lib/repo: Add repo locking mechanism

Currently ostree has no method of guarding against concurrent pruning.
When there are multiple repo writers, it's possible to have a pull or
commit race against a prune and end up with missing objects.

This adds a file based repo locking mechanism. The intention is to take
a shared lock when writing objects and an exclusive lock when deleting
them. In order to make use of the locking throughout the library in a
fine grained fashion, the lock acts recursively with a stack of lock
states. If the lock becomes exclusive, it will stay in that state until
the stack is unwound past the initial exclusive push. The file locking
is similar to GLnxLockFile in that it uses open file descriptor locks
but falls back to flock when needed.

The lock also attempts to be thread safe by storing the lock state in
thread local storage with GPrivate. This means that each thread will
have an independent lock for each repository it opens. There are some
drawbacks to that, but it seemed impossible to manage the lock state
coherently in the face of multithreaded access.

The API is a push/pop interface in accordance with the recursive nature
of the locking. The push interface uses an enum that's translated to
LOCK_SH or LOCK_EX as needed. Both interfaces use an internal timeout
field to decide whether to manage the lock in a blocking or non-blocking
fashion. The intention is to allow ostree applications as well as
administrators to control this timeout. For now, the default is a 30
second timeout.

Note that the timeout is handled synchronously in thread since the lock
is maintained in thread local storage. I.e., the thread that acquires
the lock needs to be the same thread that runs the operation. There may
be a way to offer an asynchronous version, but it's not clear exactly
how that would work since it would likely involve a separate thread that
invokes a callback when the locking operation completes.

https://bugzilla.gnome.org/show_bug.cgi?id=759442

Closes: #1343
Approved by: cgwalters

1 files changed, 3 insertions, 0 deletions

diff --git a/apidoc/ostree-experimental-sections.txt b/apidoc/ostree-experimental-sections.txt
index 60daaca5..61f43f28 100644
--- a/apidoc/ostree-experimental-sections.txt
+++ b/apidoc/ostree-experimental-sections.txt
@@ -90,6 +90,9 @@ ostree_repo_finder_override_get_type
 
 <SECTION>
 <FILE>ostree-misc-experimental</FILE>
+OstreeRepoLockType
+ostree_repo_lock_push
+ostree_repo_lock_pop
 ostree_repo_get_collection_id
 ostree_repo_set_collection_id
 ostree_validate_collection_id