OstreeAsyncProgress is thread-safe: it can have keys changed by one
thread while another is getting the same keys (modulo some locking
contention). However, the thread safety is done at the function call
level: if some code calls an OstreeAsyncProgress getter several times,
the key fetches are not atomic with respect to each other.
In the case of contention on the lock, this can result in consumers of
OstreeAsyncProgress data seeing an inconsistent state between the
properties they query, which could result in progress reporting
inaccuracies.
In the uncontested case, this results in the OstreeAsyncProgress lock
being locked and unlocked many times more than necessary.
Try to improve this by adding new API, which supports getting and
setting multiple keys atomically:
• ostree_async_progress_get()
• ostree_async_progress_set()
The new API uses GVariants and varargs: keys are passed as a
GVariantType string followed by arguments as for g_variant_new() or
g_variant_get(), followed by the next key, etc.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #819
Approved by: cgwalters
OstreeAsyncProgress currently does some contortions to try and avoid
allocating space for guints and guint64s (on 64-bit platforms), but this
means it uses two GHashTables. A GHashTable allocates 8 buckets even
when empty. Given that the largest usage of OstreeAsyncProgress in
libostree puts 13 uints and 5 uint64s in it, this optimisation does not
save significant (if any) memory.
Instead, change OstreeAsyncProgress to store values internally as
GVariants, and expose this with some new API:
• ostree_async_progress_get_variant()
• ostree_async_progress_set_variant()
Each GVariant is allocated on the heap. As they are immutable, they are
thread-safe once returned by a getter.
The existing API continues to work as before, except in the case where a
key is set/got as both a uint and a uint64 — there will now be a
collision (and a GVariant type checking failure) whereas previously
there was no collision. Nothing in OSTree uses OstreeAsyncProgress this
way though.
The new API can be used to share more complex data via the progress API.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #819
Approved by: cgwalters
As rpm-ostree evolves, it keeps driving API additions to libostree.
This creates a relatively tight coupling.
However, if delivering via e.g. RPM, unless one manually remembers to
increment the `Requires:` in the spec file, it's possible for the two
to become desynchronized.
RPM handles versioned symbols and will ensure a dependency if the
application starts using a newer version.
To implement this, switch to `-fvisibility=hidden`, along with an
annotation in the header, and finally add a `.sym` file.
This matches what other projects like systemd and libvirt do.
Although rather than attempting to retroactively version symbols, glom
them all onto the current one.
Several APIs in libostree were moved there from the commandline code,
and have hardcoded g_print() for progress and notifications. This
isn't useful for people who want to write PackageKit backends, custom
GUIs and the like.
From what I can tell, there isn't really a winning precedent in GLib
for progress notifications.
PackageKit has the model where the source has GObject properties that
change as async ops execute, which isn't bad...but I'd like something
a bit more general where say you can have multiple outstanding async
ops and sensibly track their state.
So, OstreeAsyncProgress is basically a threadsafe property bag with a
change notification signal.
Use this new API to move the GSConsole usage (i.e. g_print()) out from
libostree/ and into ostree/.
Philip Withnall
Colin Walters
Giuseppe Scrivano