module: add a read-only mode to the compile cache

A compile cache generated ahead of time and shipped inside an
application package should only ever be read: the package may be
immutable or covered by an integrity check, and a cache directory that
appears at run time would be a surprise. Add readOnly to
module.enableCompileCache() and NODE_COMPILE_CACHE_READONLY=1: existing
entries are loaded as before, nothing is serialized or persisted,
flushCompileCache() is a no-op, and the cache directory is used as found
rather than created, so enabling against a missing directory fails
instead of making one.

Signed-off-by: Shelley Vohr <shelley.vohr@gmail.com>

Author: codebytere

doc/api/cli.md

@@ -3729,6 +3729,15 @@ Enable the [module compile cache][] for the Node.js instance. See the documentat
 When set to 1, the [module compile cache][]  can be reused across different directory
 locations as long as the module layout relative to the cache directory remains the same.
 
+### `NODE_COMPILE_CACHE_READONLY=1`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+When set to 1, the [module compile cache][] only reads existing entries from
+its directory: nothing is written to it and it is not created if missing.
+
 ### `NODE_DEBUG=module[,…]`
 
 <!-- YAML

doc/api/module.md

@@ -421,6 +421,15 @@ There are two ways to enable the portable mode:
 
 2. Setting the environment variable: [`NODE_COMPILE_CACHE_PORTABLE=1`][]
 
+### Read-only compile cache
+
+A cache that was generated ahead of time, for example at build time to be
+shipped inside an application package, can be enabled with `readOnly: true`
+(or [`NODE_COMPILE_CACHE_READONLY=1`][]). Node.js then loads whatever entries
+the directory holds and never writes to it: modules without a usable entry are
+compiled as usual but not persisted, [`module.flushCompileCache()`][] is a
+no-op, and the directory is not created if it is missing.
+
 ### Limitations of the compile cache
 
 Currently when using the compile cache with [V8 JavaScript code coverage][], the
@@ -494,6 +503,9 @@ The following constants are returned as the `status` field in the object returne
 <!-- YAML
 added: v22.8.0
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/00000
+    description: Add the `readOnly` option.
   - version:
      - v25.4.0
      - v24.15.0
@@ -520,6 +532,11 @@ changes:
     the cache can be reused even if the project directory is moved. This is a best-effort
     feature. If not specified, it will depend on whether the environment variable
     [`NODE_COMPILE_CACHE_PORTABLE=1`][] is set.
+  * `readOnly` {boolean} Optional. If `true`, existing cache entries in `directory` are
+    used but nothing is ever written to it, and the directory is not created when it does
+    not exist (enabling then fails). Meant for caches generated ahead of time and shipped
+    with an application. If not specified, it will depend on whether the environment
+    variable [`NODE_COMPILE_CACHE_READONLY=1`][] is set.
 * Returns: {Object}
   * `status` {integer} One of the [`module.constants.compileCacheStatus`][]
   * `message` {string|undefined} If Node.js cannot enable the compile cache, this contains
@@ -2059,6 +2076,7 @@ returned object contains the following keys:
 [`--require`]: cli.md#-r---require-module
 [`NODE_COMPILE_CACHE=dir`]: cli.md#node_compile_cachedir
 [`NODE_COMPILE_CACHE_PORTABLE=1`]: cli.md#node_compile_cache_portable1
+[`NODE_COMPILE_CACHE_READONLY=1`]: cli.md#node_compile_cache_readonly1
 [`NODE_DISABLE_COMPILE_CACHE=1`]: cli.md#node_disable_compile_cache1
 [`NODE_V8_COVERAGE=dir`]: cli.md#node_v8_coveragedir
 [`Object.freeze()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze

doc/node.1

@@ -1862,6 +1862,10 @@ module compile cache for details.
 When set to 1, the module compile cache  can be reused across different directory
 locations as long as the module layout relative to the cache directory remains the same.
 .
+.It Ev NODE_COMPILE_CACHE_READONLY Ar 1
+When set to 1, the module compile cache only reads existing entries from
+its directory: nothing is written to it and it is not created if missing.
+.
 .It Ev NODE_DEBUG Ar module[,…]
 \fB','\fR-separated list of core modules that should print debug information.
 .

