prov/efa: failure to enumerate unconditionally when FI_PROV_ATTR_ONLY is present.

[aws-nslick]

Describe the bug

Docs for fi_getinfo state the following, emphasis mine:

FLAGS

FI_PROV_ATTR_ONLY

Indicates that the caller is only querying for what providers are potentially available. All providers will return exactly one fi_info struct, regardless of whether that provider is usable on the current platform or not. The returned fi_info struct will contain default values for all members, with the exception of fabric_attr. The fabric_attr member will have the prov_name and prov_version values filled in.

EFA fails to appear in the results unconditionally.

To Reproduce

Test program:

// g++ -std=c++2a -DFMT_HEADER_ONLY=1 -L/opt/amazon/efa/lib -isystem/opt/amazon/efa/include test.cc -o test -lfabric
#include <cstdint>
#include <fmt/core.h>
#include <fmt/format.h>
#include <rdma/fabric.h>
#include <stdexcept>
#include <string>
#include <unordered_map>

int main() {
  static constexpr auto get = [](std::uint32_t major_version = FI_MAJOR_VERSION,
                                 std::uint32_t minor_version =
                                     FI_MINOR_VERSION) {
    fi_info *provider_list{nullptr};
    int ret = fi_getinfo(FI_VERSION(major_version, minor_version), nullptr,
                         nullptr, FI_PROV_ATTR_ONLY, nullptr, &provider_list);

    if (ret < 0 || !provider_list) {
      throw std::runtime_error("fi_getinfo(FI_PROV_ATTR_ONLY) failed");
    }
    struct version_pair {
      std::uint32_t major_version{};
      std::uint32_t minor_version{};
    };
    std::unordered_map<std::string, version_pair> providers;
    for (auto *current = provider_list; current != nullptr;
         current = current->next)
      providers.emplace(
          current->fabric_attr->prov_name,
          version_pair{FI_MAJOR(current->fabric_attr->prov_version) / 100,
                       FI_MAJOR(current->fabric_attr->prov_version) % 100});

    fi_freeinfo(provider_list);
    return providers;
  };

  for (auto &&[name, version] : get()) {
    auto &[maj, min] = version;
    fmt::print(stdout, "prov/{}: {}.{}\n", name, maj, min);
  }

  return 0;
};

Example output on a host where no EFA devices are present:

$ ./asdftest
prov/ofi_mrail: 1.22
prov/sm2: 1.22
prov/udp: 1.22
prov/ofi_rxm: 1.22
prov/ofi_hook_hmem: 1.22
prov/shm: 1.22
prov/tcp: 1.22
prov/ofi_hook_trace: 1.22
prov/ofi_hook_perf: 1.22
prov/ofi_hook_dmabuf_peer_mem: 1.22
prov/psm3: 3.7
prov/sockets: 1.22
prov/ofi_hook_debug: 1.22
prov/ofi_hook_noop: 1.22
prov/off_coll: 1.22
$ ( ./asdftest | grep efa ) || echo "EFA not in output!"
EFA not in output!

Example output on a host where EFA is available:

$ ssh host-with-efa-available '(./asdftest | grep "prov/efa") || echo "EFA not in output!"'
prov/efa: 1.22