[zwl-williamzha]

Problem

After the v4.9.0 sync (#137), moduleContexts.put stores whatever a module's first hook returns, including nil:

existingCtx, ok := mc.ctxs[moduleName]
if !ok {
    mc.ctxs[moduleName] = mCtx   // may be nil
    return
}
existingCtx.SetAll(mCtx.GetAll()) // no-op on a nil receiver

Once nil is stored, every later stage's SetAll does nothing, so all context data the module writes afterwards is silently dropped. v3 handled this: it replaced a nil entry (ok && existingCtx != nil). Upstream's move to *ModuleContext in prebid#4603 lost that check.

Impact on MSP stage: ad_cache's entrypoint hook doesn't set a context, so the placement, user and impression ids set in processed_auction never reached later stages. Because of that, ad-cache ranking, dynamic bidder floors, FB demote, the fraud-user check and last-bid-floor writes were all skipped, and msp_app_level_fill_counter / msp_bidder_fill_counter stopped. test_mode has the same bug for test traffic. Prod is unaffected because it's still on the pre-sync build.

Fix

Replace the stored entry when it is nil (if !ok || existingCtx == nil), which restores v3's behavior.

Testing

• New hooks/hookexecution/context_test.go:
  • TestModuleContextsPutAfterNil fails without the fix and passes with it.
  • TestModuleContextsPutMergesAndKeepsNilHarmless covers normal merging, including a nil result after data is stored.
• go test -race ./hooks/... passes.

The same fix should go to upstream prebid-server, since it affects any v4 host with a module like this.

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