Write into a marked block of a text file instead of over it, so a shared hosts file keeps every line that is not the mesh's (hq issue 128)

internal/apply/apply.go

@@ -669,6 +669,9 @@ func applyAccess(r *declaration.Access) (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 == declaration.IntoBlock {
+		return applyBlock(r, previous)
+	}
 	if r.Into != "" {
 		return applyInto(r, previous)
 	}
@@ -1049,6 +1052,9 @@ func remove(ctx context.Context, sys system.System, a store.Applied, run Runner)
 		return "removed", "no longer declared, and empty", nil
 
 	case declaration.TypeFile:
+		if a.Into != nil && a.Into.Format == declaration.IntoBlock {
+			return removeBlock(a)
+		}
 		if a.Into != nil {
 			return removeInto(a)
 		}

internal/apply/block.go

@@ -0,0 +1,313 @@
+package apply
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/novox/mesh-host/internal/declaration"
+	"github.com/novox/mesh-host/internal/store"
+)
+
+// A file written into a marked block, never over (novox/hq issue 128, ADR 0102).
+//
+// **The file is the machine's; the mesh owns lines in it.** The machine's hosts file is the case
+// that needed it. The mesh wrote it whole — its own header, localhost, the machine's name and every
+// name in the mesh — and on a workstation that file is shared: the distribution's lines, a local
+// development tool's own marked blocks rewritten whenever its projects change, the operator's
+// hand-added names. Written whole, all of those went at the next change to the mesh's names, with
+// no failure anywhere: the tool believed it had written its block, and the mesh believed it owned
+// the file. It is ADR 0102's failure exactly, in a file ADR 0102's JSON verb cannot speak.
+//
+// So the host finds the lines between `# BEGIN mesh <id>` and `# END mesh <id>`, rewrites those and
+// nothing else, and records what they held before. Every line outside the markers is kept byte for
+// byte — including another tool's `# BEGIN …` blocks, which are that tool's. Undeclared, the region
+// is given back what it held, or taken out with its markers when it held nothing, and a file the
+// mesh created goes only if nothing but whitespace is left.
+
+// applyBlock writes a file's declared lines into its region of the file already at its path.
+func applyBlock(r *declaration.File, previous store.Applied) (Outcome, error) {
+	out := begin(r)
+	opening, closing := declaration.BlockMarkers(r.ID)
+	want := blockBody(r.Content)
+
+	raw, err := os.ReadFile(r.Path)
+	existed := err == nil
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
+		return out, err
+	}
+	existing := string(raw)
+	lines := linesOf(existing)
+	at, found, err := regionIn(lines, opening, closing)
+	if err != nil {
+		// Refused, never guessed at: markers the host cannot pair are markers it cannot write
+		// between without risking lines that are not the mesh's.
+		return out, fmt.Errorf("%s: %w; it was left as it is", r.Path, err)
+	}
+
+	// A record of a block is carried; anything else — no record, a file once written whole, one
+	// once written into as JSON — is a file the host is seeing for the first time as a block.
+	rec := store.Into{Format: declaration.IntoBlock}
+	recorded := previous.Into != nil && previous.Into.Format == declaration.IntoBlock
+	if recorded && existed {
+		rec.Created = previous.Into.Created
+		rec.Region = previous.Into.Region
+		rec.Separated = previous.Into.Separated
+		rec.At = previous.Into.At
+	} else {
+		// A file gone since the last apply is made again, and made by the mesh: what it held
+		// before went with it, so there is nothing to give back but the file's absence.
+		rec.Created = !existed
+		if found {
+			// **What the host may have written itself is not the machine's** — the same reasoning
+			// as a key in a JSON file (novox/hq ADR 0102). With no record, a region already holding
+			// exactly the declared lines cannot be told from one this host wrote a moment ago and
+			// died before saving; remembered as the machine's, it would be put back on undeclare
+			// for ever. So it is the mesh's, and undeclaring takes it out.
+			if held := at.body(lines); held != want {
+				rec.Region = &held
+			}
+		}
+	}
+
+	// Drift: the machine no longer holds, between the mesh's markers, what this host last put
+	// there. Judged only against a record of a block: a digest of a whole file says nothing about
+	// a region of it.
+	drifted := recorded && previous.Wrote != "" && existed &&
+		(!found || digestOf(at.body(lines)) != previous.Wrote)
+
+	var next string
+	switch {
+	case !existed:
+		next = regionOf(opening, closing, want)
+	case found:
+		// Where it is, whatever At says: the region is never moved, because moving it moves the
+		// machine's lines around it.
