New function to ensure compatibility with non-standard terminals

[mertalpt]

Proposed solution to issue #505 . Maintainers' input is required for some design decisions.

Problem Summary
Terminal capabilities are stored in terminfo databases and used by libraries such as ncurses to give terminal-independent methods of terminal controls.

Default Fedora images pulled by toolbox lack the necessary terminfo entries to support less common terminals that result in unusable shells inside containers. The solution is to provide the terminfo entry of the current terminal described by the TERM environment variable.

Possible Approaches

• Add ncurses-base and ncurses-term packages to Fedora images to provide a comprehensive terminfo database that is likely to cover all terminals that can be downloaded through a package manager. It is a simple fix but other images that might be used with toolbox need to include those packages to fix this issue for all possible toolbox users. Also, it will not cover terminals that are not found in ncurses packages. But it might be left as an user responsibility in such edge cases.
• Copy the terminfo entry corresponding to user's terminal described by TERM inside the container terminfo database. This is the approach I took with this pull request. Read below for implementation problems.

Implementation Problems

• Standard terminfo database is stored in a directory (/usr/share/terminfo) that requires root privileges for write access. Hence, a read lock cannot be acquired without superuser. Same holds true for every other non-privileged program and it is very unlikely a terminfo entry will ever be changed while toolbox is reading from it. But the possibility is there.
• In the current implementation the host file system is searched at every container entrance in case the necessary terminfo entry gets updated after container creation. This is a tradeoff made in performance against the possibility of a terminal update introducing a change to the corresponding terminfo entry that may cause issues. It can be made so that the host file system is searched only if any terminfo entry for the current terminal is missing from the container.
• I assumed that for any container the host file system (e.g. for "/usr/share/terminfo") can always be accessed through the path "/run/host". I do not know for sure if this assumption is safe in general.

Implementation Commentary

1. terminfo entry for the current terminal is searched in the paths with the logic specified by the "Fetching Compiled Descriptions" section of the terminfo manual page.
2. Using "podman-exec" command, necessary directories are created and the terminfo entry is copied into the container.
3. Error cases are checked and reported during the process.
4. The function is called inside run() after copy_etc_profile_d_* calls.

This process adds around 0.2 seconds to the execution time of "toolbox enter" in my somewhat mid-level laptop.

[softwarefactory-project-zuul]

Build failed.

• shellcheck : FAILURE in 2m 23s
• system-test-fedora-31 : SUCCESS in 8m 10s
• system-test-fedora-32 : SUCCESS in 3m 08s
• system-test-fedora-rawhide : FAILURE in 4m 48s

[debarshiray]

Let's keep the discussion in one place in #505

Either way, the POSIX shell implementation of Toolbox is deprecated. It's fine if you want to keep it updated, but any change has to go into the Go implementation.

[debarshiray]

Closing, in favour of #544

[debarshiray]

Note that this is a Bash-ism, while this file requires POSIX shell.