Pass a check's changed paths in a file so a change of thousands of files can start (issue 1247)

One environment string over 128 KiB made the container fail to start with no words; the gate reads --changed-file, and a command that cannot start now says its error.

cmd/mesh-controller/merge_gate.go

@@ -154,6 +154,24 @@ type mergeWidthOf struct {
 // wideRebuild is how many modules a rebuild may take before the gate says so as a warning.
 var wideRebuild = 12
 
+// changedPaths are the paths --changed (comma-separated) and --changed-file (one per line) name together.
+func changedPaths(list, file string) ([]string, error) {
+	out := splitList(list)
+	if file == "" {
+		return out, nil
+	}
+	raw, err := os.ReadFile(file)
+	if err != nil {
+		return nil, fmt.Errorf("--changed-file: %w", err)
+	}
+	for _, line := range strings.Split(string(raw), "\n") {
+		if line = strings.TrimSpace(line); line != "" {
+			out = append(out, line)
+		}
+	}
+	return out, nil
+}
+
 // mergeGateCommand is `gate`:
 //
 //	merge-gate --facts <file> --store <postgres> [--repository owner/repo --tree <dir>] [--changed a,b] [--json]
@@ -167,6 +185,8 @@ func mergeGateCommand(ctx context.Context, args []string) error {
 	repository := set.String("repository", "", "owner/repository of the change")
 	tree := set.String("tree", "", "the change's checkout: every module.json in it replaces the mesh's")
 	changed := set.String("changed", "", "the paths the change touches, comma-separated, for its rebuild's width")
+	changedFile := set.String("changed-file", "", "a file of the paths the change touches, one per line; the same as "+
+		"--changed, for a list too long for an argument (novox/hq issue 1247)")
 	group := set.String("group", "", "a delivery group's other heads, as JSON [{repository, tree, changed}], composed "+
 		"with this one as one future state (novox/hq ADR 0239)")
 	asJSON := set.Bool("json", false, "print the verdict as JSON")
@@ -198,7 +218,11 @@ func mergeGateCommand(ctx context.Context, args []string) error {
 		// verdict at all. What is said goes beside the report.
 		os.Stdout = os.Stderr
 	}
-	v, err := judgeChange(ctx, mergeCheckInput{facts: f, repository: *repository, tree: *tree, changed: splitList(*changed),
+	changedList, err := changedPaths(*changed, *changedFile)
+	if err != nil {
+		return err
+	}
+	v, err := judgeChange(ctx, mergeCheckInput{facts: f, repository: *repository, tree: *tree, changed: changedList,
 		group: others, admin: *admin, say: out, beside: os.Getenv("MESH_CHECK_BESIDE")})
 	os.Stdout = stdout
 	if err != nil {

cmd/mesh-controller/merge_gate_changed_test.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestTheGateJudgesAFileOfPathsAsItDoesTheSameListOfThem(t *testing.T) { // 1247/AC3
+	paths := []string{"modules/a/x.go", "modules/a/y.go", "modules/b/z.go", "README.md"}
+	file := filepath.Join(t.TempDir(), "changed.txt")
+	if err := os.WriteFile(file, []byte(strings.Join(paths, "\n")+"\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	fromFile, err := changedPaths("", file)
+	if err != nil {
+		t.Fatal(err)
+	}
+	fromList, err := changedPaths(strings.Join(paths, ","), "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(fromFile, fromList) || len(fromFile) != len(paths) {
+		t.Errorf("the file gives %v, the list %v", fromFile, fromList)
+	}
+	if _, err := changedPaths("", filepath.Join(t.TempDir(), "none")); err == nil {
+		t.Error("a file that is not there is read as no paths")
+	}
+}