lib/internal/modules/helpers.js

@@ -466,21 +466,25 @@ function stringify(body) {
  * after this method is called.
  * This method accepts either:
  * - A string: path to the cache directory.
- * - An options object `{directory?: string, portable?: boolean}`:
+ * - An options object `{directory?: string, portable?: boolean, readOnly?: boolean}`:
  *   - `directory`: A string path to the cache directory.
  *   - `portable`: If `portable` is true, the cache directory will be considered relative.
  *     Defaults to `NODE_COMPILE_CACHE_PORTABLE === '1'`.
+ *   - `readOnly`: If `readOnly` is true, existing cache entries are used but nothing is
+ *     written, and the cache directory is not created. Defaults to
+ *     `NODE_COMPILE_CACHE_READONLY === '1'`.
  * If cache directory is undefined, it defaults to the `NODE_COMPILE_CACHE` environment variable.
  * If `NODE_COMPILE_CACHE` isn't set, it defaults to `path.join(os.tmpdir(), 'node-compile-cache')`.
- * @param {string | { directory?: string, portable?: boolean } | undefined} options
+ * @param {string | { directory?: string, portable?: boolean, readOnly?: boolean } | undefined} options
  * @returns {{status: number, message?: string, directory?: string}}
  */
 function enableCompileCache(options) {
   let portable;
+  let readOnly;
   let directory;
 
   if (typeof options === 'object' && options !== null) {
-    ({ directory, portable } = options);
+    ({ directory, portable, readOnly } = options);
   } else {
     directory = options;
   }
@@ -490,7 +494,10 @@ function enableCompileCache(options) {
   if (portable === undefined) {
     portable = process.env.NODE_COMPILE_CACHE_PORTABLE === '1';
   }
-  const nativeResult = _enableCompileCache(directory, portable);
+  if (readOnly === undefined) {
+    readOnly = process.env.NODE_COMPILE_CACHE_READONLY === '1';
+  }
+  const nativeResult = _enableCompileCache(directory, portable, readOnly);
   const result = { status: nativeResult[0] };
   if (nativeResult[1]) {
     result.message = nativeResult[1];

src/compile_cache.cc

@@ -250,7 +250,8 @@ CompileCacheEntry* CompileCacheHandler::GetOrInsert(Local<String> code,
   // If the portable cache is enabled and it seems possible to compute the
   // relative position from an absolute path, we use the relative position
   // in the cache key.
-  if (portable_ == EnableOption::PORTABLE && IsAbsoluteFilePath(file_path)) {
+  if (HasOption(portable_, EnableOption::PORTABLE) &&
+      IsAbsoluteFilePath(file_path)) {
     // Normalize the path to ensure it is consistent.
     std::string normalized_file_path = NormalizeFileURLOrPath(env, file_path);
     if (normalized_file_path.empty()) {
@@ -325,6 +326,10 @@ void CompileCacheHandler::MaybeSaveImpl(CompileCacheEntry* entry,
     Debug("keeping the in-memory entry\n");
     return;
   }
+  if (read_only_) {
+    Debug("read-only, not serializing\n");
+    return;
+  }
   Debug("%s the in-memory entry\n",
         entry->cache == nullptr ? "initializing" : "refreshing");
 
@@ -349,6 +354,9 @@ void CompileCacheHandler::MaybeSave(CompileCacheEntry* entry,
 
 void CompileCacheHandler::MaybeSave(CompileCacheEntry* entry,
                                     std::string_view transpiled) {
+  if (read_only_) {
+    return;
+  }
   CHECK(entry->type == CachedCodeType::kStrippedTypeScript);
   Debug("[compile cache] saving transpilation cache for %s %s\n",
         entry->type_name(),
@@ -381,6 +389,10 @@ void CompileCacheHandler::MaybeSave(CompileCacheEntry* entry,
  */
 void CompileCacheHandler::Persist() {
   DCHECK(!compile_cache_dir_.empty());
+  if (read_only_) {
+    Debug("[compile cache] read-only, skipping persistence\n");
+    return;
+  }
 
   // TODO(joyeecheung): do this using a separate event loop to utilize the
   // libuv thread pool and do the file system operations concurrently.
@@ -546,10 +558,11 @@ CompileCacheEnableResult CompileCacheHandler::Enable(Environment* env,
         cache_tag,
         cache_dir_with_tag);
 
