Add interval-based update mode for test dataset generation

Add optional interval-based random content updates to the cdssync
migration test dataset generator and document the new behavior.

This allows the dataset to be created once and then updated either
continuously or every N seconds while preserving the intended
special-case file structure.

cdssync/generate_migration_test_dataset.sh

@@ -5,7 +5,7 @@ set -euo pipefail
 usage() {
   cat <<'EOF'
 Usage:
-  generate_migration_test_dataset.sh TARGET_DIR
+  generate_migration_test_dataset.sh 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.
@@ -14,16 +14,28 @@ Notes:
   - Existing TARGET_DIR contents are left in place unless they collide.
   - ACL and xattr cases are created only if the local tools are available.
   - Sparse files are created with logical size but low physical allocation.
+  - If UPDATE_INTERVAL_SECONDS is provided, the script keeps rewriting
+    mutable files with random content after the initial dataset creation.
+  - An interval of 0 means continuous updates with no sleep between passes.
+  - Update mode rewrites content-bearing regular files only.
+  - Update mode does not rewrite script files, sparse files, symlinks,
+    hard links, or empty files.
 EOF
 }
 
-if [[ $# -ne 1 ]]; then
+if [[ $# -lt 1 || $# -gt 2 ]]; then
   usage
   exit 1
 fi
 
 TARGET_DIR=$1
 ROOT=$(realpath -m "$TARGET_DIR")
+UPDATE_INTERVAL=${2:-}
+
+if [[ -n "$UPDATE_INTERVAL" && ! "$UPDATE_INTERVAL" =~ ^[0-9]+$ ]]; then
+  echo "UPDATE_INTERVAL_SECONDS must be a non-negative integer" >&2
+  exit 1
+fi
 
 mkdir -p "$ROOT"
 
@@ -42,6 +54,10 @@ create_dir() {
   mkdir -p "$ROOT/$1"
 }
 
+log() {
+  printf '[%s] %s\n' "$(date '+%Y-%m-%d %H:%M:%S')" "$*"
+}
+
 set_times() {
   local rel=$1
   local stamp=$2
@@ -114,6 +130,96 @@ apply_mode() {
   chmod "$2" "$ROOT/$1"
 }
 
+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"
+  else
+    echo "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"
+  else
+    echo "Skipping ACL assignment: setfacl not available"
+  fi
+}
+
+rewrite_file_with_random_data() {
+  local rel=$1
+  local path="$ROOT/$rel"
+  local size
+  local mode
+
+  size=$(stat -c '%s' "$path")
+  mode=$(stat -c '%a' "$path")
+
+  chmod u+w "$path"
+  if (( size > 0 )); then
+    head -c "$size" /dev/urandom >"$path"
+  else
+    : >"$path"
+  fi
+  chmod "$mode" "$path"
+}
+
+update_mutable_files_pass() {
+  local rel
+  local mutable_files=(
+    "regular/text_1mb_644.txt"
+    "regular/text_3mb_600.txt"
+    "regular/text_5mb_755.txt"
+    "regular/random_1mb_600.bin"
+    "regular/random_3mb_644.bin"
+    "regular/random_5mb_755.bin"
+    "regular/compressible_1mb_644.log"
+    "regular/compressible_3mb_600.log"
+    "regular/compressible_5mb_755.log"
+    "hidden/.hidden_text_1mb_644.txt"
+    "hidden/.hidden_random_3mb_600.bin"
+    "spaces in name/file with spaces text 1mb 644.txt"
+    "spaces in name/file with spaces random 3mb 600.bin"
+    "regular/longname_aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa_text_1mb_644.txt"
+    "regular/longname_bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb_random_3mb_600.bin"
+    "regular/longname_cccccccccccccccccccccccccccccccc_compressible_5mb_755.log"
+    "deep/tree/level1/level2/level3/deep_text_1mb_644.txt"
+    "deep/tree/level1/level2/level3/deep_random_3mb_600.bin"
+    "regular/dup_source_text_3mb_644.txt"
+    "regular/dup_copy_a_text_3mb_600.txt"
+    "deep/tree/level1/level2/dup_copy_b_text_3mb_755.txt"
+    "regular/old_text_1mb_644.txt"
+    "regular/recent_text_1mb_644.txt"
+    "regular/futureish_text_1mb_644.txt"
+    "readonly-dir/locked_text_1mb_444.txt"
+    "readonly-dir/locked_random_3mb_400.bin"
+    "metadata/xattr_text_1mb_644.txt"
+    "metadata/xattr_random_3mb_600.bin"
+    "metadata/acl_text_1mb_644.txt"
+  )
+
+  for rel in "${mutable_files[@]}"; do
+    rewrite_file_with_random_data "$rel"
+  done
+
+  set_acl_and_xattr_metadata
+}
+
+run_update_loop() {
+  local iteration=1
+
+  log "Starting update loop for $ROOT with interval ${UPDATE_INTERVAL}s"
+  while true; do
+    update_mutable_files_pass
+    log "Completed random update pass $iteration"
+    iteration=$((iteration + 1))
+
+    if (( UPDATE_INTERVAL > 0 )); then
+      sleep "$UPDATE_INTERVAL"
+    fi
+  done
+}
+
 make_file() {
   local path=$1
   local type=$2
@@ -232,19 +338,7 @@ create_metadata_cases() {
   make_file "metadata/acl_text_1mb_644.txt" text 1 0644
   make_file "metadata/acl_script_1mb_755.sh" script 1 0755
 
-  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"
-  else
-    echo "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"
-  else
-    echo "Skipping ACL assignment: setfacl not available"
-  fi
+  set_acl_and_xattr_metadata
 }
 
 write_summary() {
@@ -273,3 +367,7 @@ create_metadata_cases
 write_summary
 
 echo "Created migration test dataset at: $ROOT"
+
+if [[ -n "$UPDATE_INTERVAL" ]]; then
+  run_update_loop
+fi