MdeModulePkg/UefiBootManagerLib: Register var policy on notify (#1276)

## Description

Variable policy is currently set in the constructor. In some system
designs, variable policy might not be running when the constructor of
this library is executed. To make the policy registration process more
robust, move registration to a protocol notify.

- [x] Impacts functionality?
- [ ] Impacts security?
- [ ] Breaking change?
- [ ] Includes tests?
- [ ] Includes documentation?
- [x] Backport to release branch?

## How This Was Tested

- Verify notify is called on variable policy protocol installation and
register policy successfully.
- Verify subsequent calls to the protocol gracefully exit by recognizing
`EFI_ALREADY_STARTED` is returned from `RegisterBasicVariablePolicy()`.

## Integration Instructions

N/A

Signed-off-by: Michael Kubacki <[email protected]>

Author: makubacki

MdeModulePkg/Library/UefiBootManagerLib/BmBoot.c

@@ -2819,19 +2819,17 @@ EfiBootManagerGetNextLoadOptionDevicePath (
 //
 
 /**
-  Constructor for UefiBootMangerLib.
+  Variable policy protocol installation notification.
 
-  @param[in] ImageHandle  The handle of the loaded image.
-  @param[in] SystemTable  System resources and configuration
+  @param[in]    Event           The notification event.
+  @param[in]    Context         Pointer to the context registered when the event is created. Not used.
 
-  @retval EFI_SUCCESS   The constructor set the variable policy if implemented
-  @retval others        The constructor did not succeed and one or more variable policy are not set
 **/
-EFI_STATUS
+VOID
 EFIAPI
-UefiBootManagerLibConstructor (
-  IN EFI_HANDLE        ImageHandle,
-  IN EFI_SYSTEM_TABLE  *SystemTable
+OnVariablePolicyNotification (
+  IN  EFI_EVENT  Event,
+  IN  VOID       *Context
   )
 {
   EFI_STATUS                      Status;
@@ -2841,9 +2839,7 @@ UefiBootManagerLibConstructor (
   // VariablePolicy, if implemented on this system, should have been installed already.
   //
   Status = gBS->LocateProtocol (&gEdkiiVariablePolicyProtocolGuid, NULL, (VOID **)&VariablePolicy);
-  if (EFI_ERROR (Status)) {
-    DEBUG ((DEBUG_ERROR, "%a: - Unable to locate VariablePolicy - Code=%r\n", __func__, Status));
-  } else {
+  if (!EFI_ERROR (Status)) {
     Status = RegisterBasicVariablePolicy (
                VariablePolicy,
                &mBmHardDriveBootVariableGuid,
@@ -2861,10 +2857,43 @@ UefiBootManagerLibConstructor (
     // 2. Already Started.  Only the first module to register a variable policy will successfully register
     //                      a policy.  The subsequent modules will get EFI_ALREADY_STARTED.
     if (EFI_ERROR (Status) && (Status != EFI_ALREADY_STARTED) && (Status != EFI_WRITE_PROTECTED)) {
-      DEBUG ((DEBUG_ERROR, "%a: - Error setting policy for HDDP - Code=%r\n", __func__, Status));
+      DEBUG ((DEBUG_ERROR, "%a: - Error setting policy for HDDP - Status=%r\n", __func__, Status));
       ASSERT_EFI_ERROR (Status);
     }
+
+    if (Event != NULL) {
+      gBS->CloseEvent (Event);
+    }
+  } else {
+    DEBUG ((DEBUG_ERROR, "%a: - Unable to locate variable policy protocol - Status=%r\n", __func__, Status));
   }
+}
+
+/**
+  Constructor for UefiBootMangerLib.
+
+  @param[in] ImageHandle  The handle of the loaded image.
+  @param[in] SystemTable  System resources and configuration
+
+  @retval EFI_SUCCESS   The constructor set the variable policy if implemented
+  @retval others        The constructor did not succeed and one or more variable policy are not set
+**/
+EFI_STATUS
+EFIAPI
+UefiBootManagerLibConstructor (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  VOID  *Registration;
+
+  EfiCreateProtocolNotifyEvent (
+    &gEdkiiVariablePolicyProtocolGuid,
+    TPL_CALLBACK,
+    OnVariablePolicyNotification,
+    NULL,
+    &Registration
+    );
 
   return EFI_SUCCESS;
 }