Use typed context keys instead of string literals

[tmchow]

Replaces bare string context keys ("principal", "user", "requestId") with typed constants from a new ctxkeys package. This prevents silent value collisions if another package uses the same string as a context key.

The ctxkeys package is a leaf dependency with zero imports, so it can be used by auth, requestid, armadacontext, and grpc without introducing circular dependencies.

Changes:

• New internal/common/ctxkeys/ctxkeys.go with ContextKey type and three constants
• auth/common.go: principalKey now uses ctxkeys.PrincipalKey, "user" literal replaced with ctxkeys.UserKey
• requestid/interceptors.go: "requestId" literal replaced with ctxkeys.RequestIDKey
• armadacontext/armada_context.go: readers updated to use typed keys
• grpc/grpc.go: readers updated to use typed keys

Closes #4780

This PR was authored by a human with AI coding assistance.

[greptile-apps]

Greptile Summary

This PR refactors bare string context keys ("principal", "user", "requestId") into typed constants defined in a new leaf package internal/common/ctxkeys. The contextKey type is correctly unexported, meaning external packages cannot construct colliding keys, while the constants themselves remain accessible. All write sites (auth, requestid) and read sites (armadacontext, grpc) have been updated consistently.

Confidence Score: 5/5

Safe to merge — purely a refactor with no logic changes, all write/read sites updated consistently.

All five files make symmetric, mechanical substitutions of string literals with typed constants. The unexported contextKey type correctly prevents external collision. No behavioral differences introduced.

No files require special attention.

Vulnerabilities

No security concerns identified. The unexported key type prevents external packages from forging context keys, which is a minor security improvement over the previous string-literal approach.

Important Files Changed

Filename	Overview
internal/common/ctxkeys/ctxkeys.go	New leaf package with unexported contextKey type and three exported constants — correct idiomatic Go pattern for collision-safe context keys.
internal/common/auth/common.go	Local principalKey const now aliases ctxkeys.PrincipalKey; "user" write-site replaced with ctxkeys.UserKey. Write/read symmetry preserved — GetPrincipal/WithPrincipal both use principalKey.
internal/common/requestid/interceptors.go	"requestId" write-site in AddToIncomingContext replaced with ctxkeys.RequestIDKey; consistent with read sites in armadacontext and grpc.
internal/common/armadacontext/armada_context.go	Read sites in FromGrpcCtx updated to use ctxkeys.UserKey and ctxkeys.RequestIDKey — no logic change.
internal/common/grpc/grpc.go	Read sites in InterceptorLogger updated to use typed keys — no logic change.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    subgraph ctxkeys ["internal/common/ctxkeys"]
        PK["PrincipalKey (contextKey)"]
        UK["UserKey (contextKey)"]
        RK["RequestIDKey (contextKey)"]
    end

    subgraph writers ["Context Writers"]
        AUTH["auth/common.go\nCreateGrpcMiddlewareAuthFunction"]
        REQID["requestid/interceptors.go\nAddToIncomingContext"]
    end

    subgraph readers ["Context Readers"]
        ARMADA["armadacontext/armada_context.go\nFromGrpcCtx"]
        GRPC["grpc/grpc.go\nInterceptorLogger"]
        AUTHREAD["auth/common.go\nGetPrincipal / WithPrincipal"]
    end

    PK -->|principalKey alias| AUTHREAD
    UK --> AUTH
    UK --> ARMADA
    UK --> GRPC
    RK --> REQID
    RK --> ARMADA
    RK --> GRPC

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