Give the gate a short changed list as an argument, as before, so an older judge still runs (issue 1247, review round 1)

internal/builder/check.go

@@ -263,9 +263,12 @@ const (
 	// EnvChangedFile names a file holding every changed path, one per line, in the order given: the whole
 	// list however long, which one environment string cannot carry (novox/hq issue 1247).
 	EnvChangedFile = "MESH_CHECK_CHANGED_FILE"
-	EnvVerdict     = "MESH_CHECK_VERDICT"
-	EnvBeside      = "MESH_CHECK_BESIDE"
-	EnvModules     = "MESH_CHECK_MODULES"
+	// EnvChangedCut is 1 when MESH_CHECK_CHANGED was cut to directories, so the gate reads the file; unset
+	// otherwise, and the gate is given the list as before, which a judge older than --changed-file understands.
+	EnvChangedCut = "MESH_CHECK_CHANGED_CUT"
+	EnvVerdict    = "MESH_CHECK_VERDICT"
+	EnvBeside     = "MESH_CHECK_BESIDE"
+	EnvModules    = "MESH_CHECK_MODULES"
 	// EnvGroup is a delivery group's other heads, as JSON, for the gate (novox/hq ADR 0239); empty for a
 	// pull request checked alone.
 	EnvGroup = "MESH_CHECK_GROUP"
@@ -1096,7 +1099,7 @@ func gateLayer(ctx context.Context, spec CheckSpec, tree, root, gate, verdictFil
 	// 2. Every machine composed with the change.
 	if err := running(inToolchain(tree, append(env, EnvVerdict+"="+verdictFile), "sh", "-c",
 		`"$MESH_GATE" merge-gate --facts "$MESH_FACTS" --store "$MESH_GATE_POSTGRES" `+
-			`--repository "$MESH_CHECK_REPOSITORY" --tree . --changed-file "$MESH_CHECK_CHANGED_FILE" `+
+			`--repository "$MESH_CHECK_REPOSITORY" --tree . `+gateChangedFlag(env)+` `+
 			`${MESH_CHECK_GROUP:+--group "$MESH_CHECK_GROUP"} --json > "$MESH_CHECK_VERDICT"`)); err != nil {
 		if ctx.Err() != nil {
 			return "error", "the check was ended during the merge gate"
@@ -1926,6 +1929,7 @@ func changedEnvironment(root string, paths []string, say func(string)) ([]string
 		return nil, err
 	}
 	joined := strings.Join(paths, ",")
+	cut := false
 	if len(joined) >= changedEnvLimit {
 		var short []string
 		seen := map[string]bool{}
@@ -1941,8 +1945,25 @@ func changedEnvironment(root string, paths []string, say func(string)) ([]string
 			}
 		}
 		joined = strings.Join(short, ",")
+		cut = true
 		say(fmt.Sprintf("--- %d changed paths are too long for one environment string: %s holds all of them, one per "+
 			"line, and %s only each path's first two directories", len(paths), EnvChangedFile, EnvChanged))
 	}
-	return []string{EnvChanged + "=" + joined, EnvChangedFile + "=" + file}, nil
+	env := []string{EnvChanged + "=" + joined, EnvChangedFile + "=" + file}
+	if cut {
+		env = append(env, EnvChangedCut+"=1")
+	}
+	return env, nil
+}
+
+// gateChangedFlag is how the gate is given the changed paths: as the argument it always was while the list was not
+// cut, so a judge from before --changed-file still runs; from the file only when it was cut, which that judge
+// could never have started anyway.
+func gateChangedFlag(env []string) string {
+	for _, e := range env {
+		if e == EnvChangedCut+"=1" {
+			return `--changed-file "$MESH_CHECK_CHANGED_FILE"`
+		}
+	}
+	return `--changed "$MESH_CHECK_CHANGED"`
 }