Fuse-ext2 deadlocks on creating symlinks

Bringing the proposed fix over from bazaar.

[Impact]

 * Any attempt to create/overwrite symlinks in a partition mounted by fuseext2 causes the fuseext2 process to deadlock, preventing the mounted filesystem from being used at all until the userspace process is killed and the filesystem is remounted.

[Test Case]

 * dd if=/dev/zero of=partition bs=1M count=200
 * mkfs.ext2 partition
 * mkdir mount
 * fuseext2 partition mount
 * cd mount
 * touch test
 * ln -s test link - Doesn't complete
 * In another shell try to do anything else in the mount directory (ls, touch, rm) - Don't complete

[Regression Potential]

 * Since this changes the locking strategy of the code to hold locks less, the main risk is exposing a case where data can be accessed in a non-threadsafe manner, leading to unexpected behaviour.
 * The places where the change has been made are at the exit points of the function, and match where similar unlocks are made in other places in the code.
 * Since the program at risk is (by design) a userspace program, the risk of kernel data leakage is minimal.

[Other Info]

 * I can't actually work out where the upstream code for this project lives, the only upstream I could find is https://github.com/alperakcan/fuse-ext2 but that doesn't have any locking code at all (even in the reentrant branch) so I can't see where this issue came from.
 * This issue seems to apply to all versions of fuseext2 in the ubuntu repos (well, at least it's there in trusty, xenial and zesty)

Uploaded to Trusty using your changelog from the Bazaar merge proposal and a few trivial tweaks. It's in the Unapproved queue now waiting for a member of the SRU team to approve it.

Thank you for your contribution to Ubuntu!

Ah, I've found the upstream! It's https://sourceforge.net/p/view-os/code/HEAD/tree/trunk/fuse-modules/fuse-umfuse-ext2/ but it's not been touched since 2013, and it does contain the deadlock.

From what I gather of the bug description (without doing any detective work) it seems that this is not fixed in the development release of Ubuntu (Artful Aardvark) and it should be before we accept it as as a Stable Release Update.

As far as I can tell the patch is applicable without change to all shipped versions of the fuse-umfuse-ext2 codebase from Trusty onward (including Artful). I'm not sure which earlier releases it applies to, but I guess those are all EOL.

Uploaded to Trusty, Xenial, Zesty, and Artful. For the stable releases, it's sitting in the Unapproved queue, for Artful it's building and will migrate like normal.

This bug was fixed in the package fuse-umfuse-ext2 - 0.4-1.1ubuntu0.1

---------------
fuse-umfuse-ext2 (0.4-1.1ubuntu0.1) artful; urgency=high

  * Correctly release locks in op_link (LP: #1674868).

 -- Andrew Caldwell <email address hidden> Wed, 22 Mar 2017 19:42:35 +0000

Hello Andy, or anyone else affected,

Accepted fuse-umfuse-ext2 into zesty-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/fuse-umfuse-ext2/0.4-1.1ubuntu0.17.04.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed.Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested and change the tag from verification-needed-zesty to verification-done-zesty. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-zesty. In either case, details of your testing will help us make a better decision.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance!

Hello Andy, or anyone else affected,

Accepted fuse-umfuse-ext2 into xenial-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/fuse-umfuse-ext2/0.4-1.1ubuntu0.16.04.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed.Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested and change the tag from verification-needed-xenial to verification-done-xenial. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-xenial. In either case, details of your testing will help us make a better decision.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance!

Hello Andy, or anyone else affected,

Accepted fuse-umfuse-ext2 into trusty-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/fuse-umfuse-ext2/0.4-1.1ubuntu0.14.04.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed.Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested and change the tag from verification-needed-trusty to verification-done-trusty. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-trusty. In either case, details of your testing will help us make a better decision.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance!

Tested on Xenial and Trusty and the test-case passes fine.

Also confirmed on Artful and Zesty!

This bug was fixed in the package fuse-umfuse-ext2 - 0.4-1.1ubuntu0.17.04.1

---------------
fuse-umfuse-ext2 (0.4-1.1ubuntu0.17.04.1) zesty; urgency=high

  * Correctly release locks in op_link (LP: #1674868).

 -- Andrew Caldwell <email address hidden> Wed, 22 Mar 2017 19:42:35 +0000

The verification of the Stable Release Update for fuse-umfuse-ext2 has completed successfully and the package has now been released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.

This bug was fixed in the package fuse-umfuse-ext2 - 0.4-1.1ubuntu0.16.04.1

---------------
fuse-umfuse-ext2 (0.4-1.1ubuntu0.16.04.1) xenial; urgency=high

  * Correctly release locks in op_link (LP: #1674868).

 -- Andrew Caldwell <email address hidden> Wed, 22 Mar 2017 19:42:35 +0000

This bug was fixed in the package fuse-umfuse-ext2 - 0.4-1.1ubuntu0.14.04.1

---------------
fuse-umfuse-ext2 (0.4-1.1ubuntu0.14.04.1) trusty; urgency=high

  * Correctly release locks in op_link (LP: #1674868).

 -- Andrew Caldwell <email address hidden> Wed, 22 Mar 2017 19:42:35 +0000

Since 0.1 < 0.16.04, the version number in artful is now older than in xenial (bug 1780680).