* Simplify reexec on linux
The previous handling that kept a file descriptor open was designed to
keep working even after the qemu-user bug that made us add the
workaround was fixed; however, that breaks reexec with changed
credentials, so now we just use /proc/self/exe if we detect that we're
not being affected by the qemu-user bug.
* Clarify comments
- Don't assume an explicit $GOPATH is set
- Remove golint from linters - it's been deprecated for over a year
and golangci-lint prints a warning instead of running it.
Introduce new make targets to check and add license headers to files
("make lint-license" and "make fix-license"). License checking is now a part of
"make lint" as well.
Initial attempts used goheader, but it caused "make lint-go" to become about 9x
slower (if not more), plus it only targets go files. Google's addlicense is fast
enough and targets however many file types we want.
Existing files that were missing licenses got the header added, using the
current year as the license date.
* Introduce lint-license and fix-license make targets
* Ignore generated files
* Add license to go files
* Replace irregular licenses with standard copyright/license
* Add license to proto files
* Install addlicense in build.assets Dockerfile
To execute an SSH command, Teleport re-executes itself and execs the
command from this child process:
teleport -> teleport exec -> sh -c "user command"
Both parent teleport processes could exit unexpectedly (from SIGKILL or
even connection interruption).
Make sure all child processes get cleaned up and not orphaned to PID 1:
- teleport exec via SIGQUIT to request graceful shutdown
- user command via SIGKILL because it might ignore other signals