+		next = strings.Join(lines[:at.begin+1], "") + want + strings.Join(lines[at.end:], "")
+	case r.At == declaration.AtStart:
+		// Above everything, and one blank line between the region and the machine's first line
+		// unless there is one already — a line in some files means what the lines above it say.
+		rec.At, rec.Separated = declaration.AtStart, false
+		next = regionOf(opening, closing, want)
+		if existing != "" && !strings.HasPrefix(existing, "\n") {
+			next += "\n"
+			rec.Separated = true
+		}
+		next += existing
+	default:
+		// At the end, apart from whatever is there: the file's last line is ended if it was not,
+		// and one blank line separates the region from the machine's lines unless there is one.
+		rec.At, rec.Separated = "", false
+		next = existing
+		if next != "" && !strings.HasSuffix(next, "\n") {
+			next += "\n"
+		}
+		if next != "" && next != "\n" && !strings.HasSuffix(next, "\n\n") {
+			next += "\n"
+			rec.Separated = true
+		}
+		next += regionOf(opening, closing, want)
+	}
+
+	same := existed && next == existing
+	if !same {
+		var info os.FileInfo
+		mode := os.FileMode(0o644)
+		if info, err = os.Stat(r.Path); err == nil {
+			mode = info.Mode().Perm() // the machine's file keeps the machine's mode
+		} else if mode, err = modeOf(r.Mode, mode); err != nil {
+			return out, err
+		}
+		if err := os.MkdirAll(filepath.Dir(r.Path), 0o755); err != nil {
+			return out, err
+		}
+		if err := writeAtomically(r.Path, []byte(next), mode); err != nil {
+			return out, err
+		}
+		if existed {
+			// The write is a new file renamed over the old, so it belongs to whoever wrote it. The
+			// machine's file keeps the machine's owner, as it keeps its mode.
+			if err := keepOwner(r.Path, info); err != nil {
+				return out, err
+			}
+		} else if err := own(r.Path, r.Owner); err != nil {
+			return out, err
+		}
+	}
+
+	// Read back: the region holds what was declared, and nothing outside it moved.
+	written, err := os.ReadFile(r.Path)
+	if err != nil {
+		return out, fmt.Errorf("wrote into %s and cannot read it back: %w", r.Path, err)
+	}
+	if string(written) != next {
+		return out, fmt.Errorf("%s does not hold the mesh's region as written after writing into it", r.Path)
+	}
+
+	out.into = &rec
+	out.wrote = digestOf(want)
+	switch {
+	case !existed:
+		out.Action = "created"
+		out.Detail = "written into; the file was not there"
+	case same:
+		out.Action = "unchanged"
+	case drifted:
+		out.Action = "corrected"
+		out.Detail = "the mesh's region had been changed on the machine; every line outside it was kept"
+	case !found:
+		out.Action = "updated"
+		where := "end"
+		if rec.At == declaration.AtStart {
+			where = "start"
+		}
+		out.Detail = "the mesh's region added at the " + where + "; every other line kept as it was"
+	default:
+		out.Action = "updated"
+		out.Detail = "the mesh's region rewritten; every line outside it kept as it was"
+	}
+	return out, nil
+}
+
+// removeBlock gives back what a file written into a block held before the mesh's region.
+func removeBlock(a store.Applied) (string, string, error) {
+	raw, err := os.ReadFile(a.Target)
+	if errors.Is(err, os.ErrNotExist) {
+		return "forgotten", "no longer there", nil
+	}
+	if err != nil {
+		return "", "", err
+	}
+	info, err := os.Stat(a.Target)
+	if err != nil {
+		return "", "", err
+	}
+	opening, closing := declaration.BlockMarkers(a.ID)
+	lines := linesOf(string(raw))
+	at, found, err := regionIn(lines, opening, closing)
+	if err != nil {
+		return "kept", err.Error() + ", so nothing was taken out of it; remove the mesh's region by hand", nil
+	}
+
+	next, action, detail := string(raw), "forgotten", "the mesh's region was no longer in it"
+	switch {
+	case found && a.Into.Region != nil:
+		next = strings.Join(lines[:at.begin+1], "") + *a.Into.Region + strings.Join(lines[at.end:], "")
+		action, detail = "restored", "no longer declared; the region was given back what it held"
+	case found:
+		from, to := at.begin, at.end+1
+		// The blank line the host put beside the region, and only that one: if what stands there
+		// now is not blank, it is somebody's, and it stays.
