[Cherry-pick release/2.15] [xccl] WA: resolve libccl.so.1 next to the already loaded libccl.so.2

src/xccl/xccl.cpp

@@ -12,6 +12,7 @@
 
 #ifndef _WIN32
 #include <dlfcn.h>
+#include <link.h>
 #endif
 
 namespace {
@@ -23,10 +24,13 @@ namespace {
 // does not inherit the search path of whoever pulled it in, so in the flat
 // PyPI/venv layout libccl.so.1 is looked up only in /etc/ld.so.cache, /lib and
 // /usr/lib; it is not found and oneCCL fails with "Could not load any plugin",
-// taking down every process group creation. Loading it here resolves it
-// through this library's RPATH -- the same one that already locates
-// libccl.so.2 -- and leaves it in the global scope for the plugin to bind
-// against.
+// taking down every process group creation. Pulling it into the global scope
+// here lets the plugin bind against the already-loaded object.
+//
+// Our own RPATH is no help: libtorch_xpu.so is only an indirect dependency, so
+// glibc resolves dlopen() against the RPATH of whatever was dlopen'd directly
+// (libtorch_global_deps.so), never ours. Derive the location from the already
+// mapped libccl.so.2 instead -- oneCCL always ships the two side by side.
 //
 // A failure here is expected on a full oneAPI/DLE layout, where the plugin
 // resolves its own dependencies, so it is deliberately not reported.
@@ -35,7 +39,20 @@ namespace {
 // is the minimum supported version.
 [[maybe_unused]] const bool preloaded_oneccl_legacy_dep = []() {
 #ifndef _WIN32
-  (void)dlopen("libccl.so.1", RTLD_NOW | RTLD_GLOBAL);
+  // libccl.so.2 is DT_NEEDED, so it is already mapped by the time this runs.
+  if (void* v2 = dlopen("libccl.so.2", RTLD_LAZY | RTLD_NOLOAD)) {
+    link_map* lm = nullptr;
+    if (dlinfo(v2, RTLD_DI_LINKMAP, static_cast<void*>(&lm)) == 0 &&
+        lm != nullptr && lm->l_name != nullptr && lm->l_name[0] != '\0') {
+      std::string path(lm->l_name);
+      const size_t slash = path.rfind('/');
+      if (slash != std::string::npos) {
+        path.replace(slash + 1, std::string::npos, "libccl.so.1");
+        (void)dlopen(path.c_str(), RTLD_NOW | RTLD_GLOBAL);
+      }
+    }
+    dlclose(v2);
+  }
 #endif
   return true;
 }();