Add `posix_filesystem_static` target and source.

This implements the static registration of a filesystem, without needing
to load the plugin DSO. Simply link this target in the binary which
needs the filesystem to have the static filesystem imported.

tensorflow/c/experimental/filesystem/plugins/posix/BUILD

@@ -27,6 +27,19 @@ cc_library(
     ],
 )
 
+# Since building pip package and API tests require a filesystem, we provide a
+# static registration target that they should link against.
+cc_library(
+    name = "posix_filesystem_static",
+    srcs = ["posix_filesystem_static.cc"],
+    visibility = ["//visibility:public"],
+    deps = [
+        ":posix_filesystem_impl",
+        "//tensorflow/c/experimental/filesystem:filesystem_interface",
+        "//tensorflow/c/experimental/filesystem:modular_filesystem",
+    ],
+)
+
 # Library implementing helper functionality, so that the above only contains
 # the API implementation for modular filesystems.
 cc_library(

tensorflow/c/experimental/filesystem/plugins/posix/posix_filesystem_static.cc

@@ -0,0 +1,33 @@
+/* Copyright 2020 The TensorFlow Authors. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+==============================================================================*/
+
+#include "tensorflow/c/experimental/filesystem/filesystem_interface.h"
+#include "tensorflow/c/experimental/filesystem/modular_filesystem_registration.h"
+#include "tensorflow/c/experimental/filesystem/plugins/posix/posix_filesystem.h"
+
+namespace tensorflow {
+
+// Register the POSIX filesystems statically.
+// Return value will be unused
+Status StaticallyRegisterLocalFilesystems() {
+  TF_FilesystemPluginInfo info;
+  TF_InitPlugin(&info);
+  return filesystem_registration::RegisterFilesystemPluginImpl(&info);
+}
+
+// Perform the actual registration
+static Status unused = StaticallyRegisterLocalFilesystems();
+
+}  // namespace tensorflow