+		if a.Into.Separated {
+			if a.Into.At == declaration.AtStart {
+				if to < len(lines) && lines[to] == "\n" {
+					to++
+				}
+			} else if from > 0 && lines[from-1] == "\n" {
+				from--
+			}
+		}
+		next = strings.Join(lines[:from], "") + strings.Join(lines[to:], "")
+		action, detail = "restored", "no longer declared; the mesh's region was taken out and every other line kept"
+	}
+
+	if a.Into.Created && strings.TrimSpace(next) == "" {
+		if err := os.Remove(a.Target); err != nil {
+			return "", "", err
+		}
+		return "removed", "no longer declared; the mesh had created it and nothing else was in it", nil
+	}
+	if next == string(raw) {
+		return action, detail, nil
+	}
+	if err := writeAtomically(a.Target, []byte(next), info.Mode().Perm()); err != nil {
+		return "", "", err
+	}
+	if err := keepOwner(a.Target, info); err != nil {
+		return "", "", err
+	}
+	return action, detail, nil
+}
+
+// blockBody is the declared lines as they stand in the region: ending in exactly one line end, or
+// nothing at all when there are no lines.
+func blockBody(content string) string {
+	trimmed := strings.TrimRight(content, "\n")
+	if trimmed == "" {
+		return ""
+	}
+	return trimmed + "\n"
+}
+
+func regionOf(begin, end, body string) string {
+	return begin + "\n" + body + end + "\n"
+}
+
+// linesOf splits text into lines that keep their line ends, so joining them again gives back
+// exactly the bytes that were read — a last line without one included.
+func linesOf(text string) []string {
+	return strings.SplitAfter(text, "\n")
+}
+
+// region is where the mesh's markers stand, as indices into the lines of a file.
+type region struct{ begin, end int }
+
+// body is what stands between the markers.
+func (r region) body(lines []string) string {
+	return strings.Join(lines[r.begin+1:r.end], "")
+}
+
+// regionIn finds the mesh's markers for one resource. A line is a marker only if it is exactly the
+// marker, so another tool's block and another resource's region are never it. Markers that do not
+// form one pair — a begin with no end, an end before its begin, either twice — are an error rather
+// than a best guess, because a guess is how the host would rewrite lines that are not its own.
+func regionIn(lines []string, begin, end string) (region, bool, error) {
+	at := region{begin: -1, end: -1}
+	for i, line := range lines {
+		switch strings.TrimSuffix(line, "\n") {
+		case begin:
+			if at.begin >= 0 {
+				return at, false, fmt.Errorf("%q is in it more than once", begin)
+			}
+			at.begin = i
+		case end:
+			if at.end >= 0 {
+				return at, false, fmt.Errorf("%q is in it more than once", end)
+			}
+			at.end = i
+		}
+	}
+	switch {
+	case at.begin < 0 && at.end < 0:
+		return at, false, nil
+	case at.begin < 0:
+		return at, false, fmt.Errorf("%q is in it with no %q before it", end, begin)
+	case at.end < 0:
+		return at, false, fmt.Errorf("%q is in it with no %q after it", begin, end)
+	case at.end < at.begin:
+		return at, false, fmt.Errorf("%q stands before %q", end, begin)
+	}
+	return at, true, nil
+}
+
+// keepOwner gives a file rewritten through a new one back to whoever owned what it replaced.
+// Changed only where it differs, so a host that is not root can still write a file it owns.
+func keepOwner(path string, was os.FileInfo) error {
+	uid, gid, ok := ownerOf(was)
+	if !ok {
+		return nil
+	}
+	now, err := os.Stat(path)
+	if err != nil {
+		return err
+	}
+	if u, g, ok := ownerOf(now); ok && u == uid && g == gid {
+		return nil
+	}
+	if err := os.Chown(path, uid, gid); err != nil {
+		return fmt.Errorf("cannot give %s back to its owner %d:%d: %w", path, uid, gid, err)
+	}
+	return nil
+}

internal/apply/block_test.go

@@ -0,0 +1,471 @@
+package apply
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/novox/mesh-host/internal/store"
+)
+
+// Defends novox/hq issue 128 and ADR 0102: a text file the mesh shares with software it did not
+// install is written into a marked block, never over — every line outside the mesh's markers is
+// the machine's and is kept byte for byte, and undeclaring gives the file back.
