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// Copyright 2014 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package com.google.devtools.build.lib.analysis;
import com.google.common.collect.UnmodifiableIterator;
import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
import com.google.devtools.build.lib.syntax.Label;
import com.google.devtools.build.lib.syntax.SkylarkModule;
import javax.annotation.Nullable;
/**
* Objects that implement this interface bundle multiple {@link TransitiveInfoProvider} interfaces.
*
* <p>This interface (together with {@link TransitiveInfoProvider} is the cornerstone of the data
* model of the analysis phase.
*
* <p>The computation a configured target does is allowed to depend on the following things:
* <ul>
* <li>The associated Target (which will usually be a Rule)
* <li>Its own configuration (the configured target does not have access to other configurations,
* e.g. the host configuration, though)
* <li>The transitive info providers and labels of its direct dependencies.
* </ul>
*
* <p>And these are the only inputs. Notably, a configured target is not supposed to access
* other configured targets, the transitive info collections of configured targets it does not
* directly depend on, the actions created by anyone else or the contents of any input file. We
* strive to make it impossible for configured targets to do these things.
*
* <p>A configured target is expected to produce the following data during its analysis:
* <ul>
* <li>A number of Artifacts and Actions generating them
* <li>A set of {@link TransitiveInfoProvider}s that it passes on to the targets directly dependent
* on it
* </ul>
*
* <p>The information that can be passed on to dependent targets by way of
* {@link TransitiveInfoProvider} is subject to constraints (which are detailed in the
* documentation of that class).
*
* <p>Configured targets are currently allowed to create artifacts at any exec path. It would be
* better if they could be constrained to a subtree based on the label of the configured target,
* but this is currently not feasible because multiple rules violate this constraint and the
* output format is part of its interface.
*
* <p>In principle, multiple configured targets should not create actions with conflicting
* outputs. There are still a few exceptions to this rule that are slated to be eventually
* removed, we have provisions to handle this case (Action instances that share at least one
* output file are required to be exactly the same), but this does put some pressure on the design
* and we are eventually planning to eliminate this option.
*
* <p>These restrictions together make it possible to:
* <ul>
* <li>Correctly cache the analysis phase; by tightly constraining what a configured target is
* allowed to access and what it is not, we can know when it needs to invalidate a particular
* one and when it can reuse an already existing one.
* <li>Serialize / deserialize individual configured targets at will, making it possible for
* example to swap out part of the analysis state if there is memory pressure or to move them in
* persistent storage so that the state can be reconstructed at a different time or in a
* different process. The stretch goal is to eventually facilitate cross-uses caching of this
* information.
* </ul>
*
* <p>Implementations of build rules should <b>not</b> hold on to references to the
* {@link TransitiveInfoCollection}s representing their direct prerequisites in order to reduce
* their memory footprint (otherwise, the referenced object could refer one of its direct
* dependencies in turn, thereby making the size of the objects reachable from a single instance
* unbounded).
*
* @see TransitiveInfoProvider
*/
@SkylarkModule(name = "Target", doc =
"A BUILD target. It is essentially a <code>struct</code> with the following fields:"
+ "<ul>"
+ "<li><h3>label</h3><code><a class=\"anchor\" href=\"#modules.Label\">Label</a> "
+ "Target.label</code><br>The identifier of the target.</li>"
+ "<li><h3>files</h3><code><a class=\"anchor\" href=\"#modules.set\">set</a> Target.files"
+ "</code><br>The (transitive) set of <a class=\"anchor\" href=\"#modules.File\">File</a>s "
+ "produced by this target.</li>"
+ "<li><h3>Extra providers</h3>For rule targets all additional providers provided by this "
+ "target are accessible as <code>struct</code> fields. These extra providers are defined "
+ "in the <code>struct</code> returned by the rule implementation function.</li>"
+ "</ul>")
public interface TransitiveInfoCollection extends Iterable<TransitiveInfoProvider> {
/**
* Returns the transitive information provider requested, or null if the provider is not found.
* The provider has to be a TransitiveInfoProvider Java class.
*/
@Nullable <P extends TransitiveInfoProvider> P getProvider(Class<P> provider);
/**
* Returns the label associated with this prerequisite.
*/
Label getLabel();
/**
* <p>Returns the {@link BuildConfiguration} for which this transitive info collection is defined.
* Configuration is defined for all configured targets with exception of {@link
* InputFileConfiguredTarget} and {@link PackageGroupConfiguredTarget} for which it is always
* <b>null</b>.</p>
*/
@Nullable BuildConfiguration getConfiguration();
/**
* Returns the transitive information requested or null, if the information is not found.
* The transitive information has to have been added using the Skylark framework.
*/
@Nullable Object get(String providerKey);
/**
* Returns an unmodifiable iterator over the transitive info providers in the collections.
*/
@Override
UnmodifiableIterator<TransitiveInfoProvider> iterator();
}
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