feat(framework) Add SuperExec logcatcher (#3584)

Signed-off-by: Robert Steiner <[email protected]>
Co-authored-by: Robert Steiner <[email protected]>
Co-authored-by: Chong Shen Ng <[email protected]>
Co-authored-by: Charles Beauville <[email protected]>
Co-authored-by: jafermarq <[email protected]>
Co-authored-by: Taner Topal <[email protected]>
Co-authored-by: Daniel J. Beutel <[email protected]>

src/py/flwr/superexec/exec_servicer.py

@@ -15,6 +15,9 @@
 """SuperExec API servicer."""
 
 
+import select
+import threading
+import time
 from collections.abc import Generator
 from logging import ERROR, INFO
 from typing import Any
@@ -33,6 +36,8 @@
 
 from .executor import Executor, RunTracker
 
+SELECT_TIMEOUT = 1  # Timeout for selecting ready-to-read file descriptors (in seconds)
+
 
 class ExecServicer(exec_pb2_grpc.ExecServicer):
     """SuperExec API servicer."""
@@ -59,13 +64,66 @@ def StartRun(
 
         self.runs[run.run_id] = run
 
+        # Start a background thread to capture the log output
+        capture_thread = threading.Thread(
+            target=_capture_logs, args=(run,), daemon=True
+        )
+        capture_thread.start()
+
         return StartRunResponse(run_id=run.run_id)
 
-    def StreamLogs(
+    def StreamLogs(  # pylint: disable=C0103
         self, request: StreamLogsRequest, context: grpc.ServicerContext
     ) -> Generator[StreamLogsResponse, Any, None]:
         """Get logs."""
-        logs = ["a", "b", "c"]
+        log(INFO, "ExecServicer.StreamLogs")
+
+        # Exit if `run_id` not found
+        if request.run_id not in self.runs:
+            context.abort(grpc.StatusCode.NOT_FOUND, "Run ID not found")
+
+        last_sent_index = 0
         while context.is_active():
-            for i in range(len(logs)):  # pylint: disable=C0200
+            # Yield n'th row of logs, if n'th row < len(logs)
+            logs = self.runs[request.run_id].logs
+            for i in range(last_sent_index, len(logs)):
                 yield StreamLogsResponse(log_output=logs[i])
+            last_sent_index = len(logs)
+
+            # Wait for and continue to yield more log responses only if the
+            # run isn't completed yet. If the run is finished, the entire log
+            # is returned at this point and the server ends the stream.
+            if self.runs[request.run_id].proc.poll() is not None:
+                log(INFO, "All logs for run ID `%s` returned", request.run_id)
+                return
+
+            time.sleep(1.0)  # Sleep briefly to avoid busy waiting
+
+
+def _capture_logs(
+    run: RunTracker,
+) -> None:
+    while True:
+        # Explicitly check if Popen.poll() is None. Required for `pytest`.
+        if run.proc.poll() is None:
+            # Select streams only when ready to read
+            ready_to_read, _, _ = select.select(
+                [run.proc.stdout, run.proc.stderr],
+                [],
+                [],
+                SELECT_TIMEOUT,
+            )
+            # Read from std* and append to RunTracker.logs
+            for stream in ready_to_read:
+                line = stream.readline().rstrip()
+                if line:
+                    run.logs.append(f"{line}")
+
+        # Close std* to prevent blocking
+        elif run.proc.poll() is not None:
+            log(INFO, "Subprocess finished, exiting log capture")
+            if run.proc.stdout:
+                run.proc.stdout.close()
+            if run.proc.stderr:
+                run.proc.stderr.close()
+            break

src/py/flwr/superexec/exec_servicer_test.py

@@ -16,11 +16,11 @@
 
 
 import subprocess
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, Mock
 
 from flwr.proto.exec_pb2 import StartRunRequest  # pylint: disable=E0611
 
-from .exec_servicer import ExecServicer
+from .exec_servicer import ExecServicer, _capture_logs
 
 
 def test_start_run() -> None:
@@ -50,3 +50,20 @@ def test_start_run() -> None:
     response = servicer.StartRun(request, context_mock)
 
     assert response.run_id == 10
+
+
+def test_capture_logs() -> None:
+    """Test capture_logs function."""
+    run_res = Mock()
+    run_res.logs = []
+    with subprocess.Popen(
+        ["echo", "success"],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        text=True,
+    ) as proc:
+        run_res.proc = proc
+        _capture_logs(run_res)
+
+    assert len(run_res.logs) == 1
+    assert run_res.logs[0] == "success"

src/py/flwr/superexec/executor.py

@@ -15,7 +15,7 @@
 """Execute and monitor a Flower run."""
 
 from abc import ABC, abstractmethod
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from subprocess import Popen
 from typing import Optional
 
@@ -28,6 +28,7 @@ class RunTracker:
 
     run_id: int
     proc: Popen  # type: ignore
+    logs: list[str] = field(default_factory=list)
 
 
 class Executor(ABC):