Allow 'default' entry in provide map to decide what happens when a re…

…quire doesn't match

src/build/filegroup.go

@@ -108,8 +108,8 @@ func buildFilegroup(state *core.BuildState, target *core.BuildTarget) (bool, err
 	}
 	changed := false
 	outDir := target.OutDir()
-	localSources := target.AllSourceLocalPaths(state.Graph)
-	for i, source := range target.AllSourceFullPaths(state.Graph) {
+	localSources := target.AllSourceLocalPaths(state.Graph, state.Config.FeatureFlags.FFDefaultProvides)
+	for i, source := range target.AllSourceFullPaths(state.Graph, state.Config.FeatureFlags.FFDefaultProvides) {
 		out := filepath.Join(outDir, localSources[i])
 		fileChanged, err := theFilegroupBuilder.Build(state, target, source, out)
 		if err != nil {
@@ -154,8 +154,8 @@ func buildFilegroup(state *core.BuildState, target *core.BuildTarget) (bool, err
 // This is a small optimisation to ensure we don't need to recalculate them unnecessarily.
 func copyFilegroupHashes(state *core.BuildState, target *core.BuildTarget) {
 	outDir := target.OutDir()
-	localSources := target.AllSourceLocalPaths(state.Graph)
-	for i, source := range target.AllSourceFullPaths(state.Graph) {
+	localSources := target.AllSourceLocalPaths(state.Graph, state.Config.FeatureFlags.FFDefaultProvides)
+	for i, source := range target.AllSourceFullPaths(state.Graph, state.Config.FeatureFlags.FFDefaultProvides) {
 		if out := filepath.Join(outDir, localSources[i]); out != source {
 			state.PathHasher.CopyHash(source, out)
 		}

src/core/build_env.go

@@ -69,7 +69,7 @@ func TargetEnvironment(state *BuildState, target *BuildTarget) BuildEnv {
 // We read this as being slightly more POSIX-compliant than not having it set at all...
 func BuildEnvironment(state *BuildState, target *BuildTarget, tmpDir string) BuildEnv {
 	env := TargetEnvironment(state, target)
-	sources := target.AllSourcePaths(state.Graph)
+	sources := target.AllSourcePaths(state.Graph, state.Config.FeatureFlags.FFDefaultProvides)
 	outEnv := target.GetTmpOutputAll(target.Outputs())
 	abs := filepath.IsAbs(tmpDir)
 
@@ -92,7 +92,7 @@ func BuildEnvironment(state *BuildState, target *BuildTarget, tmpDir string) Bui
 	}
 	// Named source groups if the target declared any.
 	for name, srcs := range target.NamedSources {
-		paths := target.SourcePaths(state.Graph, srcs)
+		paths := target.SourcePaths(state.Graph, srcs, state.Config.FeatureFlags.FFDefaultProvides)
 		// TODO(macripps): Quote these to prevent spaces from breaking everything (consider joining with NUL or sth?)
 		env = append(env, "SRCS_"+strings.ToUpper(name)+"="+strings.Join(paths, " "))
 	}

src/core/build_target.go

@@ -491,28 +491,28 @@ func (target *BuildTarget) StartTestSuite() {
 }
 
 // AllSourcePaths returns all the source paths for this target
-func (target *BuildTarget) AllSourcePaths(graph *BuildGraph) []string {
-	return target.allSourcePaths(graph, BuildInput.Paths)
+func (target *BuildTarget) AllSourcePaths(graph *BuildGraph, ffDefaultProvide bool) []string {
+	return target.allSourcePaths(graph, BuildInput.Paths, ffDefaultProvide)
 }
 
 // AllSourceFullPaths returns all the source paths for this target, with a leading
 // plz-out/gen etc if appropriate.
-func (target *BuildTarget) AllSourceFullPaths(graph *BuildGraph) []string {
-	return target.allSourcePaths(graph, BuildInput.FullPaths)
+func (target *BuildTarget) AllSourceFullPaths(graph *BuildGraph, ffDefaultProvide bool) []string {
+	return target.allSourcePaths(graph, BuildInput.FullPaths, ffDefaultProvide)
 }
 
