compliance package documentation

Bug: 68860345
Bug: 151177513
Bug: 151953481

Test: m all
Test: m systemlicense
Test: m listshare; out/soong/host/linux-x86/bin/listshare ...
Test: m checkshare; out/soong/host/linux-x86/bin/checkshare ...
Test: m dumpgraph; out/soong/host/linux-x86/dumpgraph ...
Test: m dumpresolutions; out/soong/host/linux-x86/dumpresolutions ...

where ... is the path to the .meta_lic file for the system image. In my
case if

$ export PRODUCT=$(realpath $ANDROID_PRODUCT_OUT --relative-to=$PWD)

... can be expressed as:

${PRODUCT}/gen/META/lic_intermediates/${PRODUCT}/system.img.meta_lic

Change-Id: I5d48eababce7bba39795d3668eee86b332cbe43d

tools/compliance/Android.bp

@@ -51,6 +51,7 @@ bootstrap_go_package {
         "actionset.go",
         "condition.go",
         "conditionset.go",
+        "doc.go",
         "graph.go",
         "policy/policy.go",
         "policy/resolve.go",

tools/compliance/doc.go

@@ -0,0 +1,77 @@
+// Copyright 2021 Google LLC
+//
+// 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 compliance provides an approved means for reading, consuming, and
+analyzing license metadata graphs.
+
+Assuming the license metadata and dependencies are fully and accurately
+recorded in the build system, any discrepancy between the official policy for
+open source license compliance and this code is a bug in this code.
+
+A few principal types to understand are LicenseGraph, LicenseCondition, and
+ResolutionSet.
+
+LicenseGraph
+------------
+
+A LicenseGraph is an immutable graph of the targets and dependencies reachable
+from a specific set of root targets. In general, the root targets will be the
+artifacts in a release or distribution. While conceptually immutable, parts of
+the graph may be loaded or evaluated lazily.
+
+LicenseCondition
+----------------
+
+A LicenseCondition is an immutable tuple pairing a condition name with an
+originating target. e.g. Per current policy, a static library licensed under an
+MIT license would pair a "notice" condition with the static library target, and
+a dynamic license licensed under GPL would pair a "restricted" condition with
+the dynamic library target.
+
+ResolutionSet
+-------------
+
+A ResolutionSet is an immutable set of `AttachesTo`, `ActsOn`, `Resolves`
+tuples describing how license conditions apply to targets.
+
+`AttachesTo` is the trigger for acting. Distribution of the target invokes
+the policy.
+
+`ActsOn` is the target to share, give notice for, hide etc.
+
+`Resolves` is the license condition that the action resolves.
+
+Remember: Each license condition pairs a condition name with an originating
+target so each resolution in a ResolutionSet has two targets it applies to and
+one target from which it originates, all of which may be the same target.
+
+For most condition types, `ActsOn` and `Resolves.Origin` will be the same
+target. For example, a notice condition policy means attribution or notice must
+be given for the target where the condition originates. Likewise, a proprietary
+condition policy means the privacy of the target where the condition originates
+must be respected. i.e. The thing acted on is the origin.
+
+Restricted conditions are different. The infectious nature of restricted often
+means sharing code that is not the target where the restricted condition
+originates. Linking an MIT library to a GPL library implies a policy to share
+the MIT library despite the MIT license having no source sharing requirement.
+
+In this case, one or more resolution tuples will have the MIT license module in
+`ActsOn` and the restricted condition originating at the GPL library module in
+`Resolves`. These tuples will `AttachTo` every target that depends on the GPL
+library because shipping any of those targets trigger the policy to share the
+code.
+*/
+package compliance