Handle temporary RPC failures and daemon restarts without corrupting state (#35)

* Handle temporary RPC failures and daemon restarts without corrupting state

watch_status()'s per-iteration RPC read had no error handling: an
unexpected failure (the daemon crashing or restarting mid-call, or any
other transient RPC hiccup) propagated straight out of the loop and
crashed the whole container, indistinguishable from a real fatal
error.

The read is now wrapped: an unexpected exception logs a warning and
retries on the next tick, leaving prev_status untouched so the next
successful read still reports every real change. A stop signal or
another DscError still ends the loop exactly as before - the wrapper
explicitly re-raises DscError first, since GracefulShutdown is one and
must not be swallowed by the broader except below it.

Verified against the real image: a container survives seaf-daemon
being killed outright, logging the failure instead of exiting.

* Log a traceback on RPC failure and strengthen the state test

Copilot review:
- The RPC failure warning logged only the message; add exc_info so a
  repeated failure in production is actually debuggable.
- The 'prior state is not corrupted' test asserted the warning fired
  but never proved a real change after the failure was still detected
  and logged, which is the actual property being claimed.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>

tests/smoke.sh

@@ -119,6 +119,49 @@ else
 fi
 docker rm -f dsc-smoke >/dev/null
 
+echo "== a killed daemon does not crash the watch loop"
+docker rm -f dsc-smoke >/dev/null 2>&1 || true
+docker run -d $PLATFORM_ARG --name dsc-smoke "$IMAGE" python3 -c "
+import logging
+logging.basicConfig(format='%(asctime)s %(message)s', level=logging.INFO)
+from dsc.client import SeafileClient
+
+c = SeafileClient('example.invalid', 'u', 'pw', '/dsc', token='tok')
+c.init_config()
+c.start_daemon()
+c.watch_status()  # the real thing, not a stub: this is what is under test
+" >/dev/null
+
+started=0
+for _ in $(seq 1 30); do
+    logs=$(docker logs dsc-smoke 2>&1)
+    if printf '%s' "$logs" | grep -q "Seafile daemon is ready"; then started=1; break; fi
+    sleep 1
+done
+
+if [ "$started" -eq 0 ]; then
+    fail "the daemon never became ready, kill scenario not measured"
+else
+    # No procps in the image, so find the PID by hand instead of pkill.
+    docker exec dsc-smoke sh -c '
+        for p in /proc/[0-9]*; do
+            [ "$(cat "$p/comm" 2>/dev/null)" = "seaf-daemon" ] && kill -9 "${p#/proc/}"
+        done' >/dev/null 2>&1
+    recovered=0
+    for _ in $(seq 1 30); do
+        logs=$(docker logs dsc-smoke 2>&1)
+        if printf '%s' "$logs" | grep -q "Could not read sync status"; then recovered=1; break; fi
+        sleep 1
+    done
+    if [ "$recovered" -eq 1 ] && [ "$(docker inspect -f '{{.State.Running}}' dsc-smoke)" = "true" ]; then
+        pass "the watch loop logged the failure and kept the container running"
+    else
+        fail "the container did not survive a killed daemon"
+        docker logs dsc-smoke 2>&1 | sed 's/^/        /' | tail -20
+    fi
+fi
+docker rm -f dsc-smoke >/dev/null
+
 echo "== the HEALTHCHECK command through the real entrypoint"
 # Runs through main(), unlike the fixture above: HOME/UID/GID come from the
 # same drop_privileges() path a real deployment uses, so .ccnet ends up
@@ -130,7 +173,8 @@ docker run -d $PLATFORM_ARG --name dsc-smoke \
 
 ready=0
 for _ in $(seq 1 30); do
-    if docker logs dsc-smoke 2>&1 | grep -q "Seafile daemon is ready"; then ready=1; break; fi
+    logs=$(docker logs dsc-smoke 2>&1)
+    if printf '%s' "$logs" | grep -q "Seafile daemon is ready"; then ready=1; break; fi
     sleep 1
 done