internal/apply/apply.go

@@ -322,7 +322,14 @@ func ApplyKeeping(
 		if o, isOpening := resource.(*declaration.Opening); isOpening {
 			outcome, err = applyOpening(ctx, o, run, fw)
 		} else {
-			outcome, err = applyOne(ctx, sys, resource, run, changed, declares, was, unseal)
+			// A file this host has no record of, under any id, is the machine's until the mesh
+			// writes over it — on any node, adopted or not: its original is kept first.
+			var keepFound Keep
+			if f, isFile := resource.(*declaration.File); isFile && was.ID == "" &&
+				!known.Recorded(string(declaration.TypeFile), f.Path) {
+				keepFound = keep
+			}
+			outcome, err = applyOne(ctx, sys, resource, run, changed, declares, was, unseal, keepFound)
 		}
 		if err != nil {
 			failed := &Error{Resource: resource.Identity(), Err: err, Done: report}
@@ -423,12 +430,12 @@ type Unseal func(sealed string) ([]byte, error)
 
 func applyOne(ctx context.Context, sys system.System, r declaration.Resource, run Runner,
 	changed map[string]bool, declares map[string]string, previous store.Applied,
-	unseal Unseal) (Outcome, error) {
+	unseal Unseal, keepFound Keep) (Outcome, error) {
 	switch res := r.(type) {
 	case *declaration.Directory:
 		return applyDirectory(res)
 	case *declaration.File:
-		return applyFile(res, previous, unseal)
+		return applyFile(res, previous, unseal, keepFound)
 	case *declaration.Service:
 		return applyService(ctx, sys, res, run, changed)
 	case *declaration.Package:
@@ -573,7 +580,9 @@ func applyAccess(r *declaration.Access) (Outcome, error) {
 	return out, nil
 }
 
-func applyFile(r *declaration.File, previous store.Applied, unseal Unseal) (Outcome, error) {
+// keepFound, when not nil, is where the original of a file this host has no record of is kept
+// before it is written over (novox/hq ADR 0100): once, never overwritten, and named in the outcome.
+func applyFile(r *declaration.File, previous store.Applied, unseal Unseal, keepFound Keep) (Outcome, error) {
 	if r.Into != "" {
 		return applyInto(r, previous)
 	}
@@ -670,7 +679,15 @@ func applyFile(r *declaration.File, previous store.Applied, unseal Unseal) (Outc
 	drifted := existed && previous.Wrote != "" && digestOf(string(existing)) != previous.Wrote
 	modeSame := existed && beforeMode == mode.Perm()
 
+	kept := ""
 	if !contentSame {
+		if existed && keepFound != nil {
+			// Before anything is written: a keep that fails stops the write, since the
+			// original could not be had back otherwise.
+			if kept, err = keepFound(r.Path, existing, beforeMode); err != nil {
+				return out, fmt.Errorf("keeping the original of %s before writing over it: %w", r.Path, err)
+			}
+		}
 		if err := os.MkdirAll(filepath.Dir(r.Path), 0o755); err != nil {
 			return out, err
 		}
@@ -734,6 +751,12 @@ func applyFile(r *declaration.File, previous store.Applied, unseal Unseal) (Outc
 	default:
 		out.Action = "unchanged"
 	}
+	if kept != "" {
+		if out.Detail != "" {
+			out.Detail += "; "
+		}
+		out.Detail += "the file found here, which the mesh had no record of, was kept at " + kept
+	}
 	return out, nil
 }
 

internal/apply/hold_test.go

@@ -472,8 +472,40 @@ func TestAConvergedNodeStillReplacesWhatItFinds(t *testing.T) {
 	if len(state.Held) != 0 || outcomeOf(report, "hello-web.page").Action == "held" {
 		t.Errorf("a converged node held something: %+v", state.Held)
 	}
-	if _, err := os.Stat(filepath.Join(dir, "kept")); !errors.Is(err, os.ErrNotExist) {
-		t.Error("a converged node kept originals")
+	// What it writes over that it has no record of, it keeps first — on any node.
+	o := outcomeOf(report, "hello-web.page")
+	kept := o.Detail[strings.Index(o.Detail, "kept at ")+len("kept at "):]
+	if got, err := os.ReadFile(kept); err != nil || string(got) != "the predecessor's page\n" {
+		t.Errorf("the file written over was not kept, or not named: %q (%s) %v", got, o.Detail, err)
+	}
+}
+
+func TestAFileWrittenOverIsKeptOnceAndOnlyWhenTheHostHasNoRecordOfIt(t *testing.T) {
+	dir := t.TempDir()
+	conf := filepath.Join(dir, "nftables.conf")
+	_ = os.WriteFile(conf, []byte("# the distribution's own\n"), 0o644)
+	decl := func(content string) *declaration.Declaration {
+		return parse(t, `{"declaration":1,"resources":[{"id":"nftables.config","type":"file","path":"`+conf+
+			`","content":"`+content+`"}]}`)
+	}
+	m := &machine{containers: map[string]*fakeContainer{}}
+	report, state := applyAdopted(t, decl("table inet mesh {}\\n"), store.State{}, m, dir)
+	o := outcomeOf(report, "nftables.config")
+	if !strings.Contains(o.Detail, "had no record of, was kept at ") {
+		t.Fatalf("writing over an unrecorded file did not keep it: %+v", o)
+	}
+	kept := o.Detail[strings.Index(o.Detail, "kept at ")+len("kept at "):]
+	if got, _ := os.ReadFile(kept); string(got) != "# the distribution's own\n" {
+		t.Errorf("the kept original is %q", got)
+	}
+
+	// Now the mesh's: a later change keeps nothing more, and the first original stays.
+	report, _ = applyAdopted(t, decl("table inet mesh { }\\n"), state, m, dir)
+	if o := outcomeOf(report, "nftables.config"); o.Action != "updated" || strings.Contains(o.Detail, "kept at") {
+		t.Errorf("a file the mesh wrote was kept again: %+v", o)
+	}
+	if got, _ := os.ReadFile(kept); string(got) != "# the distribution's own\n" {
+		t.Errorf("the first original was overwritten: %q", got)
 	}
 }