A changed file touches exactly the modules whose build reads it (hq issue 280, ADR 0237)

The builder reads a module's own directory (the repository for one built from its root)
and a repository its recipe packages, nothing else. A file in no module's directory was
read as shared code and rebuilt everything built from the repository: 103 modules for a
merge-check.sh added at the catalogue's root. It now touches nothing, in the merge
handler, the release planner and the pull request's check alike, and the gate says so.

cmd/mesh-controller/merge_gate.go

@@ -108,9 +108,9 @@ type mergeComposed struct {
 type mergeWidthOf struct {
 	Modules []string `json:"modules"`
 	Tiers   int      `json:"tiers"`
-	// Shared is the changed paths read as shared code: in no directory of a module the mesh holds, so
-	// everything built from the repository is rebuilt for them.
-	Shared []string `json:"shared,omitempty"`
+	// Unread is the changed paths no module's build reads — in no module's directory — which rebuild
+	// nothing (issue 280): said, so a reader sees why a change to them moves no module.
+	Unread []string `json:"unread,omitempty"`
 }
 
 // wideRebuild is how many modules a rebuild may take before the gate says so as a warning.
@@ -312,11 +312,11 @@ func judgeChange(ctx context.Context, in mergeCheckInput) (mergeVerdict, error)
 			v.Notes = append(v.Notes, "the width of the rebuild could not be worked out: "+err.Error())
 		} else {
 			v.Width = &w
-			if len(w.Shared) > 0 && len(w.Modules) > wideRebuild {
-				v.Warnings = append(v.Warnings, fmt.Sprintf("a merge rebuilds %d module(s), because %s read as shared "+
-					"code: in no directory of a module the mesh holds (issue 278)", len(w.Modules),
-					readableList(w.Shared)))
-			} else if len(w.Modules) > wideRebuild {
+			if len(w.Unread) > 0 {
+				v.Notes = append(v.Notes, fmt.Sprintf("%s read by no module's build, and rebuild nothing (issue 280)",
+					readableList(w.Unread)))
+			}
+			if len(w.Modules) > wideRebuild {
 				v.Warnings = append(v.Warnings, fmt.Sprintf("a merge rebuilds %d module(s) in %d tier(s)",
 					len(w.Modules), w.Tiers))
 			}
@@ -1016,7 +1016,8 @@ func rebuildWidth(f snapshot.Facts, repository string, paths []string, tree stri
 		}
 		sort.Strings(w.Modules)
 	}
-	// The paths read as shared: in no directory of a module the mesh holds from this repository.
+	// The paths no build reads: in no directory of a module the mesh holds from this repository, or one
+	// the tree holds.
 	var dirs []string
 	for _, e := range from {
 		if d := strings.Trim(e.Source.Path, "/"); d != "" {
@@ -1032,7 +1033,7 @@ func rebuildWidth(f snapshot.Facts, repository string, paths []string, tree stri
 			inModule = inModule || strings.HasPrefix(p, d)
 		}
 		if !inModule && len(dirs) > 0 {
-			w.Shared = append(w.Shared, p)
+			w.Unread = append(w.Unread, p)
 		}
 	}
 	return w, nil