+
+const namesID = "mesh-wireguard.fact-node-names"
+
+func blockDecl(t *testing.T, path, content string, extra ...string) string {
+	t.Helper()
+	more := ""
+	for _, e := range extra {
+		more += "," + e
+	}
+	return fmt.Sprintf(`{"declaration":1,"resources":[
+	  {"id":%q,"type":"file","path":%q,"into":"block","content":%q%s}
+	]}`, namesID, path, content, more)
+}
+
+func applyBlockDecl(t *testing.T, raw string, known store.State) (Report, store.State) {
+	t.Helper()
+	report, state, err := Apply(context.Background(), archHost(t), parse(t, raw), known, store.OriginDeclared, nil, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return report, state
+}
+
+func undeclare(t *testing.T, known store.State) (Report, store.State) {
+	t.Helper()
+	report, state, err := Apply(context.Background(), archHost(t), somethingElse(t), known, store.OriginDeclared, nil, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return report, state
+}
+
+func readText(t *testing.T, path string) string {
+	t.Helper()
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return string(raw)
+}
+
+func marked(id, body string) string {
+	return "# BEGIN mesh " + id + "\n" + body + "# END mesh " + id + "\n"
+}
+
+// A workstation's hosts file, the way issue 128 found it: the distribution's lines, a development
+// tool's own marked blocks, and the operator's hand-added names.
+const workstationHosts = "127.0.0.1\tlocalhost\n" +
+	"127.0.1.1\tg14.localdomain g14\n" +
+	"\n" +
+	"# BEGIN devtool project-a\n" +
+	"127.0.0.1 a.test api.a.test\n" +
+	"# END devtool project-a\n" +
+	"# BEGIN devtool project-b\n" +
+	"127.0.0.1 b.test\n" +
+	"# END devtool project-b\n" +
+	"192.168.1.20 printer   # the operator's\n"
+
+const meshNames = "10.42.0.1 ace\n10.42.0.2 novox\n"
+
+func TestABlockKeepsEveryLineOutsideItsMarkers(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "hosts")
+	if err := os.WriteFile(path, []byte(workstationHosts), 0o640); err != nil {
+		t.Fatal(err)
+	}
+	report, state := applyBlockDecl(t, blockDecl(t, path, meshNames), store.State{})
+	want := workstationHosts + "\n" + marked(namesID, meshNames)
+	if got := readText(t, path); got != want {
+		t.Fatalf("the file after writing into it:\n%q\nwant\n%q", got, want)
+	}
+	if got := report.Outcomes[0].Action; got != "updated" {
+		t.Errorf("adding the region was %q", got)
+	}
+	if info, _ := os.Stat(path); info.Mode().Perm() != 0o640 {
+		t.Errorf("the machine's file mode was changed to %o", info.Mode().Perm())
+	}
+	rec, _ := state.Find(namesID)
+	if rec.Into == nil || rec.Into.Format != "block" || rec.Into.Region != nil || rec.Into.Created {
+		t.Fatalf("recorded as %+v", rec.Into)
+	}
+
+	// Again, with nothing changed: nothing written.
+	report, state = applyBlockDecl(t, blockDecl(t, path, meshNames), state)
+	if got := report.Outcomes[0].Action; got != "unchanged" {
+		t.Errorf("a second apply was %q", got)
+	}
+
+	// The development tool rewrites its block, and the operator adds a line after the mesh's
+	// region; the mesh's names change. Only the region moves.
+	edited := strings.Replace(readText(t, path), "127.0.0.1 b.test\n", "127.0.0.1 b.test c.test\n", 1) +
+		"10.0.0.5 nas   # added after\n"
+	_ = os.WriteFile(path, []byte(edited), 0o640)
+	changed := meshNames + "10.42.0.3 shanks\n"
+	report, state = applyBlockDecl(t, blockDecl(t, path, changed), state)
+	want = strings.Replace(edited, marked(namesID, meshNames), marked(namesID, changed), 1)
+	if got := readText(t, path); got != want {
+		t.Fatalf("rewriting the region moved something else:\n%q\nwant\n%q", got, want)
+	}
+	if got := report.Outcomes[0].Action; got != "updated" {
+		t.Errorf("rewriting the region was %q", got)
+	}
+
+	// Undeclared: the region, its markers and the blank line the host put before it go; every
+	// other line is where it was.
