Plan 03: Result Pattern for Completion Handlers & Consistent Logging

Objective

Add Result pattern to EventPublisher, RetentionHandler, and FollowUpHandler (S4), standardize their log formats (S5), add error handling to fetch/scrape Schedulers, and wire Result usage into FetchRunner for visibility.

Context

• @.claude/skills/sm-pipeline-stage/SKILL.md -- Pipeline architecture, completion handler patterns
• @.claude/skills/sm-architecture/SKILL.md -- Module structure, Result pattern usage across codebase
• @.claude/skills/tdd-cycle/SKILL.md -- TDD workflow
• RetentionHandler (30 lines) catches StandardError, logs with generic prefix, returns nil
• FollowUpHandler (43 lines) has per-item rescue, inconsistent log format, returns nil
• EventPublisher (20 lines) has no error handling at all, returns nil
• FetchRunner calls all three but ignores return values -- cannot detect handler failures
• Existing Result patterns: ItemCreator::Result, Enqueuer::Result, ItemScraper::Result, FeedFetcher::Result
• Fetch Scheduler (86 lines) and Scrape Scheduler (41 lines) have no error handling

Tasks

Task 1: Write handler Result tests (TDD red)

Create/update test files:

• test/lib/source_monitor/fetching/completion/retention_handler_test.rb: test returns Result with :applied on success; test returns Result with :failed and error on StandardError; test :skipped when result status is not :fetched
• test/lib/source_monitor/fetching/completion/follow_up_handler_test.rb: test returns Result with :applied and enqueued_count; test returns Result with :failed on error; test per-item errors captured in errors array
• test/lib/source_monitor/fetching/completion/event_publisher_test.rb: test returns Result with :published on success; test returns Result with :failed on error

Task 2: Add Result structs to completion handlers

Modify three files:

• lib/source_monitor/fetching/completion/retention_handler.rb: Add Result = Struct.new(:status, :removed_total, :error, keyword_init: true) with success? helper. Return Result from call.
• lib/source_monitor/fetching/completion/follow_up_handler.rb: Add Result = Struct.new(:status, :enqueued_count, :errors, keyword_init: true) with success? helper. Return Result from call. Collect per-item errors into errors array.
• lib/source_monitor/fetching/completion/event_publisher.rb: Add Result = Struct.new(:status, :error, keyword_init: true) with success? helper. Wrap dispatch in begin/rescue, return Result.

Task 3: Standardize logging in handlers and schedulers

Modify completion handlers (same files as Task 2):

• RetentionHandler: change log prefix from [SourceMonitor] to [SourceMonitor::Fetching::Completion::RetentionHandler]
• FollowUpHandler: change log prefix to [SourceMonitor::Fetching::Completion::FollowUpHandler] (consistent format)

Modify schedulers:

• lib/source_monitor/scheduler.rb (fetch): wrap run body in begin/rescue StandardError, log at warn level with [SourceMonitor::Scheduler] prefix
• lib/source_monitor/scraping/scheduler.rb (scrape): wrap run body in begin/rescue StandardError, log at warn level with [SourceMonitor::Scraping::Scheduler] prefix

Task 4: Wire Result usage in FetchRunner

Modify lib/source_monitor/fetching/fetch_runner.rb:

• Capture return values from retention_handler.call, follow_up_handler.call, event_publisher.call
• Log at warn level when any handler returns a non-success Result
• No behavioral change -- FetchRunner continues execution regardless (handlers are best-effort)

Task 5: Verify

• bin/rails test test/lib/source_monitor/fetching/completion/ -- all completion handler tests pass
• bin/rails test test/lib/source_monitor/fetching/fetch_runner_test.rb -- FetchRunner tests pass
• bin/rails test -- full suite passes
• bin/rubocop -- zero offenses
• Verify consistent log format: grep -r "\[SourceMonitor::" lib/source_monitor/fetching/completion/ shows uniform prefixes