rpc : fix pre-rdma macOS versions (#27815)

ggml/src/ggml-rpc/CMakeLists.txt

@@ -34,10 +34,14 @@ if (GGML_RPC_RDMA)
         find_library(RDMA_LIB ${RDMA_LIB_NAME} REQUIRED)
     endif()
     target_compile_definitions(ggml-rpc PRIVATE GGML_RPC_RDMA)
-    target_link_libraries(ggml-rpc PRIVATE ${RDMA_LIB})
     if (APPLE)
+        # librdma.dylib only exists on macOS 26.2 and later. Link it weakly so a build made
+        # where it exists still loads where it does not; checked at runtime before use.
+        target_link_options(ggml-rpc PRIVATE "LINKER:-weak_library,${RDMA_LIB}")
         target_compile_definitions(ggml-rpc PRIVATE GGML_RPC_RDMA_APPLE)
         target_sources(ggml-rpc PRIVATE transport-apple.cpp)
+    else()
+        target_link_libraries(ggml-rpc PRIVATE ${RDMA_LIB})
     endif()
     message(STATUS "  RDMA transport enabled (${RDMA_DESC})")
 else()

ggml/src/ggml-rpc/transport-apple.cpp

@@ -8,6 +8,7 @@
 #include <cstdlib>
 #include <cstring>
 #include <string>
+#include <dlfcn.h>
 #include <poll.h>
 #include <sys/socket.h>
 #include <unistd.h>
@@ -184,11 +185,28 @@ static uint8_t rdma_first_active_port(struct ibv_context * ctx, struct ibv_port_
     return 0;
 }
 
+// librdma.dylib is weak-linked, so its symbols are null when it is absent. Nothing may
+// call one before this has returned true.
+static bool rdma_library_present() {
+    static const bool present = [] {
+        void * handle = dlopen("/usr/lib/librdma.dylib", RTLD_LAZY);
+        if (handle == nullptr) {
+            return false;
+        }
+        dlclose(handle);
+        return true;
+    }();
+    return present;
+}
+
 // Called before the endpoints are exchanged: pick the local device facing this
 // peer, create a UC QP and register the frame rings. RDMA is point-to-point, so
 // the device is the one whose GID equals the bootstrap connection's local
 // address, i.e. the one cabled to the peer.
 std::unique_ptr<apple_rdma> apple_rdma::probe(int fd, const uint8_t * target_gid, uint8_t * caps) {
+    if (!rdma_library_present()) {
+        return nullptr;
+    }
     int ndev = 0;
     ibv_device ** devs = ibv_get_device_list(&ndev);
     if (!devs) return nullptr;