fix(release): keep generated release notes when the write-back fails

.changeset/preserve-generated-changelog.md

@@ -0,0 +1,2 @@
+---
+---

scripts/changeset-version.test.ts

@@ -1,5 +1,13 @@
 import { afterEach, describe, expect, test } from "bun:test";
-import { existsSync, mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
+import {
+  chmodSync,
+  existsSync,
+  mkdirSync,
+  mkdtempSync,
+  readFileSync,
+  rmSync,
+  writeFileSync,
+} from "node:fs";
 import { tmpdir } from "node:os";
 import path from "node:path";
 import { versionPackages } from "./changeset-version";
@@ -74,4 +82,35 @@ describe("canonical changelog versioning", () => {
     );
     expect(existsSync(path.join(paths.packageRoot, "CHANGELOG.md"))).toBe(false);
   });
+
+  test("keeps the generated changelog when it cannot be returned to the root", () => {
+    if (process.platform === "win32" || process.getuid?.() === 0) {
+      // Both make the read-only repository root writable anyway, so the write-back would
+      // succeed and the test could not observe the failure it exists to cover.
+      return;
+    }
+
+    const paths = createTestWorkspace();
+    const staged = path.join(paths.packageRoot, "CHANGELOG.md");
+    const canonical = path.join(paths.repoRoot, "CHANGELOG.md");
+
+    try {
+      // Deny the write-back specifically, after Changesets has already consumed the
+      // `.changeset/*.md` entries, so the staged file is the only copy of the new notes.
+      // The target file itself must be read-only: `copyFileSync` overwrites through the
+      // existing inode, which a read-only parent directory does not prevent.
+      expect(() =>
+        versionPackages(paths, () => {
+          writeFileSync(staged, "# Changelog\n\n## 1.1.0\n\n- Generated once.\n");
+          chmodSync(canonical, 0o400);
+          return 0;
+        }),
+      ).toThrow("The generated history is preserved at");
+    } finally {
+      chmodSync(canonical, 0o600);
+    }
+
+    // The release notes survive the failure and are still the generated ones.
+    expect(readFileSync(staged, "utf8")).toContain("## 1.1.0");
+  });
 });

scripts/changeset-version.ts

@@ -6,6 +6,11 @@
  * Changesets writes a changelog beside each publishable manifest. Hunk intentionally keeps its
  * release history at the repository root, so the root file is staged beside `packages/hunk` only
  * for the duration of the Changesets command and copied back after a successful version update.
+ *
+ * The staged copy is removed once it is reproducible again, never before. After Changesets runs it
+ * holds the only copy of the new release notes — the `.changeset/*.md` entries behind them are
+ * already consumed — so a failed write-back leaves it in place and says where to find it rather
+ * than cleaning up the one artifact the release cannot regenerate.
  */
 
 import { copyFileSync, existsSync, rmSync } from "node:fs";
@@ -42,6 +47,7 @@ export function versionPackages(
   }
 
   copyFileSync(canonicalChangelog, packageChangelog);
+  let generatedChangelog = false;
   try {
     const exitCode = runChangesets(paths.repoRoot);
     if (exitCode !== 0) {
@@ -50,9 +56,23 @@ export function versionPackages(
     if (!existsSync(packageChangelog)) {
       throw new Error("Changesets did not produce the hunkdiff package changelog.");
     }
+    // Past this point the staged file is the only copy of the new release notes: Changesets
+    // has already consumed the `.changeset/*.md` entries that produced them.
+    generatedChangelog = true;
     copyFileSync(packageChangelog, canonicalChangelog);
-  } finally {
     rmSync(packageChangelog, { force: true });
+  } catch (error) {
+    // Clean up the staging copy only while it is still reproducible. If the write-back
+    // failed, deleting it would destroy the generated history for good and leave the root
+    // holding pre-version content, so it stays behind for the operator to recover from.
+    if (!generatedChangelog) {
+      rmSync(packageChangelog, { force: true });
+      throw error;
+    }
+    throw new Error(
+      `Failed to return the generated changelog to ${canonicalChangelog}. The generated history is preserved at ${packageChangelog}; move it to the repository root before retrying.`,
+      { cause: error },
+    );
   }
 }