This commits adds rudimentary support for a cache directory, enabled by
default. The cache directory is created if it does not exist. The cache
is used if there's anything in it, newly created snapshot and index
files are written to the cache automatically.
This was a bit tricky: We start the ssh binary, but we want it to ignore
SIGINT. In contrast, restic itself should process SIGINT and clean up
properly. Before, we used `setsid()` to give the ssh process its own
process group, but that means it cannot prompt the user for a password
because the tty is gone.
So, now we're passing in two functions that ignore SIGINT just before
the ssh process is started and re-install it after start.
The option is named --exclude-if-present and accepts a parameter
filename[:content]. Directories are excluded and their contents is not
backed up if they contain a file with the specified name and,
optionally, that starts with the specified content. The tagfile itself
is never excluded.
There is also a shortcut --exclude-caches that works in the same way as
the likewise-named option of tar(1): Directories are recognized as cache
if they contain a file named "CACHEDIR.TAG.
Closes#317.
By default (i.e., without profile.NoShutdownHook), profile.Start listens
for SIGINT and will stop the profile and call os.Exit(0).
restic already listens for SIGINT and runs its own cleanup handlers
before calling os.Exit(0).
As is, these handlers are racing when an interrupt occurs, and in my
experience, restic tends to win the race, resulting in an unusable
profile.
Eliminate the race and properly stop profiles on interrupt by disabling
package profile's signal handler and instead stop the profile in a
restic cleanup handler.
An exclude filter is basically a 'wildcard but foo', so even if a
childMayMatch, other children of a dir may not, therefore childMayMatch
does not matter, but we should not go down unless the dir is selected
for restore.
