feat(agent): add PageBroker Unix socket server

[dfeigin-nv]

Adds the PageBroker Unix-socket daemon. It accepts one framed protobuf request per connection and rejects malformed or oversized frames.

Validation: make -C agent/pagebroker daemon test.

Summary by CodeRabbit

• New Features

  • Added a pagebroker daemon that listens for local socket connections and processes requests.
  • Added container support for building and running pagebroker.
  • Added automatic resource management for file descriptors.
  • Expanded build and test targets to include daemon functionality.

• Refactor

  • Improved code formatting without changing behavior.

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📒 Files selected for processing (6)

• agent/pagebroker/Dockerfile
• agent/pagebroker/Makefile
• agent/pagebroker/checkpoint_transaction_descriptor.hpp
• agent/pagebroker/daemon.cpp
• agent/pagebroker/file_descriptor.cpp
• agent/pagebroker/file_descriptor.hpp

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Walkthrough

The change adds a Unix-domain socket pagebroker daemon, RAII file-descriptor handling, Makefile targets for daemon and test builds, and a multi-stage Docker image with the compiled runtime.

Changes

Pagebroker daemon

Layer / File(s)	Summary
Descriptor ownership agent/pagebroker/file_descriptor.hpp, agent/pagebroker/file_descriptor.cpp	Adds a non-copyable FileDescriptor class that closes nonnegative descriptors during destruction.
Socket protocol and daemon lifecycle agent/pagebroker/daemon.cpp, agent/pagebroker/checkpoint_transaction_descriptor.hpp	Adds Unix-domain socket setup, directory creation, size-prefixed request validation, Broker dispatch, serialized responses, and a persistent accept loop.
Build and runtime packaging agent/pagebroker/Makefile, agent/pagebroker/Dockerfile	Adds daemon and expanded test builds. The multi-stage image contains pagebroker and the runtime Protobuf library.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: 🟠 High · up to a4d21

The daemon change still carries high-impact merge risks: stalled clients or repeated accept failures can impair availability, an incorrect socket path can delete a regular file, and the runtime image may fail to build because of an invalid package name. These issues should be fixed before merging.

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant pagebroker
  participant Broker
  participant staging_directory
  Client->>pagebroker: Send size-prefixed request over Unix socket
  pagebroker->>Broker: Dispatch parsed request
  Broker->>staging_directory: Use staging directory
  Broker-->>pagebroker: Return serialized response
  pagebroker-->>Client: Send size-prefixed response

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🚥 Pre-merge checks | ✅ 7

✅ Passed checks (7 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title uses the required feat prefix, clearly describes the PageBroker Unix socket server, and is under 72 characters.
Docstring Coverage	✅ Passed	Docstring check was indeterminate for this PR — some files could not be analyzed in time. Not blocking.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Breaking Api Changes	✅ Passed	The API diff changes only constants.go by adding constants; no exported fields, JSON tags, Go field types, or XValidation markers changed.
Rbac Least Privilege	✅ Passed	The PR changes only PageBroker files. Existing kubebuilder markers and Helm RBAC templates are unchanged and contain no wildcard verbs or resources.

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[dfeigin-nv]

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Actionable comments posted: 5

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Inline comments:
In `@agent/pagebroker/daemon.cpp`:
- Around line 70-94: Update HandleConnection and the accepted-socket setup in
the accept loop to enforce bounded receive and send timeouts for every
connection, covering ReadAll and WriteAll. Close the connection when either
deadline expires or I/O fails, while preserving the existing request validation
and response flow.
- Around line 139-143: Update the accept loop around the FileDescriptor
construction so only EINTR retries immediately; handle EMFILE, ENFILE, ENOMEM,
and ENOBUFS with bounded backoff and recovery logic before retrying, while
preserving error reporting for other failures.
- Line 121: Replace the unconditional unlink in the daemon startup path with
lstat-based validation that removes the path only when it already exists as a
socket; also require and verify a service-owned socket directory to prevent
replacement between validation and removal.

In `@agent/pagebroker/Dockerfile`:
- Around line 12-14: Make the PageBroker Docker build invocation explicit by
documenting the exact command using agent/pagebroker as its build context, or
update the Dockerfile build step to use make -C pagebroker daemon when invoked
from the agent context. Anchor the change to the RUN make daemon instruction and
ensure the chosen approach works from the documented build context.
- Around line 16-22: Update the PageBroker Dockerfile to install
libprotobuf32t64, create a dedicated non-root runtime user, grant that user
access to the socket, staging, and mounted storage directories, and set USER
before ENTRYPOINT so the daemon does not run as root.

Apply the same fix in `@agent/pagebroker/Dockerfile` at line 18: The protobuf
package failure is consolidated into the runtime-image remediation.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7727381 and ed56938.

📒 Files selected for processing (6)

• agent/pagebroker/Dockerfile
• agent/pagebroker/Makefile
• agent/pagebroker/checkpoint_transaction_descriptor.hpp
• agent/pagebroker/daemon.cpp
• agent/pagebroker/file_descriptor.cpp
• agent/pagebroker/file_descriptor.hpp

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