Introduce REBAR_DEPS_PREFER_LIBS env variable to alter search behaviour

[matwey]

When REBAR_DEPS_PREFER_LIBS is set, dependencies with defined sources
are allowed to be searched for in system lib directory.

Under specific circumstances (i.e. in build environments without networking)
it is impossible to fetch deps locally. So, user needs a way to ask rebar to
search in system lib directory as well.

Signed-off-by: Matwey V. Kornilov matwey.kornilov@gmail.com

[lrascao]

+1, could you please append your name to the THANKS file, document the variable in the wiki (Environment variables section) and write a small test in inttest/ to prevent future regressions? A test in the same style as the tdeps/ would be sufficient

[matwey]

REBAR_DEPS_PREFER_LIBS

REBAR_DEPS_PREFER_LIBS is boolean environment variable. When it is set, rebar looks for all dependencies in system-wide lib directory.
In some restricted build environments all dependencies are usually already pre-installed into specific system-wide place, whereas internet access is prohibited which makes git clone useless.

[ghost]

We should explain REBAR_DEPS_FORCE_LIB in the rebar_deps help string (rebar_deps:info).

[ghost]

A short explanation can be something like this: "If REBAR_DEPS_FORCE_LIB is set, consider only deps found in the code path and skip fetching."

[ghost]

Thinking about it, would it be clearer to rename REBAR_DEPS_FORCE_LIB to REBAR_DEPS_FORCE_SYS, REBAR_DEPS_IGNORE_LOCAL or something similar. It's unclear to me what FORCE_LIB means.

[matwey]

I think IGNORE_LOCAL is not exact because local is not ignored completely, it is just scanned after system. REBAR_DEPS_FORCE_LIB is accordant with ERL_LIBS
However, I am fine with any name.

[ghost]

How about PREFER_LIB or PREFER_SYS?

[matwey]

Maybe REBAR_DEPS_PREFER_LIBS in plural?

[ghost]

Works for me. @lrascao?

[lrascao]

Sure, looks good.
@matwey could you please rebase onto master? we've merged some stuff in the meantime

[matwey]

Any news? I don't remember whether you have email notifications when I rebase the request.

[lrascao]

Sorry, got busy with stuff, will merge today