 // AllSourceLocalPaths returns the local part of all the source paths for this target,
 // i.e. without this target's package in it.
-func (target *BuildTarget) AllSourceLocalPaths(graph *BuildGraph) []string {
-	return target.allSourcePaths(graph, BuildInput.LocalPaths)
+func (target *BuildTarget) AllSourceLocalPaths(graph *BuildGraph, ffDefaultProvide bool) []string {
+	return target.allSourcePaths(graph, BuildInput.LocalPaths, ffDefaultProvide)
 }
 
 type buildPathsFunc func(BuildInput, *BuildGraph) []string
 
-func (target *BuildTarget) allSourcePaths(graph *BuildGraph, full buildPathsFunc) []string {
+func (target *BuildTarget) allSourcePaths(graph *BuildGraph, full buildPathsFunc, ffDefaultProvide bool) []string {
 	ret := make([]string, 0, len(target.Sources))
 	for _, source := range target.AllSources() {
-		ret = append(ret, target.sourcePaths(graph, source, full)...)
+		ret = append(ret, target.sourcePaths(graph, source, full, ffDefaultProvide)...)
 	}
 	return ret
 }
@@ -533,7 +533,7 @@ func (target *BuildTarget) AllURLs(state *BuildState) []string {
 // TODO(peterebden,tatskaari): Work out if we really want to have this and how the suite of *Dependencies functions
 //
 //	below should behave (preferably nicely).
-func (target *BuildTarget) resolveDependencies(graph *BuildGraph, callback func(*BuildTarget) error) error {
+func (target *BuildTarget) resolveDependencies(graph *BuildGraph, callback func(*BuildTarget) error, ffDefaultProvide bool) error {
 	var g errgroup.Group
 	target.mutex.RLock()
 	for i := range target.dependencies {
@@ -542,7 +542,7 @@ func (target *BuildTarget) resolveDependencies(graph *BuildGraph, callback func(
 			continue // already done
 		}
 		g.Go(func() error {
-			if err := target.resolveOneDependency(graph, dep); err != nil {
+			if err := target.resolveOneDependency(graph, dep, ffDefaultProvide); err != nil {
 				return err
 			}
 			for _, d := range dep.deps {
@@ -557,14 +557,15 @@ func (target *BuildTarget) resolveDependencies(graph *BuildGraph, callback func(
 	return g.Wait()
 }
 
-func (target *BuildTarget) resolveOneDependency(graph *BuildGraph, dep *depInfo) error {
+func (target *BuildTarget) resolveOneDependency(graph *BuildGraph, dep *depInfo, ffDefaultProvide bool) error {
 	t := graph.WaitForTarget(*dep.declared)
 	if t == nil {
 		return fmt.Errorf("Couldn't find dependency %s", dep.declared)
 	}
 	dep.declared = &t.Label // saves memory by not storing the label twice once resolved
 
-	labels := t.provideFor(target)
+	// TODO(jpoole): shouldn't this use the ProvideFor wrapper that handles resolving to self if there's no match?
+	labels := t.provideFor(target, ffDefaultProvide)
 
 	if len(labels) == 0 {
 		target.mutex.Lock()
@@ -595,7 +596,7 @@ func (target *BuildTarget) resolveOneDependency(graph *BuildGraph, dep *depInfo)
 // MustResolveDependencies is exposed only for testing purposes.
 // TODO(peterebden, tatskaari): See if we can get rid of this.
 func (target *BuildTarget) ResolveDependencies(graph *BuildGraph) error {
-	return target.resolveDependencies(graph, func(*BuildTarget) error { return nil })
+	return target.resolveDependencies(graph, func(*BuildTarget) error { return nil }, false)
 }
 
 // DeclaredDependencies returns all the targets this target declared any kind of dependency on (including sources and tools).
@@ -880,19 +881,19 @@ func (target *BuildTarget) GetRealOutput(output string) string {
 }
 
