Backend.ai

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* FIX/IMPROVE: entrypoint.sh for kernel containers startup
- Handle UID overlap (not only GID) correctly by renaming the image's existing account
- Allow execution as root if the agent is configured to do so.
- FIX: Ensure library preloads not modifiable by the user accounts in kernels even when they unset
"LD_PRELOAD" environment variable, by writing "/etc/ld.so.preload" file as root.

NOTE: Alpine-based images does not support this because musl-libc do not use /etc/ld* configurations
but only depend on environment variables with a few hard-coded defaults.
* FIX: Ensure dropbear (our intrinsic SSH daemon) to keep environment variables when users either open a
new SSH session or execute a remote command.
* FIX: Regression of the batch-mode execution API.
* MAINTENANCE: Update dependencies and pin Trafaret to v1.x because Trafraet v2.0 release breaks the
backward compatibility.

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* FIX: SFTP/SCP should work consistently in all images, even without `/usr/bin/scp` and `libcrypto`.
Applied static builds of OpenSSH utilities with OpenSSL and zlib included.

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* OPTIMIZE: SFTP file transfers are now 3x faster by increasing the network buffer sizes used by
dropbear.
* FIX: Regression of entrypoint.sh that caused failure of user/group creation, which resulted in
inability to use the SSH service port due to missing username.

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* FIX: entrypoint.sh for kernel containers did not work properly when the container image has an user ID
or group ID that overlaps with the given values or when the agent is configured to use root for
containers. This fixes kernel launches in macOS where the default user's group "staff" has the group
ID 20 which overlaps with the group "dialout" in Ubuntu or "games" in CentOS.

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* FIX: SSH and SFTP support now works as expected in all types of kernels, including Alpine-based ones.
The auto-generated keypair name is changed to "id_container" and now it uses RSA instead of ECDSA for
better compatibility.
* FIX: Handle rarely happened ProcessLookupError when cleaning up kernels and stat synchronizers
which has caused infinitely repeated warning "cannot read stats: sysfs unreadable for container xxxx".
* FIX: Use the canonical, normalized version number for the backend.ai-common setup dependency to silence
pip warnings during installation.

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* FIX: Regression of code execution due to wrong-ordered arguments of code execution RPC call.
* FIX: Potential memory leak and PID exhaustion due to improper termination of stat synchronizer
and its logger processes.