feat(session): lazy-on-first-write shard migration for pre-shard sessions

When resolveSession() encounters a session that exists in the global DB
but has no shard, it now creates a shard and copies messages, parts,
todos, event_sequence, and events from the global DB into it.

- Add findRoot(): walks parent chain to locate the tree root
- Add seed(): creates shard DB and bulk-copies session tree data
- Add ensureShard(): combines sessionRoot lookup with lazy migration
- Update resolveSession() to use ensureShard()
- Update sync transact routing to use ensureShard()
- Global DB copies preserved (not deleted during migration)
- Concurrent migration guarded by in-process Set
- Uses INSERT OR IGNORE for idempotent cross-process safety

Author: sjawhar

.sisyphus/evidence/task-2-cold-cache.txt

@@ -0,0 +1,7 @@
+scenario: cold-cache recovery
+session: ses_28c96c7faffeA58tT6iGGN4yuR
+message: msg_d73693884001E8bB55xfWazRV0
+file: /tmp/opencode-test-data-2946638/share/opencode/sessions/ses_28c96c7faffeA58tT6iGGN4yuR.db
+has_session: true
+shard_message: msg_d73693884001E8bB55xfWazRV0
+global_message: missing

.sisyphus/evidence/task-2-todo-consistency.txt

@@ -0,0 +1,5 @@
+scenario: todo write-read consistency
+session: ses_28c96c1f8ffevBl0g4vfD1BUBt
+todos: [{"content":"one","status":"pending","priority":"high"},{"content":"two","status":"completed","priority":"low"}]
+shard_rows: 2
+global_rows: 0

.sisyphus/evidence/task-2-write-read-consistency.txt

@@ -0,0 +1,7 @@
+scenario: sharded message write-read consistency
+session: ses_28c96c279ffe7SgVchRp36sGv4
+message: msg_d73693db4001jyt5bClJZFrIoI
+parts: 1
+read_text: immediate shard read
+shard_message: msg_d73693db4001jyt5bClJZFrIoI
+global_message: missing

.sisyphus/evidence/task-5-load-test-global.txt

@@ -0,0 +1,69 @@
+scenario: sharded
+workers: 20
+writes: 1220
+duration_ms: 3263
+throughput_per_s: 373.89
+global_stats: {"count":220,"min":0.899,"p50":4.333,"p95":113.328,"p99":550.165,"max":849.164}
+shard_stats: {"count":1000,"min":0.883,"p50":3.798,"p95":26.114,"p99":62.711,"max":112.928}
+wal_max_bytes: 1569752
+wal_files: [{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c968bf8ffeMggZgqSb7hvO15.db-wal","size":1569752},{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c968e7effeIcNZemJGdwwLNc.db-wal","size":1569752},{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c969071ffeN9yRhqeUBdWD8W.db-wal","size":1569752},{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c969115ffe0Kk43RghAsSBlS.db-wal","size":1569752},{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c968aa0ffeJogHoVBHcq4tl5.db-wal","size":1569752},{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c969149ffeSri3Fspo61Y2Pu.db-wal","size":1569752},{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c969417ffeED1Z3EvSaB4Fwa.db-wal","size":1569752},{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c9690e6ffenlDRQ05AIf6SE7.db-wal","size":1569752},{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c969258ffeAybUt7HCDJ0hJW.db-wal","size":1569752},{"file":"/tmp/opencode-test-o5nq5qgsax/data/opencode/sessions/ses_28c968e8effeAXB2FE25uK8aib.db-wal","size":1569752}]
+worker_0_exit: 0
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+worker_17_stderr: 
+worker_18_exit: 0
+worker_18_stdout: {"idx":18,"mode":"sharded","session":"ses_28c969115ffe0Kk43RghAsSBlS","started":1775757782762,"ended":1775757784067,"global":[34.843,107.323,12.024,2.011,1.299,27.815,1.431,12.164,26.223,3.588,9.944],"shard":[21.935,4.322,18.862,18.087,39.275,22.41,13.612,17.577,34.684,5.348,26.114,10.036,14.849,43.918,2.316,1.604,3.798,1.862,10.436,30.17,9.614,1.327,41.133,1.732,12.921,1.476,22.116,8.461,1.556,1.175,28.962,1.727,34.644,1.847,2.19,8.026,3.233,1.025,4.063,6.373,2.514,1.758,5.114,1.61,8.918,1.287,1.243,12.607,1.327,17.319],"errors":[],"counts":{"created":1,"updated":10,"messages":50,"parts":50}}
+worker_18_stderr: 
+worker_19_exit: 0
+worker_19_stdout: {"idx":19,"mode":"sharded","session":"ses_28c968e7effeIcNZemJGdwwLNc","started":1775757783425,"ended":1775757784748,"global":[39.63,124.165,1.838,15.648,115.895,5.019,16.486,6.808,5.645,2.697,40.751],"shard":[9.524,19.001,11.102,23.118,1.99,50.94,17.09,17.43,13.819,7.045,1.661,8.76,1.965,28.123,5.036,6.266,1.958,11.052,1.458,13.136,5.862,1.519,5.649,1.405,15.628,1.656,8.72,1.639,6.71,1.391,9.309,1.118,1.863,1.248,5.555,2.58,7.311,1.226,1.097,4.396,2.039,4.456,4.441,112.928,8.828,1.49,10.802,2.01,11.25,62.711],"errors":[],"counts":{"created":1,"updated":10,"messages":50,"parts":50}}
+worker_19_stderr: 