 // SourcePaths returns the source paths for a given set of sources.
-func (target *BuildTarget) SourcePaths(graph *BuildGraph, sources []BuildInput) []string {
+func (target *BuildTarget) SourcePaths(graph *BuildGraph, sources []BuildInput, ffDefaultProvide bool) []string {
 	ret := make([]string, 0, len(sources))
 	for _, source := range sources {
-		ret = append(ret, target.sourcePaths(graph, source, BuildInput.Paths)...)
+		ret = append(ret, target.sourcePaths(graph, source, BuildInput.Paths, ffDefaultProvide)...)
 	}
 	return ret
 }
 
 // sourcePaths returns the source paths for a single source.
-func (target *BuildTarget) sourcePaths(graph *BuildGraph, source BuildInput, f buildPathsFunc) []string {
+func (target *BuildTarget) sourcePaths(graph *BuildGraph, source BuildInput, f buildPathsFunc, ffDefaultProvide bool) []string {
 	if label, ok := source.nonOutputLabel(); ok {
 		ret := []string{}
-		for _, providedLabel := range graph.TargetOrDie(label).ProvideFor(target) {
+		for _, providedLabel := range graph.TargetOrDie(label).ProvideFor(target, ffDefaultProvide) {
 			ret = append(ret, f(providedLabel, graph)...)
 		}
 		return ret
@@ -1195,20 +1196,25 @@ func (target *BuildTarget) AddProvide(language string, label BuildLabel) {
 }
 
 // ProvideFor returns the build label that we'd provide for the given target.
-func (target *BuildTarget) ProvideFor(other *BuildTarget) []BuildLabel {
-	if p := target.provideFor(other); len(p) > 0 {
+func (target *BuildTarget) ProvideFor(other *BuildTarget, ffDefaultProvide bool) []BuildLabel {
+	if p := target.provideFor(other, ffDefaultProvide); len(p) > 0 {
 		return p
 	}
 	return []BuildLabel{target.Label}
 }
 
 // provideFor is like ProvideFor but returns an empty slice if there is a direct dependency.
 // It's a small optimisation to save allocating extra slices.
-func (target *BuildTarget) provideFor(other *BuildTarget) []BuildLabel {
+func (target *BuildTarget) provideFor(other *BuildTarget, ffDefaultProvide bool) []BuildLabel {
 	target.mutex.RLock()
 	defer target.mutex.RUnlock()
+	var defaultValue []BuildLabel
+	if def, ok := target.Provides["default"]; ok && ffDefaultProvide {
+		defaultValue = []BuildLabel{def}
+	}
+
 	if target.Provides == nil || len(other.Requires) == 0 {
-		return nil
+		return defaultValue
 	}
 	// Never do this if the other target has a data or tool dependency on us.
 	for _, data := range other.Data {
@@ -1228,6 +1234,10 @@ func (target *BuildTarget) provideFor(other *BuildTarget) []BuildLabel {
 			ret = append(ret, label)
 		}
 	}
+
+	if len(ret) == 0 {
+		return defaultValue
+	}
 	return ret
 }
 

src/core/build_target_benchmark_test.go

@@ -17,7 +17,7 @@ func BenchmarkProvideFor(b *testing.B) {
 	b.Run("Simple", func(b *testing.B) {
 		b.ReportAllocs()
 		for i := 0; i < b.N; i++ {
-			result = target1.provideFor(target2)
+			result = target1.provideFor(target2, true)
 		}
 	})
 
@@ -26,7 +26,7 @@ func BenchmarkProvideFor(b *testing.B) {
 	b.Run("NoMatch", func(b *testing.B) {
 		b.ReportAllocs()
 		for i := 0; i < b.N; i++ {
-			result = target2.provideFor(target1)
+			result = target2.provideFor(target1, true)
 		}
 	})
 
@@ -35,15 +35,15 @@ func BenchmarkProvideFor(b *testing.B) {
 	b.Run("OneMatch", func(b *testing.B) {
 		b.ReportAllocs()
 		for i := 0; i < b.N; i++ {
-			result = target2.provideFor(target1)
+			result = target2.provideFor(target1, true)
 		}
 	})
 
 	target1.Requires = []string{"go", "go_src"}
 	b.Run("TwoMatches", func(b *testing.B) {
 		b.ReportAllocs()
 		for i := 0; i < b.N; i++ {
-			result = target2.provideFor(target1)
+			result = target2.provideFor(target1, true)
 		}
 	})
 
