Cquery (Configurable Query)

Overview

cquery is a command complementary to query that properly handles configurations. While cquery returns answers that are more correct, it does not replace query as it does not support all of query's functions.

cquery achieves configuration awareness by running after the analysis phase of a build, unlike traditional query, which runs after the loading phase.

The most common use for cquery is obtaining answers that correctly evaluate select() statements in configurable attributes. For example: 

$ cat > tree/BUILD <<EOF
sh_library(
    name = "ash",
    deps = select({
        ":excelsior": [":manna-ash"],
        ":americana": [":white-ash"],
        "//conditions:default": [":common-ash"],
    }),
)
sh_library(name = "manna-ash")
sh_library(name = "white-ash")
sh_library(name = "common-ash")
config_setting(
    name = "excelsior",
    values = {"define": "species=excelsior"},
)
config_setting(
    name = "americana",
    values = {"define": "species=americana"},
)
EOF

#Traditional query
$ bazel query "deps(//tree:ash)" --define species=excelsior --noimplicit_deps
//tree:ash
//tree:white-ash
//tree:manna-ash
//tree:common-ash

#cquery
$ bazel cquery "deps(//tree:ash)" --define species=excelsior --noimplicit_deps
//tree:ash (hash-of-config)
//tree:manna-ash (hash-of-config)

Keep in mind that cquery works only on the configured target graph. Because of this, it does not have insight into artifacts like build actions nor access to test_suite rules as they are not configured targets.

Basic Syntax

A simple example of the syntax for cquery is as follows:

bazel cquery "function(//target)"

The query expression (in quotes) consists of the following:

  • function(...) is the function to run on the target. cquery supports most of the same functions as traditional query.
  • //target is the expression fed to the function. In this example, the expression is a simple target, but the query language also allows nesting of functions. See the Query How-To for examples.

Note that cquery requires a target on which to run the loading and analysis phases. Unless otherwise specified, cquery parses the target(s) listed in the query expression. See the --universe_scope option below for querying targets built under other targets.

Functions

Of the set of functions supported by the traiditional query, cquery supports all but siblings, buildfiles, and tests. The functions listed below are cquery-specific.

config

expr ::= config(expr, word)

The config operator attempts to return the result of the first argument, configured in the configuration specified by the second argument. Today, the second argument can only take in three options target, host, or null but we hope to expand this functionality to be able to input custom configurations (or custom configuration diffs from the default target configuration).

$ bazel cquery config(//foo, host) --universe_scope=//bar

Note that the above example query will return //foo configured in the host configuration if and only if //foo exists in the host configuration in the universe of this query (which we've set to the transitive closure of //bar).

If not all results of the first argument can be found in the specified configuration, then only those that can be found are returned. If no results of the first argument can be found in the specified configuration, an error is thrown.

Options

Build Options

cquery runs on top of a regular Bazel build and thus inherits the set of options available during a build.

Cquery options

--universe_scope (comma-separated list)

Often, the dependencies of configured targets go through transitions, which causes their configuration to differ from their dependent. This flag allows you to query a target as if it were built as a dependency or a transitive dependency of another target. For example: 

# x/BUILD
genrule(
     name = "my_gen",
     srcs = ["x.in"],
     outs = ["x.cc"],
     cmd = "$(locations :tool) $< >$@",
     tools = [":tool"],
)
cc_library(
    name = "tool",
)

Genrules configure their tools in the host configuration so the following queries would produce the following outputs:

Query Target Built Output
bazel cquery "//x:tool" //x:tool //x:tool(targetconfig)
bazel cquery "//x:tool" --universe_scope="//x:my_gen" //x:my_gen //x:tool(hostconfig)

If this flag is set, its contents are built. If it's not set, all targets mentioned in the query expression are built instead. The transitive closure of the built targets are used as the universe of the query. Either way, the targets to be built must be buildable at the top level (that is, compatible with top-level options). cquery returns results in the transitive closure of these top-level targets.

Even if it's possible to build all targets in a query expression at the top level, it may be beneficial to not do so. For example, explicitly setting --universe_scope could prevent building targets multiple times in configurations you don't care about. It could also help specify which configuration version of a target you're looking for (since it's not currently possible to fully specify this any other way). We recommend that you set this flag if your query expression is more complex than deps(//foo).

--implicit_deps (boolean, default=True)

Setting this flag to false filters out all results that aren't explicitly set in the BUILD file and instead set elsewhere by Bazel.

--tool_deps (boolean, default=True)

Setting this flag to false filters out all configured targets for which the path from the queried target to them crosses a transition between the target configuration and the non-target configurations. If the queried target is in the target configuration, setting --notool_deps will only return targets that also are in the target configuration. If the queried target is in a non-target configuration, setting --notool_deps will only return targets also in non-target configurations.

Output Formats

By default, cquery outputs results in a dependency-ordered list of label and configuration pairs. There are other options for exposing the results as well.

Transitions

Configuration transitions are used to build targets underneath the top level targets in different configurations than the top level targets. For example, a target might impose a transition to the host transition on all dependencies in its tools attribute. These are known as attribute transitions. Rules can also impose transitions on their own configurations, known as rule class transitions. This output format outputs information about these transitions such as what type they are and the effect they have on build options.

This output format is triggered by the --transitions flag which by default is set to NONE. It can be set to FULL or LITE mode. FULL mode outputs information about rule class transitions and attribute transitions including a detailed diff of the options before and after the transition. LITE mode outputs the same information without the options diff.

Known Issues

  • Inaccessible configurations. With the exception of the host configuration being printed as HOST, Configurations are currently output as hashes and there is no way for the user to input them (and thus to directly specify the configuration to query a target in).
  • No support for aspects, output options, or recursive target patterns (/...).
  • Non-deterministic output. Cquery does not automatically wipe the build graph from previous commands and is therefore prone to picking up results from past queries. For example, genquery exerts a host transition on its tools attribute - that is, it configures its tools in the host configuration. We can see the lingering effects of that transition below.
$ cat > foo/BUILD <<<EOF
genrule(
    name = "my_gen",
    srcs = ["x.in"],
    outs = ["x.cc"],
    cmd = "$(locations :tool) $< >$@",
    tools = [":tool"],
)
cc_library(
    name = "tool",
)
EOF

$ bazel cquery "//foo:tool"
tool(target_config)

$ bazel cquery "deps(//foo:my_gen)"
my_gen (target_config)
tool (host_config)
...

$ bazel cquery "//foo:tool"
tool(host_config)

Workaround: change any startup option to force re-analysis of configured targets. For example, add --test_arg=<whatever> to your build command.

Updates

The Bazel configurability team is continuously improving cquery. If you want to ask questions, stay updated, or get involved, contact juliexxia@google.com