tools/compliance/policy_resolve.go

// 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

import (
	"sync"
)

var (
	// AllResolutions is a TraceConditions function that resolves all
	// unfiltered license conditions.
	AllResolutions = TraceConditions(func(tn *TargetNode) LicenseConditionSet { return tn.licenseConditions })
)

// TraceConditions is a function that returns the conditions to trace for each
// target node `tn`.
type TraceConditions func(tn *TargetNode) LicenseConditionSet

// ResolveBottomUpConditions performs a bottom-up walk of the LicenseGraph
// propagating conditions up the graph as necessary according to the properties
// of each edge and according to each license condition in question.
//
// e.g. if a "restricted" condition applies to a binary, it also applies to all
// of the statically-linked libraries and the transitive closure of their static
// dependencies; even if neither they nor the transitive closure of their
// dependencies originate any "restricted" conditions. The bottom-up walk will
// not resolve the library and its transitive closure, but the later top-down
// walk will.
func ResolveBottomUpConditions(lg *LicenseGraph) {
	TraceBottomUpConditions(lg, AllResolutions)
}

// TraceBottomUpConditions performs a bottom-up walk of the LicenseGraph
// propagating trace conditions from `conditionsFn` up the graph as necessary
// according to the properties of each edge and according to each license
// condition in question.
func TraceBottomUpConditions(lg *LicenseGraph, conditionsFn TraceConditions) {

	// short-cut if already walked and cached
	lg.onceBottomUp.Do(func() {
		// amap identifes targets previously walked. (guarded by mu)
		amap := make(map[*TargetNode]struct{})

		// mu guards concurrent access to amap
		var mu sync.Mutex

		var walk func(target *TargetNode, treatAsAggregate bool) LicenseConditionSet

		walk = func(target *TargetNode, treatAsAggregate bool) LicenseConditionSet {
			priorWalkResults := func() (LicenseConditionSet, bool) {
				mu.Lock()
				defer mu.Unlock()

				if _, alreadyWalked := amap[target]; alreadyWalked {
					if treatAsAggregate {
						return target.resolution, true
					}
					if !target.pure {
						return target.resolution, true
					}
					// previously walked in a pure aggregate context,
					// needs to walk again in non-aggregate context
				} else {
					target.resolution |= conditionsFn(target)
					amap[target] = struct{}{}
				}
				target.pure = treatAsAggregate
				return target.resolution, false
			}
			cs, alreadyWalked := priorWalkResults()
			if alreadyWalked {
				return cs
			}

			c := make(chan LicenseConditionSet, len(target.edges))
			// add all the conditions from all the dependencies
			for _, edge := range target.edges {
				go func(edge *TargetEdge) {
					// walk dependency to get its conditions
					cs := walk(edge.dependency, treatAsAggregate && edge.dependency.IsContainer())

					// turn those into the conditions that apply to the target
					cs = depConditionsPropagatingToTarget(lg, edge, cs, treatAsAggregate)

					c <- cs
				}(edge)
			}
			for i := 0; i < len(target.edges); i++ {
				cs |= <-c
			}
			mu.Lock()
			target.resolution |= cs
			mu.Unlock()

			// return conditions up the tree
			return cs
		}

		// walk each of the roots
		for _, rname := range lg.rootFiles {
			rnode := lg.targets[rname]
			_ = walk(rnode, rnode.IsContainer())
		}
	})
}

// ResolveTopDownCondtions performs a top-down walk of the LicenseGraph
// propagating conditions from target to dependency.
//
// e.g. For current policy, none of the conditions propagate from target to
// dependency except restricted. For restricted, the policy is to share the
// source of any libraries linked to restricted code and to provide notice.
func ResolveTopDownConditions(lg *LicenseGraph) {
	TraceTopDownConditions(lg, AllResolutions)
}

// TraceTopDownCondtions performs a top-down walk of the LicenseGraph
// propagating trace conditions returned by `conditionsFn` from target to
// dependency.
func TraceTopDownConditions(lg *LicenseGraph, conditionsFn TraceConditions) {

	// short-cut if already walked and cached
	lg.onceTopDown.Do(func() {
		wg := &sync.WaitGroup{}
		wg.Add(1)

		// start with the conditions propagated up the graph
		TraceBottomUpConditions(lg, conditionsFn)

		// amap contains the set of targets already walked. (guarded by mu)
		amap := make(map[*TargetNode]struct{})

		// mu guards concurrent access to amap
		var mu sync.Mutex

		var walk func(fnode *TargetNode, cs LicenseConditionSet, treatAsAggregate bool)

		walk = func(fnode *TargetNode, cs LicenseConditionSet, treatAsAggregate bool) {
			defer wg.Done()
			continueWalk := func() bool {
				mu.Lock()
				defer mu.Unlock()

				depcs := fnode.resolution
				_, alreadyWalked := amap[fnode]
				if alreadyWalked {
					if cs.IsEmpty() {
						return false
					}
					if cs.Difference(depcs).IsEmpty() {
						// no new conditions

						// pure aggregates never need walking a 2nd time with same conditions
						if treatAsAggregate {
							return false
						}
						// non-aggregates don't need walking as non-aggregate a 2nd time
						if !fnode.pure {
							return false
						}
						// previously walked as pure aggregate; need to re-walk as non-aggregate
					}
				}
				fnode.resolution |= conditionsFn(fnode)
				fnode.resolution |= cs
				fnode.pure = treatAsAggregate
				amap[fnode] = struct{}{}
				cs = fnode.resolution
				return true
			}()
			if !continueWalk {
				return
			}
			// for each dependency
			for _, edge := range fnode.edges {
				// dcs holds the dpendency conditions inherited from the target
				dcs := targetConditionsPropagatingToDep(lg, edge, cs, treatAsAggregate, conditionsFn)
				dnode := edge.dependency
				// add the conditions to the dependency
				wg.Add(1)
				go walk(dnode, dcs, treatAsAggregate && dnode.IsContainer())
			}
		}

		// walk each of the roots
		for _, rname := range lg.rootFiles {
			rnode := lg.targets[rname]
			wg.Add(1)
			// add the conditions to the root and its transitive closure
			go walk(rnode, NewLicenseConditionSet(), rnode.IsContainer())
		}
		wg.Done()
		wg.Wait()
	})
}