Cquery (Configurable Query)

Overview

cquery is a variant of query that correctly handles select() and build options' effects on the build graph.

It achieves this by running over the results of Bazel's analysis phase, which integrates these effects. query, by constrast, runs over the results of Bazel's loading phase, before options are evaluated.

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: query doesn't know which select() branch will be chosen
# so it conservatively lists all of them.
$ bazel query "deps(//tree:ash)" --noimplicit_deps
//tree:ash
//tree:white-ash
//tree:manna-ash
//tree:common-ash

# cquery: cquery lets you set build options at the command line and chooses
# the exact dependencies that implies.
$ bazel cquery "deps(//tree:ash)" --define species=excelsior --noimplicit_deps
//tree:ash (hash-of-config)
//tree:manna-ash (hash-of-config)

Each result includes information about the configuration in which the target was built. For most targets, this is an (opaque) hash of the build options and values in that configuration. Host-configured targets are marked (HOST). Non-generated source files, like those commonly found in srcs, have no need for configuration and are marked (null).

Since cquery runs over the configured target graph. it doesn't have insight into artifacts like build actions nor access to test_suite rules as they are not configured targets. For the former, see aquery.

Basic Syntax

A simple cquery call looks like:

bazel cquery "function(//target)"

The query expression "function(//target)" consists of the following:

  • function(...) is the function to run on the target. cquery supports most of query's functions, plus a few new ones.
  • //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.

cquery requires a target to run through the loading and analysis phases. Unless otherwise specified, cquery parses the target(s) listed in the query expression. See --universe_scope for querying dependencies of top-level build targets.

Target Pattern Evaluation

//foo has a different meaning for cquery than for query. This is because cquery evaluates configured targets and the build graph may have multiple configured versions of //foo.

For cquery, a target pattern in the query expression evaluates to every configured target with a label that matches that pattern. Output is deterministic, but cquery makes no ordering guarantee beyond the core query ordering contract.

This produces subtler results for query expressions than with query. For example, the following can produce multiple results: 

# Analyzes //foo in the target configuration, but also analyzes
# //genrule_with_foo_as_tool which depends on a host-configured
# //foo. So there are two configured target instances of //foo in
# the build graph.
$ bazel cquery //foo --universe_scope=//foo,//genrule_with_foo_as_tool
//foo (9f87702f17541d32a56488e3b8be8a98e52b06945f7)
//foo (HOST)

If you want to precisely declare which instance to query over, use the config function.

See query's target pattern documentation for more information on target patterns.

Functions

Of the set of functions supported by query, cquery supports all but siblings, buildfiles, and tests.

cquery also introduces the following new functions:

config

expr ::= config(expr, word)

The config operator attempts to find the configured target for the label denoted by the first argument and configuration specified by the second argument.

Valid values for the second argument are "target", "host", "null", or a custom configuration hash. Hashes can be retrieved from $ bazel config or a prevous cquery's output.

Examples: 

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

$ bazel cquery "deps(//foo)"
//bar (HOST)
//baz (3732cc8c6d40d97)

$ bazel cquery "config(//baz, 3732cc8c6d40d97)"

If not all results of the first argument can be found in the specified configuration, only those that can be found are returned. If no results can be found in the specified configuration, the query fails.

Options

Build Options

cquery runs over 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 

--transitions=lite
--transitions=full

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.

Proto

--output=proto

This option causes the resulting targets to be printed in a binary protocol buffer form. The definition of the protocol buffer can be found at src/main/protobuf/analysis.proto.

--[no]proto:include_configurations

By default, cquery results return configuration information as part of each configured target. If you'd like to omit this information and get proto output that is formatted exactly like query's proto output, set this flag to false .

See query's proto output documentation for more proto output-related options.

Note: while selects are resolved both at the top level of returned targets and within attributes, all possible inputs for selects are still included as rule_input fields.

cquery vs. query

cquery and query complement each other and excel in different niches. Consider the following to decide which is right for you:

  • cquery follows specific select() branches to model the exact graph you build. query doesn't know which branch the build chooses, so overapproximates by including all branches.
  • cquery's precision requires building more of the graph than query does. Specifically, cquery evaluates configured targets while query only evaluates targets. This takes more time and uses more memory.
  • cquery's intepretation of the query language introduces ambiguity that query avoids. For example, if "//foo" exists in two configurations, which one should cquery "deps(//foo)" use? The config function can help with this.
  • As a newer tool, cquery lacks support for certain use cases. See Known Issues for details.

Known Issues

  • All targets that cquery "builds" must have the same configuration.

    Before evaluating queries, cquery triggers a build up to just before the point where build actions would execute. The targets it "builds" are by default selected from all labels that appear in the query expression (this can be overridden with --universe_scope). These must have the same configuration.

    While these generally share the top-level "target" configuration, rules can change their own configuration with incoming edge transitions. This is where cquery falls short.

    Workaround: If possible, set --universe_scope to a stricter scope. For example: 

    # This command attempts to build the transitive closures of both //foo and
# //bar. //bar uses an incoming edge transition to change its --cpu flag.
$ bazel cquery 'somepath(//foo, //bar)'
ERROR: Error doing post analysis query: Top-level targets //foo and //bar
have different configurations (top-level targets with different
configurations is not supported)

# This command only builds the transitive closure of //foo, under which
# //bar should exist in the correct configuration.
$ bazel cquery 'somepath(//foo, //bar)' --universe_scope=//foo
  • No support for aspects.
  • No support for --output=graph or --output=xml.
  • No support for 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 the Bazel team at one of these channels.