tool/listpkgs: add --affected-by-tag

For paring back build tag variant CI runs' set of packages to test.

Updates tailscale/corp#28679

Change-Id: Iba46fd1f58c1eaee1f7888ef573bc8b14fa73208
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tool/listpkgs/listpkgs.go

@@ -26,6 +26,7 @@ var (
 	withTagsAllStr    = flag.String("with-tags-all", "", "if non-empty, a comma-separated list of builds tags to require (a package will only be listed if it contains all of these build tags)")
 	withoutTagsAnyStr = flag.String("without-tags-any", "", "if non-empty, a comma-separated list of build constraints to exclude (a package will be omitted if it contains any of these build tags)")
 	shard             = flag.String("shard", "", "if non-empty, a string of the form 'N/M' to only print packages in shard N of M (e.g. '1/3', '2/3', '3/3/' for different thirds of the list)")
+	affectedByTag     = flag.String("affected-by-tag", "", "if non-empty, only list packages whose test binary would be affected by the presence or absence of this build tag")
 )
 
 func main() {
@@ -41,6 +42,10 @@ func main() {
 		Mode: packages.LoadFiles,
 		Env:  os.Environ(),
 	}
+	if *affectedByTag != "" {
+		cfg.Mode |= packages.NeedImports
+		cfg.Tests = true
+	}
 	if *goos != "" {
 		cfg.Env = append(cfg.Env, "GOOS="+*goos)
 	}
@@ -62,6 +67,11 @@ func main() {
 		withAll = strings.Split(*withTagsAllStr, ",")
 	}
 
+	var affected map[string]bool // PkgPath → true
+	if *affectedByTag != "" {
+		affected = computeAffected(pkgs, *affectedByTag)
+	}
+
 	seen := map[string]bool{}
 	matches := 0
 Pkg:
@@ -69,6 +79,17 @@ Pkg:
 		if pkg.PkgPath == "" { // malformed (shouldn’t happen)
 			continue
 		}
+		if affected != nil {
+			// Skip synthetic packages created by Tests: true:
+			// - for-test variants like "foo [foo.test]" (ID != PkgPath)
+			// - test binary packages like "foo.test" (PkgPath ends in ".test")
+			if pkg.ID != pkg.PkgPath || strings.HasSuffix(pkg.PkgPath, ".test") {
+				continue
+			}
+			if !affected[pkg.PkgPath] {
+				continue
+			}
+		}
 		if seen[pkg.PkgPath] {
 			continue // suppress duplicates when patterns overlap
 		}
@@ -96,7 +117,7 @@ Pkg:
 		if *shard != "" {
 			var n, m int
 			if _, err := fmt.Sscanf(*shard, "%d/%d", &n, &m); err != nil || n < 1 || m < 1 {
-				log.Fatalf("invalid shard format %q; expected 'N/M'", *shard)
+				log.Fatalf("invalid shard format %q; expected ‘N/M’", *shard)
 			}
 			if m > 0 && (matches-1)%m != n-1 {
 				continue // not in this shard
@@ -112,6 +133,62 @@ Pkg:
 	}
 }
 
+// computeAffected returns the set of package paths whose test binaries would
+// differ with vs without the given build tag. It finds packages that directly
+// mention the tag, then propagates transitively via reverse dependencies.
+func computeAffected(pkgs []*packages.Package, tag string) map[string]bool {
+	// Build a map from package ID to package for quick lookup.
+	byID := make(map[string]*packages.Package, len(pkgs))
+	for _, pkg := range pkgs {
+		byID[pkg.ID] = pkg
+	}
+
+	// First pass: find directly affected package IDs.
+	directlyAffected := make(map[string]bool)
+	for _, pkg := range pkgs {
+		if hasBuildTag(pkg, tag) {
+			directlyAffected[pkg.ID] = true
+		}
+	}
+
+	// Build reverse dependency graph: importedID → []importingID.
+	reverseDeps := make(map[string][]string)
+	for _, pkg := range pkgs {
+		for _, imp := range pkg.Imports {
+			reverseDeps[imp.ID] = append(reverseDeps[imp.ID], pkg.ID)
+		}
+	}
+
+	// BFS from directly affected packages through reverse deps.
+	affectedIDs := make(map[string]bool)
+	queue := make([]string, 0, len(directlyAffected))
+	for id := range directlyAffected {
+		affectedIDs[id] = true
+		queue = append(queue, id)
+	}
+	for len(queue) > 0 {
+		id := queue[0]
+		queue = queue[1:]
+		for _, rdep := range reverseDeps[id] {
+			if !affectedIDs[rdep] {
+				affectedIDs[rdep] = true
+				queue = append(queue, rdep)
+			}
+		}
+	}
+
+	// Map affected IDs back to PkgPaths. For-test variants like
+	// "foo [foo.test]" share the same PkgPath as "foo", so the
+	// result naturally deduplicates.
+	affected := make(map[string]bool)
+	for id := range affectedIDs {
+		if pkg, ok := byID[id]; ok {
+			affected[pkg.PkgPath] = true
+		}
+	}
+	return affected
+}
+
 func isThirdParty(pkg string) bool {
 	return strings.HasPrefix(pkg, "tailscale.com/tempfork/")
 }
@@ -194,7 +271,7 @@ func getFileTags(filename string) (tagSet, error) {
 	mu.Lock()
 	defer mu.Unlock()
 	fileTags[filename] = ts
-	return tags, nil
+	return ts, nil
 }
 
 func fileMentionsTag(filename, tag string) (bool, error) {