutts authored 8 years ago and Mikhail Glushenkov committed 8 years ago

File globs can now be absolute, e.g. starting with / or c:\
Also allow homedir relative, ie ~/

Globs can also have a trailing slash, in which case they only match
directories, not files.

Previously whether globs could match dirs was not totally consistent.
The matchFileGlob would match dirs, but the file monitor globs would
not. The file monitor globs can now match dirs (or with a trailing
slash, only match dirs). File monitors now also detect changes in the
file type, ie switching from file to dir or the other way around.

The file monitor are now pretty consistent between single file monitors
and globs monitors. They now have equivalent capabilities and share
code. For a single file or for a glob we can now control what we
monitor if the path is a file or a dir. In both cases we can monitor
mere existence, non-existence or modification time. For files we can
additionally monitor content hash.

File monitors now also detect changes in the file type, ie switching
from file to dir or the other way around.

New tests cover all these new file monitor cases. There are also new
tests for glob syntax, covering printing/parsing round trips.

(cherry picked from commit f6c1e71c)

If we take the snapshot after the action has completed then we have a
problem. The problem is that files might have changed while the action
was running but /after/ the action read them. If we take the snapshot
after the action completes then we will miss these changes.

The solution is to record a timestamp before beginning execution of the
action and then we make the conservative assumption that any file that
has changed since then has already changed, ie the file monitor state
for these files will be such that checkFileMonitorChanged will report
that they have changed.

Makes use of this in the Rebuild monad so everything using this will get
the feature for free. Also adds a test.

Changed the representation of files that have already changed by the
time we take the snapshot. We had two extra constructor, but now instead
we represent it with the normal constructors but with a Maybe ModTime.
The reason is that it's easier to extend to the globbing case.

See https://github.com/haskell/cabal/commit/a6a7ac59b01ebdc2fdf5a410adb5ae097ce92714#commitcomment-15937386.