sea: snapshot support in single executable applications

doc/api/single-executable-applications.md

@@ -6,6 +6,10 @@
 added:
   - v19.7.0
   - v18.16.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/46824
+    description: Added support for "useSnapshot".
 -->
 
 > Stability: 1 - Experimental: This feature is being designed and will change.
@@ -169,14 +173,46 @@ The configuration currently reads the following top-level fields:
 {
   "main": "/path/to/bundled/script.js",
   "output": "/path/to/write/the/generated/blob.blob",
-  "disableExperimentalSEAWarning": true // Default: false
+  "disableExperimentalSEAWarning": true, // Default: false
+  "useSnapshot": false  // Default: false
 }
 ```
 
 If the paths are not absolute, Node.js will use the path relative to the
 current working directory. The version of the Node.js binary used to produce
 the blob must be the same as the one to which the blob will be injected.
 
+### Startup snapshot support
+
+The `useSnapshot` field can be used to enable startup snapshot support. In this
+case the `main` script would not be when the final executable is launched.
+Instead, it would be run when the single executable application preparation
+blob is generated on the building machine. The generated preparation blob would
+then include a snapshot capturing the states initialized by the `main` script.
+The final executable with the preparation blob injected would deserialize
+the snapshot at run time.
+
+When `useSnapshot` is true, the main script must invoke the
+[`v8.startupSnapshot.setDeserializeMainFunction()`][] API to configure code
+that needs to be run when the final executable is launched by the users.
+
+The typical pattern for an application to use snapshot in a single executable
+application is:
+
+1. At build time, on the building machine, the main script is run to
+   initialize the heap to a state that's ready to take user input. The script
+   should also configure a main function with
+   [`v8.startupSnapshot.setDeserializeMainFunction()`][]. This function will be
+   compiled and serialized into the snapshot, but not invoked at build time.
+2. At run time, the main function will be run on top of the deserialized heap
+   on the user machine to process user input and generate output.
+
+The general constraints of the startup snapshot scripts also apply to the main
+script when it's used to build snapshot for the single executable application,
+and the main script can use the [`v8.startupSnapshot` API][] to adapt to
+these constraints. See
+[documentation about startup snapshot support in Node.js][].
+
 ## Notes
 
 ### `require(id)` in the injected module is not file based
@@ -249,6 +285,9 @@ to help us document them.
 [`process.execPath`]: process.md#processexecpath
 [`require()`]: modules.md#requireid
 [`require.main`]: modules.md#accessing-the-main-module
+[`v8.startupSnapshot.setDeserializeMainFunction()`]: v8.md#v8startupsnapshotsetdeserializemainfunctioncallback-data
+[`v8.startupSnapshot` API]: v8.md#startup-snapshot-api
+[documentation about startup snapshot support in Node.js]: cli.md#--build-snapshot
 [fuse]: https://www.electronjs.org/docs/latest/tutorial/fuses
 [postject]: https://github.com/nodejs/postject
 [signtool]: https://learn.microsoft.com/en-us/windows/win32/seccrypto/signtool

lib/internal/main/mksnapshot.js

@@ -16,6 +16,10 @@ const {
   anonymousMainPath,
 } = internalBinding('mksnapshot');
 
+const { isExperimentalSeaWarningNeeded } = internalBinding('sea');
+
+const { emitExperimentalWarning } = require('internal/util');
+
 const {
   getOptionValue,
 } = require('internal/options');
@@ -126,6 +130,7 @@ function requireForUserSnapshot(id) {
   return require(normalizedId);
 }
 
+
 function main() {
   prepareMainThreadExecution(true, false);
   initializeCallbacks();
@@ -167,6 +172,10 @@ function main() {
 
   const serializeMainArgs = [process, requireForUserSnapshot, minimalRunCjs];
 
+  if (isExperimentalSeaWarningNeeded()) {
+    emitExperimentalWarning('Single executable application');
+  }
+
   if (getOptionValue('--inspect-brk')) {
     internalBinding('inspector').callAndPauseOnStart(
       runEmbedderEntryPoint, undefined, ...serializeMainArgs);

lib/internal/process/pre_execution.js

@@ -174,7 +174,7 @@ function patchProcessObject(expandArgv1) {
     __proto__: null,
     enumerable: true,
     // Only set it to true during snapshot building.
-    configurable: getOptionValue('--build-snapshot'),
+    configurable: isBuildingSnapshot(),
     value: process.argv[0],
   });
 

src/node.cc

@@ -292,6 +292,17 @@ MaybeLocal<Value> StartExecution(Environment* env, StartExecutionCallback cb) {
 
   CHECK(!env->isolate_data()->is_building_snapshot());
 
+#ifndef DISABLE_SINGLE_EXECUTABLE_APPLICATION
+  if (sea::IsSingleExecutable()) {
+    sea::SeaResource sea = sea::FindSingleExecutableResource();
+    // The SEA preparation blob building process should already enforce this,
+    // this check is just here to guard against the unlikely case where
+    // the SEA preparation blob has been manually modified by someone.
+    CHECK_IMPLIES(sea.use_snapshot(),
+                  !env->snapshot_deserialize_main().IsEmpty());
+  }
+#endif
+
   // TODO(joyeecheung): move these conditions into JS land and let the
   // deserialize main function take precedence. For workers, we need to
