L127: C++: SPIFFE Bundle Map support in Providers (#506)

Author: gtcooke94

L127-cpp-spiffe-root-provider.md

@@ -0,0 +1,59 @@
+L127: C++: SPIFFE Bundle Map support in Root Providers
+----
+* Author(s): gtcooke94
+* Approver: markdroth
+* Status: Implemented
+* Implemented in: core, cpp
+* Last updated: 2025-08-11
+* Discussion at: https://groups.google.com/g/grpc-io/c/G47BjLsF4JQ
+
+## Abstract
+
+The purpose of this proposal is to add public API support for SPIFFE bundle maps in root certificate file watcher providers. [A87] details the broader internals for this support.
+
+## Background
+
+gRPC supports SPIFFE bundle maps as root certificate material per [A87]. Public APIs to configure these roots are needed.
+
+### Related Proposals:
+* [A87]
+
+[A87]: A87-mtls-spiffe-support.md
+
+## Proposal
+
+This document proposes to extend the C-Core and C++ APIs as follows:
+
+
+### C-Core
+In the C-core API, we will add a new `spiffe_bundle_map_path` parameter to the `grpc_tls_certificate_provider_file_watcher_create()` function, which will now look like this:
+
+```
+GRPCAPI grpc_tls_certificate_provider*
+grpc_tls_certificate_provider_file_watcher_create(
+    const char* private_key_path, const char* identity_certificate_path,
+    const char* root_cert_path, const char* spiffe_bundle_map_path, unsigned int refresh_interval_sec);
+```
+
+If the `spiffe_bundle_map_path` is set, the `root_cert_path` will be ignored. This holds even in the case where the `spiffe_bundle_map_path` ends up being invalid.
+
+### C++
+While the existing C++ API is marked experimental, we don't _want_ to break existing users. Thus, we will add a constructor with the `spiffe_bundle_map_path` argument to the `FileWatcherCertificateProvider`.
+In order to not break current users, we will make the existing constructors support this by supplying an empty SPIFFE bundle map path.
+```
+
+FileWatcherCertificateProvider(const std::string& private_key_path,
+                                const std::string& identity_certificate_path,
+                                const std::string& root_cert_path,
+                                const std::string& spiffe_bundle_map_path,
+                                unsigned int refresh_interval_sec);
+```
+
+### Other Providers
+This proposal _only_ aims to support file-based SPIFFE Bundle Maps via the file watcher providers. The `StaticDataCertificateProvider` structure is left as future work. This will involve broadening the API surface to expose a type for the SPIFFE bundle map.
+
+## Implementation
+- https://github.com/grpc/grpc/pull/39148
+- https://github.com/grpc/grpc/pull/39445
+- https://github.com/grpc/grpc/pull/39708
+- https://github.com/grpc/grpc/pull/40321