fix(backend): detect circular component references and enforce reserved keywords in visitor.go. Fixes #13966

[bhuvan-somisetty]

Description of your changes:

Problem

In backend/src/v2/compiler/visitor.go, component dependency graphs are traversed using a depth-first search (pipelineDFS.dfs). Analysis of the compiler visitor pattern revealed multiple related defects:

1. Uncaught Circular Component References: pipelineDFS used a single flat visited map[string]bool map. When a sub-DAG component referenced an ancestor component (circular dependency), if state.visited[name] { return nil } returned nil silently. This allowed invalid cyclic pipeline specs to bypass API server validation and generate incomplete/corrupted Argo Workflow specs that failed with cryptic errors in Argo.
2. Unenforced Reserved root Component Name: RootComponentName ("root") was not validated against user component names, allowing user components named "root" to collide with root DAG traversal state.
3. Opaque Error Context: Error messages for missing component refs or executors omitted task name and parent DAG component context.
4. Redundant Proto Serialization Passes: GetPipelineSpec and GetDeploymentConfig performed double JSON round-tripping (protojson.Marshal -> string -> protojson.Unmarshal), adding memory allocations during large pipeline compilation.

Solution

1. Active Call-Stack Cycle Detection: Added inStack map[string]bool to pipelineDFS to track active DFS recursion. If inStack[name] is true, dfs returns a clear cycle error (circular reference detected in component graph: <name>).
2. Reserved Keyword Enforcement: Rejects user components named RootComponentName ("root") in Accept.
3. Enriched Error Context: Included task name (task.GetTaskInfo().GetName()) and parent component name in componentError.
4. Proto Unmarshaling Optimization: Direct unmarshaling from jsonBytes eliminating intermediate string conversions and buffer allocations.
5. Unit Tests: Added visitor_test.go covering cycle detection, reserved keyword checks, and error message context.

Fixes #13966

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[google-oss-prow]

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[google-oss-prow]

Hi @bhuvan-somisetty. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[bhuvan-somisetty]

Hi @hbelmiro @jeffspahr @droctothorpe,

Could you please review this backend compiler PR when you get a chance?

This PR refactors pipelineDFS in backend/src/v2/compiler/visitor.go to add active call-stack recursion tracking (inStack) for detecting circular component references during compilation. It also enforces reserved keyword checking for RootComponentName ("root"), enriches error messages with task context, and optimizes proto unmarshaling. Includes unit tests in visitor_test.go.

Thanks!

[hbelmiro]

/ok-to-test