   // move the pre-execution part into a different file that can be
@@ -1198,49 +1209,66 @@ ExitCode GenerateAndWriteSnapshotData(const SnapshotData** snapshot_data_ptr,
   return exit_code;
 }
 
-ExitCode LoadSnapshotDataAndRun(const SnapshotData** snapshot_data_ptr,
-                                const InitializationResultImpl* result) {
-  ExitCode exit_code = result->exit_code_enum();
+bool LoadSnapshotData(const SnapshotData** snapshot_data_ptr) {
   // nullptr indicates there's no snapshot data.
   DCHECK_NULL(*snapshot_data_ptr);
+
+  bool is_sea = false;
+#ifndef DISABLE_SINGLE_EXECUTABLE_APPLICATION
+  if (sea::IsSingleExecutable()) {
+    is_sea = true;
+    sea::SeaResource sea = sea::FindSingleExecutableResource();
+    if (sea.use_snapshot()) {
+      std::unique_ptr<SnapshotData> read_data =
+          std::make_unique<SnapshotData>();
+      std::string_view snapshot = sea.main_code_or_snapshot;
+      if (SnapshotData::FromBlob(read_data.get(), snapshot)) {
+        *snapshot_data_ptr = read_data.release();
+        return true;
+      } else {
+        fprintf(stderr, "Invalid snapshot data in single executable binary\n");
+        return false;
+      }
+    }
+  }
+#endif
+
   // --snapshot-blob indicates that we are reading a customized snapshot.
-  if (!per_process::cli_options->snapshot_blob.empty()) {
+  // Ignore it when we are loading from SEA.
+  if (!is_sea && !per_process::cli_options->snapshot_blob.empty()) {
     std::string filename = per_process::cli_options->snapshot_blob;
     FILE* fp = fopen(filename.c_str(), "rb");
     if (fp == nullptr) {
       fprintf(stderr, "Cannot open %s", filename.c_str());
-      exit_code = ExitCode::kStartupSnapshotFailure;
-      return exit_code;
+      return false;
     }
     std::unique_ptr<SnapshotData> read_data = std::make_unique<SnapshotData>();
     bool ok = SnapshotData::FromFile(read_data.get(), fp);
     fclose(fp);
     if (!ok) {
-      // If we fail to read the customized snapshot,
-      // simply exit with kStartupSnapshotFailure.
-      exit_code = ExitCode::kStartupSnapshotFailure;
-      return exit_code;
+      return false;
     }
     *snapshot_data_ptr = read_data.release();
-  } else if (per_process::cli_options->node_snapshot) {
-    // If --snapshot-blob is not specified, we are reading the embedded
-    // snapshot, but we will skip it if --no-node-snapshot is specified.
+    return true;
+  }
+
+  if (per_process::cli_options->node_snapshot) {
+    // If --snapshot-blob is not specified or if the SEA contains no snapshot,
+    // we are reading the embedded snapshot, but we will skip it if
+    // --no-node-snapshot is specified.
     const node::SnapshotData* read_data =
         SnapshotBuilder::GetEmbeddedSnapshotData();
-    if (read_data != nullptr && read_data->Check()) {
+    if (read_data != nullptr) {
+      if (!read_data->Check()) {
+        return false;
+      }
       // If we fail to read the embedded snapshot, treat it as if Node.js
       // was built without one.
       *snapshot_data_ptr = read_data;
     }
   }
 
-  NodeMainInstance main_instance(*snapshot_data_ptr,
-                                 uv_default_loop(),
-                                 per_process::v8_platform.Platform(),
-                                 result->args(),
-                                 result->exec_args());
-  exit_code = main_instance.Run();
-  return exit_code;
+  return true;
 }
 
 static ExitCode StartInternal(int argc, char** argv) {
@@ -1275,7 +1303,8 @@ static ExitCode StartInternal(int argc, char** argv) {
 
   std::string sea_config = per_process::cli_options->experimental_sea_config;
   if (!sea_config.empty()) {
-    return sea::BuildSingleExecutableBlob(sea_config);
+    return sea::BuildSingleExecutableBlob(
+        sea_config, result->args(), result->exec_args());
   }
 
   // --build-snapshot indicates that we are in snapshot building mode.
@@ -1290,7 +1319,15 @@ static ExitCode StartInternal(int argc, char** argv) {
   }
 
   // Without --build-snapshot, we are in snapshot loading mode.
-  return LoadSnapshotDataAndRun(&snapshot_data, result.get());
+  if (!LoadSnapshotData(&snapshot_data)) {
+    return ExitCode::kStartupSnapshotFailure;
+  }
+  NodeMainInstance main_instance(snapshot_data,
+                                 uv_default_loop(),
+                                 per_process::v8_platform.Platform(),
+                                 result->args(),
+                                 result->exec_args());
+  return main_instance.Run();
 }
 
 int Start(int argc, char** argv) {

src/node_main_instance.cc

@@ -92,12 +92,16 @@ void NodeMainInstance::Run(ExitCode* exit_code, Environment* env) {
     bool runs_sea_code = false;
 #ifndef DISABLE_SINGLE_EXECUTABLE_APPLICATION
     if (sea::IsSingleExecutable()) {
-      runs_sea_code = true;
       sea::SeaResource sea = sea::FindSingleExecutableResource();
-      std::string_view code = sea.code;
-      LoadEnvironment(env, code);
+      if (!sea.use_snapshot()) {
+        runs_sea_code = true;
+        std::string_view code = sea.main_code_or_snapshot;
+        LoadEnvironment(env, code);
+      }
     }
 #endif
+    // Either there is already a snapshot main function from SEA, or it's not
+    // a SEA at all.
     if (!runs_sea_code) {
       LoadEnvironment(env, StartExecutionCallback{});
     }