rework concurrent upload

server/mergin/sync/models.py

@@ -2,12 +2,14 @@
 #
 # SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-MerginMaps-Commercial
 from __future__ import annotations
+from contextlib import contextmanager
 import json
 import logging
 import os
+import threading
 import time
 import uuid
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from enum import Enum
 from typing import Optional, List, Dict, Set, Tuple
 from dataclasses import dataclass, asdict
@@ -17,11 +19,11 @@
 from flask_login import current_user
 from pygeodiff import GeoDiff
 from sqlalchemy import text, null, desc, nullslast, tuple_
-from sqlalchemy.dialects.postgresql import ARRAY, BIGINT, UUID, JSONB, ENUM
+from sqlalchemy.dialects.postgresql import ARRAY, BIGINT, UUID, JSONB, ENUM, insert
 from sqlalchemy.types import String
 from sqlalchemy.ext.hybrid import hybrid_property
 from pygeodiff.geodifflib import GeoDiffLibError, GeoDiffLibConflictError
-from flask import current_app
+from flask import Flask, current_app
 
 from .files import (
     DeltaChangeMerged,
@@ -44,7 +46,6 @@
     LOG_BASE,
     Checkpoint,
     generate_checksum,
-    Toucher,
     get_chunk_location,
     get_project_path,
     is_supported_type,
@@ -1805,6 +1806,11 @@ class Upload(db.Model):
         db.Integer, db.ForeignKey("user.id", ondelete="CASCADE"), nullable=True
     )
     created = db.Column(db.DateTime, default=datetime.utcnow)
+    # last ping time to determine if upload is still active
+    last_ping = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
+    transaction_id = db.Column(
+        UUID(as_uuid=True), unique=True, nullable=False, index=True
+    )
 
     user = db.relationship("User")
     project = db.relationship(
@@ -1822,28 +1828,173 @@ def __init__(self, project: Project, version: int, changes: dict, user_id: int):
         self.version = version
         self.changes = ChangesSchema().dump(changes)
         self.user_id = user_id
+        self.transaction_id = str(uuid.uuid4())
+
+    @classmethod
+    def create_upload(
+        cls, project_id: str, version: int, changes: dict, user_id: int
+    ) -> Upload | None:
+        """Create upload session, it can either create a new record or handover an existing one but with new transaction id
+        Old transaction folder is removed and new one is created.
+        """
+        now = datetime.now(timezone.utc).replace(tzinfo=None)
+        expiration = current_app.config["LOCKFILE_EXPIRATION"]
+        new_tx_id = str(uuid.uuid4())
+
+        # CTE captures the existing row's transaction_id BEFORE the upsert (pre-statement snapshot)
+        # NULL in RETURNING means fresh INSERT, non-NULL means we took over a stale upload
+        existing_cte = (
+            db.select(Upload.transaction_id)
+            .where(
+                Upload.project_id == project_id,
+                Upload.version == version,
+            )
+            .cte("existing")
+        )
+
+        stmt = (
+            insert(Upload)
+            .values(
+                id=str(uuid.uuid4()),
+                transaction_id=new_tx_id,
+                project_id=project_id,
+                version=version,
+                user_id=user_id,
+                last_ping=now,
+                changes=ChangesSchema().dump(changes),
+            )
+            .add_cte(existing_cte)
+        )
+
+        upsert_stmt = stmt.on_conflict_do_update(
+            constraint="uq_upload_project_id",
+            set_={
+                "transaction_id": new_tx_id,
+                "user_id": user_id,
+                "last_ping": now,
+                "changes": ChangesSchema().dump(changes),
+            },
+            # ONLY update if the existing row is stale
+            where=(Upload.last_ping < (now - timedelta(seconds=expiration))),
+        )
+
+        upsert_stmt = upsert_stmt.returning(
+            Upload,
+            db.select(existing_cte.c.transaction_id)
+            .scalar_subquery()
+            .label("old_transaction_id"),
+        )
+
+        result = db.session.execute(upsert_stmt).fetchone()
+        db.session.commit()
+
+        # if nothing returned, it means the WHERE clause failed (active upload)
+        if not result:
+            return
+
+        upload = result.Upload
+        old_transaction_id = result.old_transaction_id
+
+        try:
+            os.makedirs(upload.upload_dir)
+
+            # old_transaction_id is NULL on fresh INSERT, set to old UUID when taking over a stale upload
+            if old_transaction_id:
+                upload.project.sync_failed(
+                    "", "push_lost", "Push artefact removed by subsequent push", user_id
+                )
+                if os.path.exists(
+                    os.path.join(
+                        upload.project.storage.project_dir,
+                        "tmp",
+                        str(old_transaction_id),
+                    )
+                ):
+                    move_to_tmp(
+                        os.path.join(
+                            upload.project.storage.project_dir,
+                            "tmp",
+                            str(old_transaction_id),
+                        ),
+                        str(old_transaction_id),
+                    )
+        except OSError as err:
+            # filesystem setup failed after the DB row was already committed.
+            # delete the row immediately so the next attempt isn't blocked until expiration.
+            db.session.delete(upload)
+            db.session.commit()
+            logging.error(f"Failed to create upload directory: {err}")
+            return
+
+        return upload
 
