Conception/drake-master/tools/skylark

This directory contains build system files related to the Bazel build system. https://bazel.build/

Specifically, files named *.bzl are Skylark extensions. https://bazel.build/versions/master/docs/skylark/concepts.html

This folder is intended to house skylark code used by Drake. It contains both generic functions such as pathutils.bzl, and widely-used but drake-specific functions such as the drake_{cc,java,py,...}.bzl language-specific helpers.

Options shared across multiple macros

Several of the macros in Drake (e.g., drake_cc_googletest, drake_py_unittest, etc.) provide for additional options beyond what is built-in to Bazel. In cases where those options are shared across several different rules, we document the option here to avoid repetition.

allow_network

A list of components allowed to use the network, per IsNetworkingAllowed() in drake/common/network_policy.h. When empty or not set, defaults to allowing ["meshcat"] but nothing else. You can set it to ["none"] to deny everything.

We allow meshcat by default because it's mostly harmless, a bajillion tests use it by default, and it doesn't seem worthwhile to try to allow-list them all.

Note that this does not affect network sandboxing (i.e., Bazel's block-network tag). Code outside of Drake purview can still access the network in tests (e.g., license servers for commercial solvers).

num_threads

Can either be None, or else an integer.

When None or 1, adds OMP_NUM_THREADS="1" to env to disable OpenMP.

When N>1, adds OMP_NUM_THREADS="N" to env to allow for the desired level of parallelism, and also adds "cpu:N" to tags to reserve sufficient CPUs for the test. Those changes are a necessary but not sufficient condition to enable actual CPU parallelism while running the test. In addition, the overall build must also have OpenMP enabled, via --config=omp or --config=everything.

Note that setting N>1 will also be applied to any sub-processes that are launched by your test program. Ask for help on Slack if you need this flag to work correctly in the presence of sub-processes.

(Aside: Besides OMP_NUM_THREADS, the function also sets several other environment variables using alternative spellings of the same concept; the overall effect should be the same.)