// A helper function for use after incrementing an inode's lookup count.
// Ensures that the lookup count is decremented again if the caller is going to
// return in error (in which case the kernel and gcsfuse would otherwise
// disagree about the lookup count for the inode's ID), so that the inode isn't
// leaked.
//
// Typical usage:
//
//     func (fs *fileSystem) doFoo() (err error) {
//       in, err := fs.lookUpOrCreateInodeIfNotStale(...)
//       if err != nil {
//         return
//       }
//
//       defer fs.unlockAndMaybeDisposeOfInode(in, &err)
//
//       ...
//     }
//
// LOCKS_EXCLUDED(fs.mu)
// UNLOCK_FUNCTION(in)
func (fs *fileSystem) unlockAndMaybeDisposeOfInode(
	in inode.Inode,
	err *error) {
	// If there is no error, just unlock.
	if *err == nil {
		in.Unlock()
		return
	}

	// Otherwise, go through the decrement helper, which requires the file system
	// lock.
	fs.mu.Lock()
	fs.unlockAndDecrementLookupCount(in, 1)
}

// Fetch attributes for the supplied inode and fill in an appropriate
// expiration time for them.
//
// LOCKS_REQUIRED(in)
func (fs *fileSystem) getAttributes(
	ctx context.Context,
	in inode.Inode) (
	attr fuseops.InodeAttributes,
	expiration time.Time,
	err error) {
	// Call through.
	attr, err = in.Attributes(ctx)
	if err != nil {
		return
	}

	// Set up the expiration time.
	if fs.inodeAttributeCacheTTL > 0 {
		expiration = time.Now().Add(fs.inodeAttributeCacheTTL)
	}

	return
}

// Decrement the supplied inode's lookup count, destroying it if the inode says
// that it has hit zero.
//
// We require the file system lock to exclude concurrent lookups, which might
// otherwise find an inode whose lookup count has gone to zero.
//
// UNLOCK_FUNCTION(fs.mu)
// UNLOCK_FUNCTION(in)
func (fs *fileSystem) unlockAndDecrementLookupCount(
	in inode.Inode,
	N uint64) {
	name := in.Name()

	// Decrement the lookup count.
	shouldDestroy := in.DecrementLookupCount(N)

	// Update file system state, orphaning the inode if we're going to destroy it
	// below.
	if shouldDestroy {
		delete(fs.inodes, in.ID())

		// Update indexes if necessary.
		if fs.generationBackedInodes[name] == in {
			delete(fs.generationBackedInodes, name)
		}

		if fs.implicitDirInodes[name] == in {
			delete(fs.implicitDirInodes, name)
		}
	}

	// We are done with the file system.
	fs.mu.Unlock()

	// Now we can destroy the inode if necessary.
	if shouldDestroy {
		destroyErr := in.Destroy()
		if destroyErr != nil {
			log.Printf("Error destroying inode %q: %v", name, destroyErr)
		}
	}

	in.Unlock()
}