Add rsync exit code awareness + plan operation abstraction

- spawn.js: rsync() wrapper handles exit codes 0/24 as OK, 23 as fatal
- spawn.js: capture() accepts allowedExitCodes option
- run.js: all rsync calls go through rsync() wrapper
- PLAN.md: document planned operation abstraction refactor

PLAN.md

@@ -127,6 +127,40 @@ CWD or implicit defaults for directories — explicit is safer.
 All external tools (rsync, zstd, xdelta3) are spawned with explicit argument arrays.
 No shell string interpolation ever. Use Node's `child_process.spawn` or similar.
 
+### Planned: Operation Abstractions
+
+Currently dry-run logic is scattered inline throughout the run command. The intent is to refactor
+toward self-describing operation objects — each operation knows both how to describe itself (for
+dry-run) and how to execute itself. This makes the run command a clean sequence of operations,
+makes per-tool behavior easy to adjust (e.g. rsync exit code handling), and makes dry-run output
+a natural consequence of the abstraction rather than duplicated conditional logic.
+
+Sketch:
+```js
+// Each tool gets its own operation type
+const op = rsyncOp({ args: [...], allowedExitCodes: [0, 24] });
+op.describe(); // prints what it would do
+await op.run(); // executes
+
+// Run command becomes:
+const ops = buildOps(config);
+if (dry) ops.forEach(op => op.describe());
+else     for (const op of ops) await op.run();
+```
+
+Per-tool exit code handling (e.g. rsync's partial transfer codes) lives inside the operation,
+not scattered across callers.
+
+### Current: rsync Exit Code Handling
+
+rsync meaningful exit codes:
+- `0` — success
+- `23` — partial transfer due to error (fatal)
+- `24` — partial transfer due to vanished source files (acceptable in some cases)
+
+Currently basic: any non-zero exit code throws. Finer-grained handling planned as part of the
+operation abstraction refactor.
+
 ## Occasional Snapshots
 
 Delta chains are efficient but fragile over long chains. Periodic full snapshots (every N deltas,

lib/commands/run.js

@@ -3,7 +3,7 @@
  */
 import { mkdir, writeFile } from 'fs/promises';
 import { join } from 'path';
-import { run as spawn, capture } from '../spawn.js';
+import { run as spawn, rsync } from '../spawn.js';
 import { parseItemize } from '../itemize.js';
 import { getBackend } from '../backends/index.js';
 import { readState, writeState, PHASES } from '../state.js';
@@ -33,28 +33,22 @@ export async function runCommand(config) {
   // ── Phase 2: rsync PREV → PEND (local seed, with delete) ────
   await setPhase(deltas, state, PHASES.RSYNC_LOCAL, dry);
   console.log('\n── rsync PREV → PEND (local seed) ──');
-  await spawn('rsync', ['-aP', '--delete', trailingSlash(prev), trailingSlash(pend)], { dryRun: dry });
+  await rsync(['-aP', '--delete', trailingSlash(prev), trailingSlash(pend)], { dryRun: dry });
 
   // ── Phase 3: rsync SOURCE → PEND, capture change list ───────
   await setPhase(deltas, state, PHASES.RSYNC_REMOTE, dry);
   console.log('\n── rsync SOURCE → PEND ──');
 
-  let changes = [];
+  const output = await rsync(
+    ['-aP', '--itemize-changes', '--delete', trailingSlash(source), trailingSlash(pend)],
+    { dryRun: dry, capture: true },
+  );
+  const changes = dry ? [] : parseItemize(output);
   if (!dry) {
-    const rsyncArgs = [
-      '-aP',
-      '--itemize-changes',
-      '--delete',
-      trailingSlash(source),
-      trailingSlash(pend),
-    ];
-    const output = await capture('rsync', rsyncArgs);
-    changes = parseItemize(output);
     console.log(`  ${changes.length} file(s) changed`);
     for (const c of changes) console.log(`  [${c.status}] ${c.path}`);
   } else {
-    console.log(`$ rsync -aP --itemize-changes --delete ${trailingSlash(source)} ${trailingSlash(pend)}`);
+    console.log('  [dry-run] change list determined at runtime');
-    console.log('[dry-run] (change list will be determined at runtime)');
   }
 
   // ── Phase 4: Generate per-file deltas into DELTAS/tmp/N/ ────
@@ -132,7 +126,7 @@ export async function runCommand(config) {
   // ── Phase 6: Promote PEND → PREV ────────────────────────────
   await setPhase(deltas, state, PHASES.PROMOTING, dry);
   console.log('\n── Promote PEND → PREV ──');
-  await spawn('rsync', ['-aP', '--delete', trailingSlash(pend), trailingSlash(prev)], { dryRun: dry });
+  await rsync(['-aP', '--delete', trailingSlash(pend), trailingSlash(prev)], { dryRun: dry });
 
   // ── Done ─────────────────────────────────────────────────────
   state.last_complete = seq;

lib/spawn.js

@@ -29,9 +29,10 @@ export async function run(cmd, args, { dryRun = false } = {}) {
  * stderr is inherited (shown to user). Never used in dry-run context.
  * @param {string} cmd
  * @param {string[]} args
+ * @param {{ allowedExitCodes?: number[] }} opts
  * @returns {Promise<string>}
  */
-export async function capture(cmd, args) {
+export async function capture(cmd, args, { allowedExitCodes = [0] } = {}) {
   console.log(`$ ${[cmd, ...args].join(' ')}`);
 
   return new Promise((resolve, reject) => {
@@ -40,8 +41,45 @@ export async function capture(cmd, args) {
     child.stdout.on('data', chunk => chunks.push(chunk));
     child.on('error', reject);
     child.on('close', code => {
-      if (code === 0) resolve(Buffer.concat(chunks).toString('utf8'));
+      if (allowedExitCodes.includes(code)) resolve(Buffer.concat(chunks).toString('utf8'));
       else reject(new Error(`${cmd} exited with code ${code}`));
     });
   });
 }
+
+// rsync exit codes that are not errors
+const RSYNC_OK_CODES = [
+  0,  // success
+  24, // partial transfer: source files vanished mid-run (acceptable)
+];
+
+const RSYNC_ERROR_CODES = {
+  23: 'partial transfer due to error',
+};
+
+/**
+ * Run rsync with exit code awareness.
+ * @param {string[]} args
+ * @param {{ dryRun?: boolean, capture?: boolean }} opts
+ * @returns {Promise<void | string>}
+ */
+export async function rsync(args, { dryRun = false, capture: doCapture = false } = {}) {
+  console.log(`$ rsync ${args.join(' ')}`);
+  if (dryRun) return doCapture ? '' : undefined;
+
+  return new Promise((resolve, reject) => {
+    const stdio = doCapture ? ['inherit', 'pipe', 'inherit'] : 'inherit';
+    const child = spawn('rsync', args, { stdio });
+    const chunks = [];
+    if (doCapture) child.stdout.on('data', chunk => chunks.push(chunk));
+    child.on('error', reject);
+    child.on('close', code => {
+      if (RSYNC_OK_CODES.includes(code)) {
+        resolve(doCapture ? Buffer.concat(chunks).toString('utf8') : undefined);
+      } else {
+        const reason = RSYNC_ERROR_CODES[code] ?? `unknown error`;
+        reject(new Error(`rsync exited with code ${code}: ${reason}`));
+      }
+    });
+  });
+}