From 9dee0c8437d5714e54c90cc53079b67429112ae2 Mon Sep 17 00:00:00 2001
From: zwl-williamzha <zwlwilliamzha@gmail.com>
Date: Wed, 23 Sep 2026 14:24:06 -0700
Subject: [PATCH] Keep module context data when a module's first hook returns
 none

v4's moduleContexts.put stores whatever the first hook returns, so a
nil context gets stored for the module. Every later SetAll on that nil
pointer is a no-op, and all data from later stages is silently lost.
v3 replaced a nil entry instead (ok && existingCtx != nil), and the
move to *ModuleContext in #4603 dropped that.

In MSP, ad_cache's entrypoint hook returns no context, so on stage its
placement/user/imp ids never reached the bidder request or auction
response stages: ad-cache ranking, dynamic floors and FB demote were
skipped, and msp_app_level_fill_counter stopped.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
---
 hooks/hookexecution/context.go      |  2 +-
 hooks/hookexecution/context_test.go | 39 +++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 hooks/hookexecution/context_test.go

diff --git a/hooks/hookexecution/context.go b/hooks/hookexecution/context.go
index d0d8073dda..b2fbda3832 100644
--- a/hooks/hookexecution/context.go
+++ b/hooks/hookexecution/context.go
@@ -54,7 +54,7 @@ func (mc *moduleContexts) put(moduleName string, mCtx *hookstage.ModuleContext)
 	defer mc.Unlock()
 
 	existingCtx, ok := mc.ctxs[moduleName]
-	if !ok {
+	if !ok || existingCtx == nil {
 		mc.ctxs[moduleName] = mCtx
 		return
 	}
diff --git a/hooks/hookexecution/context_test.go b/hooks/hookexecution/context_test.go
new file mode 100644
index 0000000000..aaab2cebb4
--- /dev/null
+++ b/hooks/hookexecution/context_test.go
@@ -0,0 +1,39 @@
+package hookexecution
+
+import (
+	"testing"
+
+	"github.com/prebid/prebid-server/v4/hooks/hookstage"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestModuleContextsPutAfterNil(t *testing.T) {
+	mcs := &moduleContexts{ctxs: map[string]*hookstage.ModuleContext{}}
+	mcs.put("module", nil)
+
+	later := hookstage.NewModuleContext()
+	later.Set("key", "value")
+	mcs.put("module", later)
+
+	got, ok := mcs.get("module")
+	assert.True(t, ok)
+	v, found := got.Get("key")
+	assert.True(t, found)
+	assert.Equal(t, "value", v)
+}
+
+func TestModuleContextsPutMergesAndKeepsNilHarmless(t *testing.T) {
+	mcs := &moduleContexts{ctxs: map[string]*hookstage.ModuleContext{}}
+
+	first := hookstage.NewModuleContext()
+	first.Set("a", 1)
+	mcs.put("module", first)
+	mcs.put("module", nil)
+
+	second := hookstage.NewModuleContext()
+	second.Set("b", 2)
+	mcs.put("module", second)
+
+	got, _ := mcs.get("module")
+	assert.Equal(t, map[string]any{"a": 1, "b": 2}, got.GetAll())
+}