-  if (!env->permission()->is_granted(
-          env,
-          permission::PermissionScope::kFileSystemWrite,
-          cache_dir_with_tag)) [[unlikely]] {
+  const bool read_only = HasOption(option, EnableOption::READ_ONLY);
+  if (!read_only && !env->permission()->is_granted(
+                        env,
+                        permission::PermissionScope::kFileSystemWrite,
+                        cache_dir_with_tag)) [[unlikely]] {
     result.message = "Skipping compile cache because write permission for " +
                      cache_dir_with_tag + " is not granted";
     result.status = CompileCacheEnableStatus::FAILED;
@@ -566,25 +579,46 @@ CompileCacheEnableResult CompileCacheHandler::Enable(Environment* env,
     return result;
   }
 
-  fs::FSReqWrapSync req_wrap;
-  int err = fs::MKDirpSync(
-      nullptr, &(req_wrap.req), cache_dir_with_tag, 0777, nullptr);
-  if (is_debug_) {
-    Debug("[compile cache] creating cache directory %s...%s\n",
+  if (read_only) {
+    // A read-only cache is used as found and never created: without the
+    // directory there is nothing to read.
+    uv_fs_t stat_req;
+    int err =
+        uv_fs_stat(nullptr, &stat_req, cache_dir_with_tag.c_str(), nullptr);
+    // libuv normalizes st_mode across platforms; the S_ISDIR macro is not on MSVC.
+    bool is_dir = err == 0 && (stat_req.statbuf.st_mode & S_IFMT) == S_IFDIR;
+    uv_fs_req_cleanup(&stat_req);
+    Debug("[compile cache] read-only cache directory %s...%s\n",
           cache_dir_with_tag,
-          err < 0 ? uv_strerror(err) : "success");
-  }
-  if (err != 0 && err != UV_EEXIST) {
-    result.message =
-        "Cannot create cache directory: " + std::string(uv_strerror(err));
-    result.status = CompileCacheEnableStatus::FAILED;
-    return result;
+          is_dir ? "found" : "not found");
+    if (!is_dir) {
+      result.message =
+          "Cache directory does not exist (read-only): " + cache_dir_with_tag;
+      result.status = CompileCacheEnableStatus::FAILED;
+      return result;
+    }
+  } else {
+    fs::FSReqWrapSync req_wrap;
+    int err = fs::MKDirpSync(
+        nullptr, &(req_wrap.req), cache_dir_with_tag, 0777, nullptr);
+    if (is_debug_) {
+      Debug("[compile cache] creating cache directory %s...%s\n",
+            cache_dir_with_tag,
+            err < 0 ? uv_strerror(err) : "success");
+    }
+    if (err != 0 && err != UV_EEXIST) {
+      result.message =
+          "Cannot create cache directory: " + std::string(uv_strerror(err));
+      result.status = CompileCacheEnableStatus::FAILED;
+      return result;
+    }
   }
 
   result.cache_directory = absolute_cache_dir_base;
   compile_cache_dir_ = cache_dir_with_tag;
   portable_ = option;
-  if (option == EnableOption::PORTABLE) {
+  read_only_ = read_only;
+  if (HasOption(option, EnableOption::PORTABLE)) {
     normalized_compile_cache_dir_ =
         NormalizeFileURLOrPath(env, compile_cache_dir_);
   }

src/compile_cache.h

@@ -60,7 +60,22 @@ struct CompileCacheEnableResult {
   std::string message;  // Set in case of failure.
 };
 
-enum class EnableOption : uint8_t { DEFAULT, PORTABLE };
+enum class EnableOption : uint8_t {
+  DEFAULT = 0,
+  PORTABLE = 1 << 0,
+  // Only read existing cache entries: nothing is compiled into the in-memory
+  // store for persisting, nothing is written to disk, and the cache directory
+  // is not created if it does not exist.
+  READ_ONLY = 1 << 1,
+};
+
+inline constexpr EnableOption operator|(EnableOption a, EnableOption b) {
+  return static_cast<EnableOption>(static_cast<uint8_t>(a) |
+                                   static_cast<uint8_t>(b));
+}
+inline constexpr bool HasOption(EnableOption value, EnableOption flag) {
+  return (static_cast<uint8_t>(value) & static_cast<uint8_t>(flag)) != 0;
+}
 
 class CompileCacheHandler {
  public:
@@ -82,6 +97,7 @@ class CompileCacheHandler {
                  bool rejected);
   void MaybeSave(CompileCacheEntry* entry, std::string_view transpiled);
   std::string_view cache_dir() { return compile_cache_dir_; }
+  bool read_only() const { return read_only_; }
 