+	report, _ = undeclare(t, state)
+	want = strings.Replace(edited, "\n"+marked(namesID, meshNames), "", 1)
+	if got := readText(t, path); got != want {
+		t.Fatalf("undeclaring left:\n%q\nwant\n%q", got, want)
+	}
+	if got := report.Outcomes[0].Action; got != "restored" {
+		t.Errorf("undeclaring was %q", got)
+	}
+}
+
+func TestUndeclaringABlockAddedAtTheEndGivesTheFileBackExactly(t *testing.T) {
+	for name, original := range map[string]string{
+		"ending in a line":       "127.0.0.1 localhost\n",
+		"ending in a blank line": "127.0.0.1 localhost\n\n",
+		"empty":                  "",
+	} {
+		t.Run(name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "hosts")
+			_ = os.WriteFile(path, []byte(original), 0o644)
+			_, state := applyBlockDecl(t, blockDecl(t, path, meshNames), store.State{})
+			undeclare(t, state)
+			if got := readText(t, path); got != original {
+				t.Errorf("undeclaring left %q, the machine had %q", got, original)
+			}
+		})
+	}
+}
+
+func TestABlockAddedAtTheEndIsSetApartFromTheMachinesLines(t *testing.T) {
+	for name, c := range map[string]struct{ before, after string }{
+		"no line end":            {"127.0.0.1 localhost", "127.0.0.1 localhost\n\n" + marked(namesID, meshNames)},
+		"a line end":             {"127.0.0.1 localhost\n", "127.0.0.1 localhost\n\n" + marked(namesID, meshNames)},
+		"a blank line already":   {"127.0.0.1 localhost\n\n", "127.0.0.1 localhost\n\n" + marked(namesID, meshNames)},
+		"empty":                  {"", marked(namesID, meshNames)},
+		"only a blank line":      {"\n", "\n" + marked(namesID, meshNames)},
+		"content without an end": {"x\n\n", "x\n\n" + marked(namesID, meshNames)},
+	} {
+		t.Run(name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "hosts")
+			_ = os.WriteFile(path, []byte(c.before), 0o644)
+			applyBlockDecl(t, blockDecl(t, path, meshNames), store.State{})
+			if got := readText(t, path); got != c.after {
+				t.Errorf("got %q, want %q", got, c.after)
+			}
+		})
+	}
+}
+
+func TestTheRegionEndsInExactlyOneLineEnd(t *testing.T) {
+	for content, body := range map[string]string{
+		"10.42.0.1 ace":       "10.42.0.1 ace\n",
+		"10.42.0.1 ace\n":     "10.42.0.1 ace\n",
+		"10.42.0.1 ace\n\n\n": "10.42.0.1 ace\n",
+		"":                    "",
+		"\n\n":                "",
+	} {
+		path := filepath.Join(t.TempDir(), "hosts")
+		_, state := applyBlockDecl(t, blockDecl(t, path, content), store.State{})
+		if got := readText(t, path); got != marked(namesID, body) {
+			t.Errorf("content %q was written as %q", content, got)
+		}
+		// And the same content again is not a change.
+		report, _ := applyBlockDecl(t, blockDecl(t, path, content), state)
+		if got := report.Outcomes[0].Action; got != "unchanged" {
+			t.Errorf("content %q applied twice was %q", content, got)
+		}
+	}
+}
+
+func TestARegionAlreadyThereIsRewrittenInPlaceAndGivenBack(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "hosts")
+	before := "127.0.0.1 localhost\n"
+	after := "# BEGIN devtool x\n127.0.0.1 x.test\n# END devtool x\n192.168.1.20 printer\n"
+	found := "10.42.0.9 old-name\n"
+	original := before + marked(namesID, found) + after
+	_ = os.WriteFile(path, []byte(original), 0o644)
+
+	report, state := applyBlockDecl(t, blockDecl(t, path, meshNames), store.State{})
+	if got := readText(t, path); got != before+marked(namesID, meshNames)+after {
+		t.Fatalf("the region was not rewritten in place: %q", got)
+	}
+	if got := report.Outcomes[0].Action; got != "updated" {
+		t.Errorf("rewriting a found region was %q", got)
+	}
+	rec, _ := state.Find(namesID)
+	if rec.Into.Region == nil || *rec.Into.Region != found {
+		t.Fatalf("what the region held before was recorded as %v", rec.Into.Region)
+	}
+
+	// Undeclared: what the region held goes back, where it was.
+	report, _ = undeclare(t, state)
+	if got := readText(t, path); got != original {
+		t.Errorf("undeclaring left %q, the machine had %q", got, original)
+	}
+	if got := report.Outcomes[0].Action; got != "restored" {
+		t.Errorf("undeclaring was %q", got)
+	}
+}
+
+func TestARegionWithNoRecordHoldingExactlyTheDeclaredLinesIsTheMeshs(t *testing.T) {
+	// A host that wrote the region and died before saving its state: what is between the markers
+	// is exactly what the mesh declares, and remembered as the machine's it would never go.