@@ -52,15 +52,15 @@ func BenchmarkProvideFor(b *testing.B) {
 	b.Run("FourMatches", func(b *testing.B) {
 		b.ReportAllocs()
 		for i := 0; i < b.N; i++ {
-			result = target2.provideFor(target1)
+			result = target2.provideFor(target1, true)
 		}
 	})
 
 	target1.AddDatum(target2.Label)
 	b.Run("IsData", func(b *testing.B) {
 		b.ReportAllocs()
 		for i := 0; i < b.N; i++ {
-			result = target2.provideFor(target1)
+			result = target2.provideFor(target1, true)
 		}
 	})
 }

src/core/build_target_test.go

@@ -219,18 +219,41 @@ func TestProvideFor(t *testing.T) {
 	// target1 is provided directly since they have a simple dependency
 	target1 := makeTarget1("//src/core:target1", "PUBLIC")
 	target2 := makeTarget1("//src/core:target2", "PUBLIC", target1)
-	assert.Equal(t, []BuildLabel{target1.Label}, target1.ProvideFor(target2))
+	assert.Equal(t, []BuildLabel{target1.Label}, target1.ProvideFor(target2, true))
 	// Now have target2 provide target1. target3 will get target1 instead.
 	target2.Provides = map[string]BuildLabel{"whatevs": target1.Label}
 	target3 := makeTarget1("//src/core:target3", "PUBLIC", target2)
 	target3.Requires = append(target3.Requires, "whatevs")
-	assert.Equal(t, []BuildLabel{target1.Label}, target2.ProvideFor(target3))
+	assert.Equal(t, []BuildLabel{target1.Label}, target2.ProvideFor(target3, true))
 	// Now target4 has a data dependency on target2. It has the same requirement as target3 but
 	// it gets target2 instead of target1, because that's just how data deps work.
 	target4 := makeTarget1("//src/core:target4", "PUBLIC", target2)
 	target4.Data = append(target4.Data, target2.Label)
 	target4.Requires = append(target4.Requires, "whatevs")
-	assert.Equal(t, []BuildLabel{target2.Label}, target2.ProvideFor(target4))
+	assert.Equal(t, []BuildLabel{target2.Label}, target2.ProvideFor(target4, true))
+}
+
+func TestDefaultProvide(t *testing.T) {
+	goTarget := makeTarget1("//src/core:go", "PUBLIC")
+	defaultTarget := makeTarget1("//src/core:default", "PUBLIC")
+	providesGo := makeTarget1("//src/core:provider", "PUBLIC", goTarget, defaultTarget)
+
+	requireGo := makeTarget1("//src/core:req_go", "PUBLIC")
+	requirePython := makeTarget1("//src/core:req_python", "PUBLIC")
+	requireNothing := makeTarget1("//src/core:req_none", "PUBLIC")
+
+	requireGo.Requires = []string{"go"}
+	requirePython.Requires = []string{"python"}
+
+	providesGo.Provides = map[string]BuildLabel{
+		"go":      goTarget.Label,
+		"default": defaultTarget.Label,
+	}
+
+	assert.Equal(t, []BuildLabel{defaultTarget.Label}, providesGo.ProvideFor(requirePython, true))
+	assert.Equal(t, []BuildLabel{defaultTarget.Label}, providesGo.ProvideFor(requireNothing, true))
+	assert.Equal(t, []BuildLabel{providesGo.Label}, providesGo.ProvideFor(requirePython, false))
+	assert.Equal(t, []BuildLabel{goTarget.Label}, providesGo.ProvideFor(requireGo, true))
 }
 