  private:
   void ReadCacheFile(CompileCacheEntry* entry);
@@ -107,6 +123,7 @@ class CompileCacheHandler {
   std::string compile_cache_dir_;
   std::string normalized_compile_cache_dir_;
   EnableOption portable_ = EnableOption::DEFAULT;
+  bool read_only_ = false;
   std::unordered_map<uint32_t, std::unique_ptr<CompileCacheEntry>>
       compiler_cache_store_;
 };

src/env.cc

@@ -1195,8 +1195,14 @@ void Environment::InitializeCompileCache() {
           DebugCategory::COMPILE_CACHE,
           "[compile cache] using relative path\n");
   }
-  EnableCompileCache(dir_from_env,
-                     portable ? EnableOption::PORTABLE : EnableOption::DEFAULT);
+  std::string read_only_env;
+  bool read_only = credentials::SafeGetenv(
+                       "NODE_COMPILE_CACHE_READONLY", &read_only_env, this) &&
+                   read_only_env == "1";
+  EnableOption option = EnableOption::DEFAULT;
+  if (portable) option = option | EnableOption::PORTABLE;
+  if (read_only) option = option | EnableOption::READ_ONLY;
+  EnableCompileCache(dir_from_env, option);
 }
 
 CompileCacheEnableResult Environment::EnableCompileCache(

src/node_modules.cc

@@ -508,7 +508,10 @@ void EnableCompileCache(const FunctionCallbackInfo<Value>& args) {
 
   EnableOption option = EnableOption::DEFAULT;
   if (args.Length() > 1 && args[1]->IsTrue()) {
-    option = EnableOption::PORTABLE;
+    option = option | EnableOption::PORTABLE;
+  }
+  if (args.Length() > 2 && args[2]->IsTrue()) {
+    option = option | EnableOption::READ_ONLY;
   }
 
   Utf8Value value(isolate, args[0]);

test/parallel/test-compile-cache-api-readonly.js

@@ -0,0 +1,65 @@
+'use strict';
+
+// This tests module.enableCompileCache({ directory, readOnly: true }): existing
+// entries are read, nothing is written, and a missing directory is not created.
+
+const common = require('../common');
+const { spawnSyncAndAssert } = require('../common/child_process');
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+const tmpdir = require('../common/tmpdir');
+const fixtures = require('../common/fixtures');
+
+tmpdir.refresh();
+const wrapper = fixtures.path('compile-cache-wrapper-options.js');
+const target = path.join(tmpdir.path, 'target.js');
+fs.writeFileSync(target, 'module.exports = 1;');
+const other = path.join(tmpdir.path, 'other.js');
+fs.writeFileSync(other, 'module.exports = 2;');
+const directory = path.join(tmpdir.path, 'cache');
+const list = () => fs.readdirSync(directory, { recursive: true }).sort();
+const run = (options, extraEnv, requires, check) => spawnSyncAndAssert(
+  process.execPath,
+  [...requires.flatMap((r) => ['-r', r]), target],
+  {
+    env: {
+      ...process.env,
+      NODE_DEBUG_NATIVE: 'COMPILE_CACHE',
+      NODE_TEST_COMPILE_CACHE_OPTIONS: JSON.stringify(options),
+      ...extraEnv,
+    },
+  },
+  check);
+
+// Read-only against a directory that does not exist: enabling fails and
+// nothing is created.
+run({ directory, readOnly: true }, {}, [wrapper], {
+  stderr: /read-only cache directory .*\.\.\.not found/,
+});
+assert(!fs.existsSync(directory));
+
+// A normal run generates the cache for target.js.
+run({ directory }, {}, [wrapper], {
+  stderr: /writing cache for .*target\.js.*success/,
+});
+const generated = list();
+assert.notStrictEqual(generated.length, 0);
+
+// Read-only against it: target.js's entry is accepted, other.js is compiled
+// but not written, and persistence is skipped.
+run({ directory, readOnly: true }, {}, [wrapper, other], {
+  stderr: common.mustCall((output) => {
+    assert.match(output, /cache for .*target\.js was accepted/);
+    assert.match(output, /read-only, skipping persistence/);
+    assert.doesNotMatch(output, /writing cache for/);
+    return true;
+  }),
+});
+assert.deepStrictEqual(list(), generated);
+
+// The environment variable form.
+run({ directory }, { NODE_COMPILE_CACHE_READONLY: '1' }, [wrapper, other], {
+  stderr: /read-only, skipping persistence/,
+});
+assert.deepStrictEqual(list(), generated);