+	path := filepath.Join(t.TempDir(), "hosts")
+	original := "127.0.0.1 localhost\n"
+	_ = os.WriteFile(path, []byte(original+"\n"+marked(namesID, meshNames)), 0o644)
+	_, state := applyBlockDecl(t, blockDecl(t, path, meshNames), store.State{})
+	if rec, _ := state.Find(namesID); rec.Into.Region != nil {
+		t.Fatalf("the mesh's own lines were recorded as the machine's: %q", *rec.Into.Region)
+	}
+	undeclare(t, state)
+	if got := readText(t, path); !strings.HasPrefix(got, original) || strings.Contains(got, "BEGIN mesh") {
+		t.Errorf("undeclaring left the mesh's region behind: %q", got)
+	}
+}
+
+func TestADriftedRegionIsCorrected(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "hosts")
+	_ = os.WriteFile(path, []byte(workstationHosts), 0o644)
+	_, state := applyBlockDecl(t, blockDecl(t, path, meshNames), store.State{})
+	written := readText(t, path)
+
+	_ = os.WriteFile(path, []byte(strings.Replace(written, "10.42.0.2 novox\n", "10.42.0.2 novox\n6.6.6.6 evil\n", 1)), 0o644)
+	report, _ := applyBlockDecl(t, blockDecl(t, path, meshNames), state)
+	if got := report.Outcomes[0].Action; got != "corrected" {
+		t.Errorf("a region edited on the machine was %q", got)
+	}
+	if got := readText(t, path); got != written {
+		t.Errorf("the region was not put back: %q", got)
+	}
+
+	// The region taken out by hand is drift too, and it is put back.
+	_ = os.WriteFile(path, []byte(workstationHosts), 0o644)
+	report, _ = applyBlockDecl(t, blockDecl(t, path, meshNames), state)
+	if got := report.Outcomes[0].Action; got != "corrected" {
+		t.Errorf("a region removed on the machine was %q", got)
+	}
+	if got := readText(t, path); got != written {
+		t.Errorf("the region was not put back: %q", got)
+	}
+}
+
+func TestTwoRegionsInOneFileAreEachTheirOwn(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "hosts")
+	_ = os.WriteFile(path, []byte(workstationHosts), 0o644)
+	raw := fmt.Sprintf(`{"declaration":1,"resources":[
+	  {"id":"a.names","type":"file","path":%q,"into":"block","content":"10.42.0.1 ace\n"},
+	  {"id":"b.names","type":"file","path":%q,"into":"block","content":"10.43.0.1 lab\n"}
+	]}`, path, path)
+	_, state := applyBlockDecl(t, raw, store.State{})
+	want := workstationHosts + "\n" + marked("a.names", "10.42.0.1 ace\n") + "\n" + marked("b.names", "10.43.0.1 lab\n")
+	if got := readText(t, path); got != want {
+		t.Fatalf("two regions:\n%q\nwant\n%q", got, want)
+	}
+	report, state := applyBlockDecl(t, raw, state)
+	for _, o := range report.Outcomes {
+		if o.Action != "unchanged" {
+			t.Errorf("%s applied twice was %q", o.ID, o.Action)
+		}
+	}
+
+	// One undeclared: only its region goes.
+	only := fmt.Sprintf(`{"declaration":1,"resources":[
+	  {"id":"b.names","type":"file","path":%q,"into":"block","content":"10.43.0.1 lab\n"}
+	]}`, path)
+	applyBlockDecl(t, only, state)
+	want = workstationHosts + "\n" + marked("b.names", "10.43.0.1 lab\n")
+	if got := readText(t, path); got != want {
+		t.Errorf("undeclaring one region:\n%q\nwant\n%q", got, want)
+	}
+}
+
+func TestABlockInAFileThatWasNotThereIsCreatedAndRemovedWithIt(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "conf.d", "mesh.conf")
+	report, state := applyBlockDecl(t, blockDecl(t, path, meshNames, `"mode":"0600"`), store.State{})
+	if got := readText(t, path); got != marked(namesID, meshNames) {
+		t.Fatalf("a created file holds %q", got)
+	}
+	if info, _ := os.Stat(path); info.Mode().Perm() != 0o600 {
+		t.Errorf("a created file is mode %o, declared 0600", info.Mode().Perm())
+	}
+	if got := report.Outcomes[0].Action; got != "created" {
+		t.Errorf("creating was %q", got)
+	}
+	if rec, _ := state.Find(namesID); !rec.Into.Created {
+		t.Error("the mesh creating the file was not recorded")
+	}
+	report, _ = undeclare(t, state)
+	if _, err := os.Stat(path); !os.IsNotExist(err) {
+		t.Errorf("a file the mesh created, holding only its region, was left behind")
+	}
+	if got := report.Outcomes[0].Action; got != "removed" {
+		t.Errorf("undeclaring was %q", got)
+	}
+
+	// Somebody else wrote into it meanwhile: it is no longer only the mesh's, and it stays.
