add hook to observe pending sessions (#751)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

jupyter_server/services/sessions/sessionmanager.py

@@ -10,6 +10,9 @@
     # fallback on pysqlite2 if Python was build without sqlite
     from pysqlite2 import dbapi2 as sqlite3
 
+from dataclasses import dataclass, fields
+from typing import Union
+
 from tornado import web
 from traitlets import Instance, TraitError, Unicode, validate
 from traitlets.config.configurable import LoggingConfigurable
@@ -18,6 +21,132 @@
 from jupyter_server.utils import ensure_async
 
 
+class KernelSessionRecordConflict(Exception):
+    """Exception class to use when two KernelSessionRecords cannot
+    merge because of conflicting data.
+    """
+
+    pass
+
+
+@dataclass
+class KernelSessionRecord:
+    """A record object for tracking a Jupyter Server Kernel Session.
+
+    Two records that share a session_id must also share a kernel_id, while
+    kernels can have multiple session (and thereby) session_ids
+    associated with them.
+    """
+
+    session_id: Union[None, str] = None
+    kernel_id: Union[None, str] = None
+
+    def __eq__(self, other: "KernelSessionRecord") -> bool:
+        if isinstance(other, KernelSessionRecord):
+            condition1 = self.kernel_id and self.kernel_id == other.kernel_id
+            condition2 = all(
+                [
+                    self.session_id == other.session_id,
+                    self.kernel_id is None or other.kernel_id is None,
+                ]
+            )
+            if any([condition1, condition2]):
+                return True
+            # If two records share session_id but have different kernels, this is
+            # and ill-posed expression. This should never be true. Raise an exception
+            # to inform the user.
+            if all(
+                [
+                    self.session_id,
+                    self.session_id == other.session_id,
+                    self.kernel_id != other.kernel_id,
+                ]
+            ):
+                raise KernelSessionRecordConflict(
+                    "A single session_id can only have one kernel_id "
+                    "associated with. These two KernelSessionRecords share the same "
+                    "session_id but have different kernel_ids. This should "
+                    "not be possible and is likely an issue with the session "
+                    "records."
+                )
+        return False
+
+    def update(self, other: "KernelSessionRecord") -> None:
+        """Updates in-place a kernel from other (only accepts positive updates"""
+        if not isinstance(other, KernelSessionRecord):
+            raise TypeError("'other' must be an instance of KernelSessionRecord.")
+
+        if other.kernel_id and self.kernel_id and other.kernel_id != self.kernel_id:
+            raise KernelSessionRecordConflict(
+                "Could not update the record from 'other' because the two records conflict."
+            )
+
+        for field in fields(self):
+            if hasattr(other, field.name) and getattr(other, field.name):
+                setattr(self, field.name, getattr(other, field.name))
+
+
+class KernelSessionRecordList:
+    """An object for storing and managing a list of KernelSessionRecords.
+
+    When adding a record to the list, the KernelSessionRecordList
+    first checks if the record already exists in the list. If it does,
+    the record will be updated with the new information; otherwise,
+    it will be appended.
+    """
+
+    def __init__(self, *records):
+        self._records = []
+        for record in records:
+            self.update(record)
+
+    def __str__(self):
+        return str(self._records)
+
+    def __contains__(self, record: Union[KernelSessionRecord, str]):
+        """Search for records by kernel_id and session_id"""
+        if isinstance(record, KernelSessionRecord) and record in self._records:
+            return True
+
+        if isinstance(record, str):
+            for r in self._records:
+                if record in [r.session_id, r.kernel_id]:
+                    return True
+        return False
+
+    def __len__(self):
+        return len(self._records)
+
+    def get(self, record: Union[KernelSessionRecord, str]) -> KernelSessionRecord:
+        """Return a full KernelSessionRecord from a session_id, kernel_id, or
+        incomplete KernelSessionRecord.
+        """
+        if isinstance(record, str):
+            for r in self._records:
+                if record == r.kernel_id or record == r.session_id:
+                    return r
+        elif isinstance(record, KernelSessionRecord):
+            for r in self._records:
+                if record == r:
+                    return record
+        raise ValueError(f"{record} not found in KernelSessionRecordList.")
+
+    def update(self, record: KernelSessionRecord) -> None:
+        """Update a record in-place or append it if not in the list."""
+        try:
+            idx = self._records.index(record)
+            self._records[idx].update(record)
+        except ValueError:
+            self._records.append(record)
+
+    def remove(self, record: KernelSessionRecord) -> None:
+        """Remove a record if its found in the list. If it's not found,
+        do nothing.
+        """
+        if record in self._records:
+            self._records.remove(record)
+
+
 class SessionManager(LoggingConfigurable):
 
     database_filepath = Unicode(
@@ -58,6 +187,10 @@ def _validate_database_filepath(self, proposal):
         ]
     )
 
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self._pending_sessions = KernelSessionRecordList()
+
     # Session database initialized below
     _cursor = None
     _connection = None
@@ -118,15 +251,20 @@ async def create_session(
     ):
         """Creates a session and returns its model"""
         session_id = self.new_session_id()
+        record = KernelSessionRecord(session_id=session_id)
+        self._pending_sessions.update(record)
         if kernel_id is not None and kernel_id in self.kernel_manager:
             pass
         else:
             kernel_id = await self.start_kernel_for_session(
                 session_id, path, name, type, kernel_name
             )
+        record.kernel_id = kernel_id
+        self._pending_sessions.update(record)
         result = await self.save_session(
             session_id, path=path, name=name, type=type, kernel_id=kernel_id
         )
+        self._pending_sessions.remove(record)
         return result
 
     async def start_kernel_for_session(self, session_id, path, name, type, kernel_name):
@@ -305,6 +443,9 @@ async def list_sessions(self):
 
     async def delete_session(self, session_id):
         """Deletes the row in the session database with given session_id"""
+        record = KernelSessionRecord(session_id=session_id)
+        self._pending_sessions.update(record)
         session = await self.get_session(session_id=session_id)
         await ensure_async(self.kernel_manager.shutdown_kernel(session["kernel"]["id"]))
         self.cursor.execute("DELETE FROM session WHERE session_id=?", (session_id,))
+        self._pending_sessions.remove(record)