 func TestAddProvide(t *testing.T) {
@@ -240,7 +263,7 @@ func TestAddProvide(t *testing.T) {
 	target2.AddDependency(target1.Label)
 	target2.AddProvide("go", ParseBuildLabel(":target1", "src/core"))
 	target3.Requires = append(target3.Requires, "go")
-	assert.Equal(t, []BuildLabel{target1.Label}, target2.ProvideFor(target3))
+	assert.Equal(t, []BuildLabel{target1.Label}, target2.ProvideFor(target3, true))
 }
 
 func TestAddDatum(t *testing.T) {

src/core/config.go

@@ -688,6 +688,7 @@ type Configuration struct {
 	buildEnvStored *storedBuildEnv
 
 	FeatureFlags struct {
+		FFDefaultProvides bool `help:"Allows setting 'default' in the provides map to control what happens when none of the provided targets match"`
 	} `help:"Flags controlling preview features for the next release. Typically these config options gate breaking changes and only have a lifetime of one major release."`
 	Metrics struct {
 		PrometheusGatewayURL string       `help:"The gateway URL to push prometheus updates to."`

src/core/graph.go

@@ -157,10 +157,10 @@ func NewGraph() *BuildGraph {
 
 // DependentTargets returns the labels that 'from' should actually depend on when it declared a dependency on 'to'.
 // This is normally just 'to' but could be otherwise given require/provide shenanigans.
-func (graph *BuildGraph) DependentTargets(from, to BuildLabel) []BuildLabel {
+func (graph *BuildGraph) DependentTargets(from, to BuildLabel, ffDefaultProvide bool) []BuildLabel {
 	fromTarget := graph.Target(from)
 	if toTarget := graph.Target(to); fromTarget != nil && toTarget != nil {
-		return toTarget.ProvideFor(fromTarget)
+		return toTarget.ProvideFor(fromTarget, ffDefaultProvide)
 	}
 	return []BuildLabel{to}
 }

src/core/graph_test.go

@@ -39,7 +39,7 @@ func TestDependentTargets(t *testing.T) {
 	graph.AddTarget(target1)
 	graph.AddTarget(target2)
 	graph.AddTarget(target3)
-	assert.Equal(t, []BuildLabel{target3.Label}, graph.DependentTargets(target2.Label, target1.Label))
+	assert.Equal(t, []BuildLabel{target3.Label}, graph.DependentTargets(target2.Label, target1.Label, true))
 }
 
 func TestSubrepo(t *testing.T) {

src/core/state.go

@@ -1034,7 +1034,7 @@ func (state *BuildState) ActivateTarget(pkg *Package, label, dependent BuildLabe
 			}
 		}
 	} else {
-		for _, l := range state.Graph.DependentTargets(dependent, label) {
+		for _, l := range state.Graph.DependentTargets(dependent, label, state.Config.FeatureFlags.FFDefaultProvides) {
 			// We use :all to indicate a dependency needed for parse.
 			if err := state.QueueTarget(l, dependent, dependent.IsAllTargets(), mode); err != nil {
 				return err
@@ -1088,7 +1088,7 @@ func (state *BuildState) queueTarget(label, dependent BuildLabel, forceBuild boo
 	if dependent.IsAllTargets() || dependent == OriginalTarget {
 		return state.queueResolvedTarget(target, forceBuild, mode)
 	}
-	for _, l := range target.ProvideFor(state.Graph.TargetOrDie(dependent)) {
+	for _, l := range target.ProvideFor(state.Graph.TargetOrDie(dependent), state.Config.FeatureFlags.FFDefaultProvides) {
 		if l == label {
 			if err := state.queueResolvedTarget(target, forceBuild, mode); err != nil {
 				return err
@@ -1167,7 +1167,7 @@ func (state *BuildState) queueTargetAsync(target *BuildTarget, forceBuild, build
 		if err := target.resolveDependencies(state.Graph, func(t *BuildTarget) error {
 			called.SetTrue()
 			return state.queueResolvedTarget(t, forceBuild, ParseModeNormal)
-		}); err != nil {
+		}, state.Config.FeatureFlags.FFDefaultProvides); err != nil {
 			state.asyncError(target.Label, err)
 			return
 		}