Turbopack: In CI only persist at the end (#87316)

### What?

Usually we persist the database every 2 minutes inside of `next build`, so you can continue from a partial build.

But in CI that doesn't make sense, so this changes it to only persist on shutdown.

Author: sokra

crates/napi/src/next_api/turbopack_ctx.rs

@@ -204,6 +204,8 @@ pub fn create_turbo_tasks(
             BackendOptions {
                 storage_mode: Some(if std::env::var("TURBO_ENGINE_READ_ONLY").is_ok() {
                     turbo_tasks_backend::StorageMode::ReadOnly
+                } else if is_ci {
+                    turbo_tasks_backend::StorageMode::ReadWriteOnShutdown
                 } else {
                     turbo_tasks_backend::StorageMode::ReadWrite
                 }),

turbopack/crates/turbo-tasks-backend/src/backend/mod.rs

@@ -122,8 +122,11 @@ pub enum StorageMode {
     /// Queries the storage for cache entries that don't exist locally.
     ReadOnly,
     /// Queries the storage for cache entries that don't exist locally.
-    /// Keeps a log of all changes and regularly push them to the backing storage.
+    /// Regularly pushes changes to the backing storage.
     ReadWrite,
+    /// Queries the storage for cache entries that don't exist locally.
+    /// On shutdown, pushes all changes to the backing storage.
+    ReadWriteOnShutdown,
 }
 
 pub struct BackendOptions {
@@ -242,7 +245,10 @@ impl<B: BackingStorage> TurboTasksBackend<B> {
 impl<B: BackingStorage> TurboTasksBackendInner<B> {
     pub fn new(mut options: BackendOptions, backing_storage: B) -> Self {
         let shard_amount = compute_shard_amount(options.num_workers, options.small_preallocation);
-        let need_log = matches!(options.storage_mode, Some(StorageMode::ReadWrite));
+        let need_log = matches!(
+            options.storage_mode,
+            Some(StorageMode::ReadWrite) | Some(StorageMode::ReadWriteOnShutdown)
+        );
         if !options.dependency_tracking {
             options.active_tracking = false;
         }
@@ -369,7 +375,10 @@ impl<B: BackingStorage> TurboTasksBackendInner<B> {
     }
 
     fn should_persist(&self) -> bool {
-        matches!(self.options.storage_mode, Some(StorageMode::ReadWrite))
+        matches!(
+            self.options.storage_mode,
+            Some(StorageMode::ReadWrite) | Some(StorageMode::ReadWriteOnShutdown)
+        )
     }
 
     fn should_restore(&self) -> bool {
@@ -1262,7 +1271,9 @@ impl<B: BackingStorage> TurboTasksBackendInner<B> {
             }
         }
 
-        if self.should_persist() {
+        // Only when it should write regularly to the storage, we schedule the initial snapshot
+        // job.
+        if matches!(self.options.storage_mode, Some(StorageMode::ReadWrite)) {
             // Schedule the snapshot job
             let _span = trace_span!("persisting background job").entered();
             let _span = tracing::info_span!("thread").entered();