+	_, state = applyBlockDecl(t, blockDecl(t, path, meshNames), store.State{})
+	_ = os.WriteFile(path, []byte(readText(t, path)+"their = line\n"), 0o600)
+	undeclare(t, state)
+	if got := readText(t, path); got != "their = line\n" {
+		t.Errorf("undeclaring a created file somebody wrote into left %q", got)
+	}
+}
+
+func TestMarkersThatDoNotPairAreRefusedAndLeftAlone(t *testing.T) {
+	for name, text := range map[string]string{
+		"a begin with no end":   "a\n# BEGIN mesh " + namesID + "\nb\n",
+		"an end with no begin":  "a\n# END mesh " + namesID + "\n",
+		"an end before a begin": "# END mesh " + namesID + "\n# BEGIN mesh " + namesID + "\n",
+		"a begin twice":         marked(namesID, "x\n") + "# BEGIN mesh " + namesID + "\n",
+	} {
+		t.Run(name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "hosts")
+			_ = os.WriteFile(path, []byte(text), 0o644)
+			if _, _, err := Apply(context.Background(), archHost(t), parse(t, blockDecl(t, path, meshNames)),
+				store.State{}, store.OriginDeclared, nil, nil, nil); err == nil {
+				t.Fatal("markers that do not pair were written between")
+			}
+			if got := readText(t, path); got != text {
+				t.Errorf("the file was changed: %q", got)
+			}
+		})
+	}
+}
+
+func TestAMarkerOfAnotherIDIsNotThisRegion(t *testing.T) {
+	// The id is part of the marker: a region whose id merely starts with this one is not it.
+	path := filepath.Join(t.TempDir(), "hosts")
+	other := marked(namesID+"-extra", "10.9.9.9 other\n")
+	_ = os.WriteFile(path, []byte(other), 0o644)
+	_, state := applyBlockDecl(t, blockDecl(t, path, meshNames), store.State{})
+	if got := readText(t, path); got != other+"\n"+marked(namesID, meshNames) {
+		t.Fatalf("got %q", got)
+	}
+	undeclare(t, state)
+	if got := readText(t, path); got != other {
+		t.Errorf("undeclaring touched the other region: %q", got)
+	}
+}
+
+func TestABlockAtTheStartStandsAboveEverything(t *testing.T) {
+	// dhcpcd scopes every line after `interface X` to that interface, so the mesh's global options
+	// go above all of it.
+	dhcpcd := "hostname\nduid\n\ninterface enp6s0\nstatic ip_address=192.168.1.5/24\n"
+	opts := "nohook resolv.conf\ndenyinterfaces mesh0\n"
+	for name, c := range map[string]struct{ before, after string }{
+		"a file with content":     {dhcpcd, marked(namesID, opts) + "\n" + dhcpcd},
+		"an empty file":           {"", marked(namesID, opts)},
+		"a file opening blank":    {"\n" + dhcpcd, marked(namesID, opts) + "\n" + dhcpcd},
+		"a last line with no end": {"interface enp6s0", marked(namesID, opts) + "\ninterface enp6s0"},
+	} {
+		t.Run(name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "dhcpcd.conf")
+			_ = os.WriteFile(path, []byte(c.before), 0o644)
+			report, state := applyBlockDecl(t, blockDecl(t, path, opts, `"at":"start"`), store.State{})
+			if got := readText(t, path); got != c.after {
+				t.Fatalf("got %q, want %q", got, c.after)
+			}
+			if got := report.Outcomes[0].Action; got != "updated" {
+				t.Errorf("adding the region was %q", got)
+			}
+			report, state = applyBlockDecl(t, blockDecl(t, path, opts, `"at":"start"`), state)
+			if got := report.Outcomes[0].Action; got != "unchanged" {
+				t.Errorf("a second apply was %q", got)
+			}
+			undeclare(t, state)
+			if got := readText(t, path); got != c.before {
+				t.Errorf("undeclaring left %q, the machine had %q", got, c.before)
+			}
+		})
+	}
+
+	t.Run("a file that was not there", func(t *testing.T) {
