Add update-only mode for test dataset generator

Add support for running content updates against an existing migration
test dataset without recreating the filesystem structure.

Also make ACL/xattr updates non-fatal on filesystems that do not
support those operations.

cdssync/AGENTS.md

@@ -19,6 +19,9 @@ For migration test datasets in this workspace, follow this process by default:
   - omit it to create the dataset once and exit
   - use `0` for continuous random content updates
   - use any integer greater than `0` to rewrite mutable files every `N` seconds
+- The generator script also accepts `--update-only`:
+  - use it to update an existing dataset in place without recreating files, links, or directories
+  - combine it with `UPDATE_INTERVAL_SECONDS` to keep mutating an existing dataset on a fixed interval
 - If ACL/xattr coverage matters, ensure the generation host has:
   - `acl` installed for `setfacl` and `getfacl`
   - `attr` installed for `setfattr` and `getfattr`
@@ -55,3 +58,4 @@ Preferred verification commands include:
 
 - If the destination host lacks `acl` or `attr`, ACL/xattr verification will be incomplete.
 - If the destination filesystem does not support ACLs or xattrs, those attributes may not survive transfer even when the copy method is correct.
+- The generator now logs and continues when ACL/xattr assignment is unsupported on the target filesystem instead of exiting.

cdssync/generate_migration_test_dataset.sh

@@ -5,7 +5,7 @@ set -euo pipefail
 usage() {
   cat <<'EOF'
 Usage:
-  generate_migration_test_dataset.sh TARGET_DIR [UPDATE_INTERVAL_SECONDS]
+  generate_migration_test_dataset.sh [--update-only] TARGET_DIR [UPDATE_INTERVAL_SECONDS]
 
 Creates a compact filesystem migration test dataset under TARGET_DIR.
 The dataset matches the manifest in migration-test-manifest.md.
@@ -23,6 +23,12 @@ Notes:
 EOF
 }
 
+UPDATE_ONLY=0
+if [[ ${1:-} == "--update-only" ]]; then
+  UPDATE_ONLY=1
+  shift
+fi
+
 if [[ $# -lt 1 || $# -gt 2 ]]; then
   usage
   exit 1
@@ -132,17 +138,25 @@ apply_mode() {
 
 set_acl_and_xattr_metadata() {
   if (( have_setfattr )); then
-    setfattr -n user.migration_case -v "xattr-text" "$ROOT/metadata/xattr_text_1mb_644.txt"
-    setfattr -n user.migration_case -v "xattr-random" "$ROOT/metadata/xattr_random_3mb_600.bin"
+    if ! setfattr -n user.migration_case -v "xattr-text" "$ROOT/metadata/xattr_text_1mb_644.txt"; then
+      log "Skipping xattr assignment on $ROOT/metadata/xattr_text_1mb_644.txt: operation not supported"
+    fi
+    if ! setfattr -n user.migration_case -v "xattr-random" "$ROOT/metadata/xattr_random_3mb_600.bin"; then
+      log "Skipping xattr assignment on $ROOT/metadata/xattr_random_3mb_600.bin: operation not supported"
+    fi
   else
-    echo "Skipping xattr assignment: setfattr not available"
+    log "Skipping xattr assignment: setfattr not available"
   fi
 
   if (( have_setfacl )); then
-    setfacl -m u:nobody:r-- "$ROOT/metadata/acl_text_1mb_644.txt"
-    setfacl -m u:nobody:r-x "$ROOT/metadata/acl_script_1mb_755.sh"
+    if ! setfacl -m u:nobody:r-- "$ROOT/metadata/acl_text_1mb_644.txt"; then
+      log "Skipping ACL assignment on $ROOT/metadata/acl_text_1mb_644.txt: operation not supported"
+    fi
+    if ! setfacl -m u:nobody:r-x "$ROOT/metadata/acl_script_1mb_755.sh"; then
+      log "Skipping ACL assignment on $ROOT/metadata/acl_script_1mb_755.sh: operation not supported"
+    fi
   else
-    echo "Skipping ACL assignment: setfacl not available"
+    log "Skipping ACL assignment: setfacl not available"
   fi
 }
 
@@ -357,16 +371,21 @@ Notes:
 EOF
 }
 
-create_base_dirs
-create_regular_files
-create_named_variants
-create_deep_and_duplicate_cases
-create_time_and_readonly_cases
-create_links
-create_metadata_cases
-write_summary
-
-echo "Created migration test dataset at: $ROOT"
+if (( UPDATE_ONLY )); then
+  log "Running in update-only mode for $ROOT"
+  update_mutable_files_pass
+  log "Completed initial update-only pass"
+else
+  create_base_dirs
+  create_regular_files
+  create_named_variants
+  create_deep_and_duplicate_cases
+  create_time_and_readonly_cases
+  create_links
+  create_metadata_cases
+  write_summary
+  echo "Created migration test dataset at: $ROOT"
+fi
 
 if [[ -n "$UPDATE_INTERVAL" ]]; then
   run_update_loop

cdssync/migration-test-manifest.md

@@ -7,12 +7,14 @@ The generator script can also run in continuous update mode after initial creati
 - omit the interval argument to create the dataset once and exit
 - use `0` for continuous rewrites with no sleep between passes
 - use any integer greater than `0` to rewrite mutable files every `N` seconds
+- use `--update-only` to run updates against an already-existing dataset without recreating the special-case filesystem objects first
 
 Important implementation detail for update mode:
 
 - the update loop rewrites content-bearing regular files that are intended to simulate active data churn
 - it does not rewrite script files, sparse files, symlinks, hard links, or empty files
 - this preserves the special-case filesystem structure while still generating ongoing content changes
+- if ACL/xattr assignment is unsupported on the target filesystem, the script logs that condition and continues
 
 ## Recommended Root Layout