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Target Universe in BXL

BXL cquery and target universe

BXL cannot infer the target universe like in the CLI (in most cases). BXL splits up cquery functions per function (ex: ctx.cquery().kind(...)), with the exception of ctx.cquery.eval(...), which accepts literals exactly like in the CLI. For the eval query, target universe is inferred exactly like the CLI.

For all other cases, take the following query as an example:

buck2 cquery "rdeps(deps(//example:foo), deps(//example:bar))"

The target universe here should be constructed from the all the target literals and their transitive deps, which is to say deps(//example:foo, //example:bar). When you run the query, the evaluation of deps(//example:foo) and deps(//example:bar) nested in the rdeps query will happen inside the universe resulting from deps(//example:foo, //example:bar). Translating it to BXL's individual cquery functions, and let’s say we also try to use the target literals to construct the universe as the CLI target inference does:

from_node = ctx.cquery().deps("//example:foo") # universe would be //example:foo

to_node = ctx.cquery().deps("//example:bar") # universe would be //example:bar

rdeps = ctx.cquery().rdeps(from_node, to_node) # what is the universe here?

Here, the from_node query is actually evaluated in the wrong target universe because we have broken up the query steps in BXL. Instead of deps(//example:foo) being evaluated in deps(//example:foo, //example:bar), it’s evaluated with only deps(//example:foo). It’s impossible to know that there’s going to be an rdeps query later on that expects a different target universe.

Specifying target universe in BXL cquery

BXL cquery functions should only accept configured targets as inputs, with the exception of eval and testsof_with_default_platform.

BXL has a ctx.target_universe() function to construct a target_universe object, which has a lookup() function to lookup the configured targets within the target universe and return the target set. ​​The lookup functionality is useful because sometimes a single target can appear multiple times within a target universe. For example, if you specify a cxx toolchain using its unconfigured target label, it will always match against all cxx toolchains in the target universe (so at least once for target deps and once for exec deps), since cxx toolchains may have multiple configurations. Example:

def _impl:
target_universe = ctx.target_universe(["//example:foo", "//example:bar"])
to_node = target_universe.lookup("//example:foo")
from_node = target_universe.lookup("//example:bar")
rdeps = ctx.cquery().rdeps(to_node, from_node)

However, sometimes you might want a specific configuration instead of using all configurations found within a target universe, in which case you could use ctx.configured_targets(...) to specify the configuration. Or, sometimes you may want to use the specific configured target nodes resulting from other BXL calls. In these cases, you can pass the configured targets directly into cquery functions, instead of going through target universe lookup.

What does the target universe tend to be in practice?

For owner query, the universe would be constructed with the unconfigured target nodes returned from ctx.uquery().owner(...). Example:

def _impl:
unconfigured_owners = ctx.uquery().owner("foobar")
target_universe = ctx.target_universe(unconfigured_owners).target_set()
owners = ctx.cquery().owner("foobar", target_universe)

For everything else, the universe would usually be constructed using all target literals found in your query. Example:

def _impl:
target_universe = ctx.target_universe("//example:foo")
inputs = target_universe.target_set()
deps = ctx.cquery().deps(inputs)

While the above guideline should work for rdeps as well, for rdeps the universe would usually be narrowed down to the "to"/"destination" target set argument. (This is a subset of the target universe suggested for non-owner query cases). Updating the example from above:

def _impl:
target_universe = ctx.target_universe("//example:foo") # narrowed down to the "to" literals in rdeps
universe_node = target_universe.target_set()
from_node = target_universe.lookup("//example:bar")
rdeps = ctx.cquery().rdeps(universe_node, from_node)

BXL build and target universe

Note that BXL builds currently do not support target universe, but we intend to add this.