The sysroot wrapper is checking the value of SYSROOT and, if it is not set, it logs a message in a log file. Noone seems to be paying attention to this. And when the check fails it consumes a lot of time as it is show in issue 773138.

from e-mail discussion

Vapier said:
"i don't believe anyone looks at the log until something goes really wrong, and they even know this log exists.  so rarely.

it has no real value in the standalone case.  we could disable it via cros_setup_toolchains.py:_ProcessSysrootWrappers like we do with ccache support.

that said, i think fundamentally the check makes sense in our cross-compiling scenario as it usually means we'd be using the wrong sysroot to compile/link.  in fact, as i look my log now, i see packages doing it wrong like media-libs/dlm.

making the check require an env var be set wouldn't help ... in the case of packages like dlm, they'd remove that env var too.  the code walks back through its parents, so we could have it trigger when emerge shows up.  if it does, we can have it dump to stderr and then fail, otherwise just be silent."