+		path := filepath.Join(t.TempDir(), "dhcpcd.conf")
+		applyBlockDecl(t, blockDecl(t, path, opts, `"at":"start"`), store.State{})
+		if got := readText(t, path); got != marked(namesID, opts) {
+			t.Errorf("got %q", got)
+		}
+	})
+}
+
+func TestARegionAlreadyThereIsNotMovedWhereverAtSaysItGoes(t *testing.T) {
+	for _, at := range []string{`"at":"start"`, `"at":"end"`} {
+		path := filepath.Join(t.TempDir(), "dhcpcd.conf")
+		original := "hostname\n" + marked(namesID, "old\n") + "interface enp6s0\n"
+		_ = os.WriteFile(path, []byte(original), 0o644)
+		applyBlockDecl(t, blockDecl(t, path, "nohook resolv.conf\n", at), store.State{})
+		want := "hostname\n" + marked(namesID, "nohook resolv.conf\n") + "interface enp6s0\n"
+		if got := readText(t, path); got != want {
+			t.Errorf("%s: a found region was moved: %q", at, got)
+		}
+	}
+}
+
+func TestAFileWrittenIntoABlockIsNeverHeldOnAnAdoptedNode(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "hosts")
+	_ = os.WriteFile(path, []byte(workstationHosts), 0o644)
+	resource := fmt.Sprintf(`{"id":%q,"type":"file","path":%q,"into":"block","content":%q}`, namesID, path, meshNames)
+	d := adopted(t, `{"taken":[],"untaken":{"mesh-wireguard":["`+namesID+`"]}}`, resource)
+	report, state := applyAdopted(t, d, store.State{}, &machine{}, t.TempDir())
+	if got := outcomeOf(report, namesID).Action; got == "held" {
+		t.Fatal("a file written into a block was held, though it replaces nothing that was found")
+	}
+	if len(state.Held) != 0 {
+		t.Errorf("something was held: %+v", state.Held)
+	}
+	if got := readText(t, path); got != workstationHosts+"\n"+marked(namesID, meshNames) {
+		t.Errorf("the adopted node's file was not written into: %q", got)
+	}
+
+	// Held from when it was declared whole, the hold does not keep the region out.
+	path2 := filepath.Join(t.TempDir(), "hosts")
+	_ = os.WriteFile(path2, []byte(workstationHosts), 0o644)
+	known := store.State{Held: []store.Held{{ID: namesID, Module: "mesh-wireguard", Kind: "file", Target: path2}}}
+	resource2 := fmt.Sprintf(`{"id":%q,"type":"file","path":%q,"into":"block","content":%q}`, namesID, path2, meshNames)
+	d2 := adopted(t, `{"taken":[],"untaken":{"mesh-wireguard":["`+namesID+`"]}}`, resource2)
+	report, state = applyAdopted(t, d2, known, &machine{}, t.TempDir())
+	if got := outcomeOf(report, namesID).Action; got != "updated" {
+		t.Errorf("the file was %q, not written into", got)
+	}
+	if len(state.Held) != 0 {
+		t.Errorf("the old hold outlived the block declaration: %+v", state.Held)
+	}
+
+	// And the preview says the same: written into, not held.
+	for _, s := range Plan(d2, known, store.OriginDeclared) {
+		if s.ID == namesID && s.Verb == "hold" {
+			t.Errorf("the preview holds a file written into a block: %+v", s)
+		}
+	}
+}
+
+func TestTheRecordOfABlockSurvivesTheStateFile(t *testing.T) {
+	// What undeclaring needs is in the state a host saves, not only in memory.
+	path := filepath.Join(t.TempDir(), "hosts")
+	original := "a\n" + marked(namesID, "old\n")
+	_ = os.WriteFile(path, []byte(original), 0o644)
+	_, state := applyBlockDecl(t, blockDecl(t, path, meshNames, `"at":"start"`), store.State{})
+	raw, err := json.Marshal(state)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var back store.State
+	if err := json.Unmarshal(raw, &back); err != nil {
+		t.Fatal(err)
+	}
+	undeclare(t, back)
+	if got := readText(t, path); got != original {
+		t.Errorf("undeclaring from a saved state left %q", got)
+	}
+}