.sisyphus/evidence/task-5-load-test-sharded.txt

@@ -192,8 +192,8 @@ export namespace Runner {
         case "ShellThenRun":
           return [
             Effect.gen(function* () {
-              yield* Deferred.fail(st.run.done, new Cancelled()).pipe(Effect.asVoid)
               yield* stopShell(st.shell)
+              yield* Deferred.fail(st.run.done, new Cancelled()).pipe(Effect.asVoid)
               yield* idleIfCurrent()
             }),
             { _tag: "Idle" } as const,

packages/opencode/src/effect/runner.ts

@@ -134,6 +134,12 @@ export namespace Project {
       const db = <T>(fn: (d: Parameters<typeof Database.use>[0] extends (trx: infer D) => any ? D : never) => T) =>
         Effect.sync(() => Database.use(fn))
 
+      type Sync<T> = T extends Promise<any> ? never : T
+
+      const tx = <T>(
+        fn: (d: Parameters<typeof Database.transaction>[0] extends (trx: infer D) => any ? D : never) => Sync<T>,
+      ) => Effect.sync(() => Database.transaction(fn, { behavior: "immediate" }))
+
       const emitUpdated = (data: Info) =>
         Effect.sync(() =>
           GlobalBus.emit("event", {
@@ -245,7 +251,7 @@ export namespace Project {
         })
 
         // Phase 2: upsert
-        const row = yield* db((d) => d.select().from(ProjectTable).where(eq(ProjectTable.id, data.id)).get())
+        const row = yield* tx((d) => d.select().from(ProjectTable).where(eq(ProjectTable.id, data.id)).get())
         const existing = row
           ? fromRow(row)
           : {
@@ -277,9 +283,8 @@ export namespace Project {
           { concurrency: "unbounded" },
         ).pipe(Effect.map((arr) => arr.filter((x): x is string => x !== undefined)))
 
-        yield* db((d) =>
-          d
-            .insert(ProjectTable)
+        yield* tx((d) => {
+          d.insert(ProjectTable)
             .values({
               id: result.id,
               worktree: result.worktree,
@@ -307,18 +312,15 @@ export namespace Project {
                 commands: result.commands,
               },
             })
-            .run(),
-        )
+            .run()
 
-        if (data.id !== ProjectID.global) {
-          yield* db((d) =>
-            d
-              .update(SessionTable)
-              .set({ project_id: data.id })
-              .where(and(eq(SessionTable.project_id, ProjectID.global), eq(SessionTable.directory, data.worktree)))
-              .run(),
-          )
-        }
+          if (data.id === ProjectID.global) return
+
+          d.update(SessionTable)
+            .set({ project_id: data.id })
+            .where(and(eq(SessionTable.project_id, ProjectID.global), eq(SessionTable.directory, data.worktree)))
+            .run()
+        })
 
         yield* emitUpdated(result)
         return { project: result, sandbox: data.sandbox }

packages/opencode/src/project/project.ts

@@ -403,6 +403,14 @@ export namespace Session {
 
         yield* Effect.sync(() => SyncEvent.run(Event.Created, { sessionID: result.id, info: result }))
 
+        if (!input.parentID) {
+          try {
+            Database.session(result.id)
+          } catch (e) {
+            log.error("failed to create per-tree db", { error: e })
+          }
+        }
+
         const cfg = yield* config.get()
         if (!result.parentID && (Flag.OPENCODE_AUTO_SHARE || cfg.share === "auto")) {
           yield* share(result.id).pipe(Effect.ignore, Effect.forkIn(scope))
@@ -592,12 +600,13 @@ export namespace Session {
         )
       })
 
-      const messages = Effect.fn("Session.messages")(function* (input: { sessionID: SessionID; limit?: number }) {
-        if (input.limit) {
-          return MessageV2.page({ sessionID: input.sessionID, limit: input.limit }).items
-        }
-        return Array.from(MessageV2.stream(input.sessionID)).reverse()
-      })
+      const messages = Effect.fn("Session.messages")((input: { sessionID: SessionID; limit?: number }) =>
+        Effect.succeed(
+          input.limit
+            ? MessageV2.page({ sessionID: input.sessionID, limit: input.limit }).items
+            : Array.from(MessageV2.stream(input.sessionID)).reverse(),
+        ),
+      )
 
       const removeMessage = Effect.fn("Session.removeMessage")(function* (input: {
         sessionID: SessionID