Fix Agera3 retimer cold-boot crash (uninitialized productInfo + PAI platform-port factory)

Summary:
## Summary
Cold boot on Ladakh/Leh800bcls (Agera3, PAI 4.1) crashed with a SIGSEGV in
`StateDelta` construction during parallel XPHY programming, and firmware never
downloaded. Two defects, both exposed when the platform-port creation was
refactored per-vendor (the old `else -> SaiFakePlatformPort` catch-all in
`SaiPlatform::initPorts()` became a hard `throw`):

1. **Uninitialized product info.** `BspSaiPhyManager::createExternalPhy` created a
   `PlatformProductInfo` but never called `initialize()`. `Platform::getType()`
   returns `productInfo_->getType()`, so the retimer platform type was
   uninitialized garbage -> `createSaiPlatformPort()` returned null and
   `initPorts()` threw for every XPHY. (Same fix D114436354 made for Elbert; the
   retimer path was missed.)

2. **PAI build linked the fake port factory.** After the refactor, each binary
   links one vendor-specific `createSaiPlatformPort`. The PAI phy build was wired
   to `SaiPlatformInitFake.cpp`, whose factory only handles `PLATFORM_FAKE_*` and
   returns null for `LEH800BCLS`/`LADAKH800BCLS` -> throw, even once the type was
   correct.

Together these failed all XPHY inits; programming then ran against uninitialized
switches and hit the `StateDelta` null-deref crash.

## Changes
- `BspSaiPhyManager::createExternalPhy`: call `productInfo->initialize()` before
  constructing `SaiPhyPlatform` (mirrors `ElbertPhyManager`/`SaiPlatformInit`).
- New `SaiPlatformInitPai.cpp`: PAI-specific `createSaiPlatformPort`
  (`LADAKH800BCLS`/`LEH800BCLS` -> `SaiBcmPlatformPort`, plus fake fallback) and
  `chooseSaiPlatformImpl`.
- `platform.bzl` (internal buck): `brcm_pai_srcs` links `SaiPlatformInitPai.cpp`
  instead of `SaiPlatformInitFake.cpp`.
- `AgentPlatformsSai.cmake` (OSS): the `SAI_BRCM_PAI_IMPL` branch swaps the
  factory the same way -- reuse the fake port sources but drop
  `SaiPlatformInitFake.cpp` and add `SaiPlatformInitPai.cpp`. The real fake build
  (`SAI_PLATFORM_FAKE_SRC` / `else()` branch) is untouched.

No new build deps (`SaiBcmPlatformPort.cpp` + `SaiFakePlatform*.cpp` are already
in both source lists). The OSS PAI source set is now identical to the buck
`brcm_pai_srcs` (22 files, same set), so exactly one `createSaiPlatformPort` is
linked -- no duplicate/missing symbols, and the new file is wired into CMake
(OSS-003 satisfied).

Reviewed By: daiwei1983, birdsoup

Differential Revision: D115255918

fbshipit-source-id: 391c89521ff5ae94def7f0d46827dc7cadd4c73d

Author: Protick Bhowmick

cmake/AgentPlatformsSai.cmake

@@ -91,9 +91,17 @@ elseif (CHENAB_SAI_SDK)
     ${SAI_PLATFORM_CHENAB_SRC}
   )
 elseif (SAI_BRCM_PAI_IMPL)
+  # PAI (retimer / XPHY) build: same platform-port sources as the fake build,
+  # but with a PAI-specific createSaiPlatformPort that builds real ports for the
+  # Agera3 retimer boxes (LEH800BCLS/LADAKH800BCLS) instead of the fake-only
+  # factory. Mirrors brcm_pai_srcs in platform.bzl.
+  set(SAI_PLATFORM_PAI_SRC ${SAI_PLATFORM_FAKE_SRC})
+  list(REMOVE_ITEM SAI_PLATFORM_PAI_SRC
+    fboss/agent/platforms/sai/SaiPlatformInitFake.cpp)
   list(APPEND SAI_PLATFORM_SRC
     fboss/agent/platforms/sai/SaiPhyPlatform.cpp
-    ${SAI_PLATFORM_FAKE_SRC}
+    fboss/agent/platforms/sai/SaiPlatformInitPai.cpp
+    ${SAI_PLATFORM_PAI_SRC}
   )
 else()
   list(APPEND SAI_PLATFORM_SRC

fboss/agent/platforms/sai/SaiPlatformInitPai.cpp

@@ -0,0 +1,50 @@
+/*
+ *  Copyright (c) 2004-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+#include "fboss/agent/platforms/sai/SaiPlatformInitImpl.h"
+
+#include "fboss/agent/platforms/sai/SaiBcmPlatformPort.h"
+#include "fboss/agent/platforms/sai/SaiFakePlatform.h"
+#include "fboss/agent/platforms/sai/SaiFakePlatformPort.h"
+
+namespace facebook::fboss {
+
+// Broadcom PAI (retimer / XPHY) build. The retimer SaiPhyPlatform is created
+// directly by BspSaiPhyManager::createExternalPhy, so chooseSaiPlatformImpl is
+// only exercised by fake test paths here.
+std::unique_ptr<SaiPlatform> chooseSaiPlatformImpl(
+    std::unique_ptr<PlatformProductInfo>& productInfo,
+    folly::MacAddress /*localMac*/,
+    const std::string& /*platformMappingStr*/) {
+  if (productInfo->getType() == PlatformType::PLATFORM_FAKE_SAI) {
+    return std::make_unique<SaiFakePlatform>(std::move(productInfo));
+  }
+  return nullptr;
+}
+
+std::unique_ptr<SaiPlatformPort> createSaiPlatformPort(
+    const PortID& portId,
+    SaiPlatform* platform) {
+  const auto type = platform->getType();
+  // Agera3 retimer boxes: XPHY ports use the generic SaiBcmPlatformPort, same
+  // as the agent bcm build. (These are the only real platforms the PAI build
+  // instantiates.)
+  if (type == PlatformType::PLATFORM_LADAKH800BCLS ||
+      type == PlatformType::PLATFORM_LEH800BCLS) {
+    return std::make_unique<SaiBcmPlatformPort>(portId, platform);
+  }
+  if (type == PlatformType::PLATFORM_FAKE_SAI ||
+      type == PlatformType::PLATFORM_FAKE_WEDGE) {
+    return std::make_unique<SaiFakePlatformPort>(portId, platform);
+  }
+  return nullptr;
+}
+
+} // namespace facebook::fboss

fboss/lib/phy/BspSaiPhyManager.cpp

@@ -189,6 +189,7 @@ void BspSaiPhyManager::createExternalPhy(
   // Create SaiPhyPlatform for this xphy
   auto productInfo =
       std::make_unique<PlatformProductInfo>(FLAGS_fruid_filepath);
+  productInfo->initialize();
   addSaiPlatform(
       xphyID,
       std::make_unique<SaiPhyPlatform>(