Add second import guard

Mostly so that we can print a log message informing the user we loaded
MPI/CUDA

src/context.jl

@@ -34,6 +34,11 @@ Behavior can be overridden by setting the `CLIMACOMMS_CONTEXT` environment varia
 to either `MPI` or `SINGLETON`.
 """
 function context(device = device(); target_context = context_type())
+    if target_context == :MPICommsContext && mpi_ext_is_not_loaded()
+        error(
+            "Loading MPI.jl is required to use MPICommsContext. You might want to call ClimaComms.@import_required_backends",
+        )
+    end
     ContextConstructor = getproperty(ClimaComms, target_context)
     return ContextConstructor(device)
 end

src/devices.jl

@@ -46,6 +46,21 @@ function device_functional end
 device_functional(::CPUSingleThreaded) = true
 device_functional(::CPUMultiThreaded) = true
 
+function device_type()
+    env_var = get(ENV, "CLIMACOMMS_DEVICE", "CPU")
+    if env_var == "CPU"
+        return Threads.nthreads() > 1 ? :CPUMultiThreaded : :CPUSingleThreaded
+    elseif env_var == "CPUSingleThreaded"
+        return :CPUSingleThreaded
+    elseif env_var == "CPUMultiThreaded"
+        return :CPUMultiThreaded
+    elseif env_var == "CUDA"
+        return :CUDADevice
+    else
+        error("Invalid CLIMACOMMS_DEVICE: $env_var")
+    end
+end
+
 """
     ClimaComms.device()
 
@@ -60,18 +75,14 @@ Allowed values:
 The default is `CPU`.
 """
 function device()
-    env_var = get(ENV, "CLIMACOMMS_DEVICE", "CPU")
-    if env_var == "CPU"
-        return Threads.nthreads() > 1 ? CPUMultiThreaded() : CPUSingleThreaded()
-    elseif env_var == "CPUSingleThreaded"
-        return CPUSingleThreaded()
-    elseif env_var == "CPUMultiThreaded"
-        return CPUMultiThreaded()
-    elseif env_var == "CUDA"
-        return CUDADevice()
-    else
-        error("Invalid CLIMACOMMS_DEVICE: $env_var")
+    target_device = device_type()
+    if target_device == :CUDADevice && cuda_ext_is_not_loaded()
+        error(
+            "Loading CUDA.jl is required to use CUDADevice. You might want to call ClimaComms.@import_required_backends",
+        )
     end
+    DeviceConstructor = getproperty(ClimaComms, target_device)
+    return DeviceConstructor()
 end
 
 """

src/loading.jl

@@ -1,11 +1,21 @@
+import ..ClimaComms
+
 export import_required_backends
 
-function mpi_required()
+function mpi_is_required()
     return context_type() == :MPICommsContext
 end
 
-function cuda_required()
-    return device() isa CUDADevice
+function mpi_ext_is_not_loaded()
+    return isnothing(Base.get_extension(ClimaComms, :ClimaCommsMPIExt))
+end
+
+function cuda_is_required()
+    return device_type() == :CUDADevice
+end
+
+function cuda_ext_is_not_loaded()
+    return isnothing(Base.get_extension(ClimaComms, :ClimaCommsCUDAExt))
 end
 
 """
@@ -16,23 +26,13 @@ If the desired device is CUDA (as determined by `ClimaComms.device()`), try load
 """
 macro import_required_backends()
     return quote
-        @static if $mpi_required()
-            try
-                import MPI
-            catch
-                error(
-                    "Cannot load MPI.jl. Make sure it is included in your environment stack.",
-                )
-            end
+        @static if $mpi_is_required() && $mpi_ext_is_not_loaded()
+            import MPI
+            @info "Loaded MPI.jl"
         end
-        @static if $cuda_required()
-            try
-                import CUDA
-            catch
-                error(
-                    "Cannot load CUDA.jl. Make sure it is included in your environment stack.",
-                )
-            end
+        @static if $cuda_is_required() && $cuda_ext_is_not_loaded()
+            import CUDA
+            @info "Loaded CUDA.jl"
         end
     end
 end