     @property
     def upload_dir(self):
-        return os.path.join(self.project.storage.project_dir, "tmp", self.id)
+        return os.path.join(
+            self.project.storage.project_dir, "tmp", str(self.transaction_id)
+        )
 
-    @property
-    def lockfile(self):
-        return os.path.join(self.upload_dir, "lockfile")
-
-    def is_active(self):
-        """Check if upload is still active because there was a ping (lockfile update) from underlying process"""
-        return os.path.exists(self.lockfile) and (
-            time.time() - os.path.getmtime(self.lockfile)
-            < current_app.config["LOCKFILE_EXPIRATION"]
+    def _heartbeat_task(self, app: Flask, stop_event: threading.Event, timeout: int):
+        """
+        Background task: Runs as a Thread, it is compatible with Sync (direct) or Gevent (monkey-patch) worker type.
+        Uses a fresh engine connection to stay pool-efficient.
+        """
+        # manual context push is required for background execution
+        with app.app_context():
+            while not stop_event.is_set():
+                try:
+                    # db.engine.begin() is efficient and isolated, it immediately returns a connection to the pool
+                    with db.engine.begin() as conn:
+                        conn.execute(
+                            db.text(
+                                "UPDATE upload SET last_ping = NOW() WHERE id = :id"
+                            ),
+                            {"id": self.id},
+                        )
+                except Exception as e:
+                    logging.exception(
+                        f"Upload heartbeat failed for ID {self.project_id} and version {self.version}: {e}"
+                    )
+
+                # wait for x seconds, but wake up immediately if stop_event is set
+                stop_event.wait(timeout)
+
+    @contextmanager
+    def heartbeat(self, timeout: int = 5):
+        """
+        Context manager to be used inside a Flask route.
+
+        Example of usage:
+        -----------------
+        with upload.heartbeat(interval):
+            do_something_slow
+        """
+        # we need to pass a real Flask app object to the thread
+        app = current_app._get_current_object()
+        stop_event = threading.Event()
+
+        bg = threading.Thread(
+            target=self._heartbeat_task, args=(app, stop_event, timeout), daemon=True
         )
 
+        bg.start()
+        try:
+            yield
+        finally:
+            # signal the loop to stop
+            stop_event.set()
+
+            # wait for the task to finish its last SQL call.
+            # in Gevent, this yields to other requests (non-blocking), while in Sync, this blocks the current thread for up to 2s
+            # this is to protect main thread / greenlet from zombie bg processes
+            bg.join(timeout=2)
+
     def clear(self):
         """Clean up pending upload.
         Uploaded files and table records are removed, and another upload can start.
         """
         try:
-            move_to_tmp(self.upload_dir, self.id)
+            move_to_tmp(self.upload_dir, str(self.transaction_id))
             db.session.delete(self)
             db.session.commit()
         except Exception:
@@ -1864,7 +2015,7 @@ def process_chunks(
         to_remove = [i.path for i in file_changes if i.change == PushChangeType.DELETE]
         current_files = [f for f in self.project.files if f.path not in to_remove]
 
-        with Toucher(self.lockfile, 5):
+        with self.heartbeat(5):
             for f in file_changes:
                 if f.change == PushChangeType.DELETE:
                     continue

server/mergin/sync/permissions.py

@@ -271,17 +271,18 @@ def check_project_permissions(
     return None
 
 
-def get_upload(transaction_id):
-    upload = Upload.query.get_or_404(transaction_id)
+def get_upload_or_fail(transaction_id: str) -> Upload:
+    if not is_valid_uuid(transaction_id):
+        abort(404)
+    upload = Upload.query.filter_by(transaction_id=transaction_id).first_or_404()
     # upload to 'removed' projects is forbidden
     if upload.project.removed_at:
         abort(404)
 
     if upload.user_id != current_user.id:
         abort(403, "You do not have permissions for ongoing upload")
 
-    upload_dir = os.path.join(upload.project.storage.project_dir, "tmp", transaction_id)
-    return upload, upload_dir
+    return upload
 
 
 def projects_query(permission, as_admin=True, public=True):

server/mergin/sync/public_api.yaml

@@ -699,7 +699,7 @@ paths:
        - do integrity check comparing uploaded file sizes with what was expected
        - move uploaded files to new version dir and applying sync changes (e.g. geodiff apply_changeset)
        - bump up version in database
-       - remove artifacts (chunks, lockfile) by moving them to tmp directory"
+       - remove artifacts (chunks) by moving them to tmp directory"
       operationId: push_finish
       parameters:
         - name: transaction_id