Add support for dynamically loaded DAOS libraries (#4)

Currently if DAOS libraries are not installed on a node, the
libtensorflow_io_plugins.so will fail to load due to unsatisfied
externals, and all modular filesystems are then unusable, not
just DFS.  This PR changes the DFS plugin to dynamically load
the DAOS libraries so that the DFS filesystem is available if
DAOS is installed, but the other modular filesystems are still
available if DAOS is not installed.

The checks for the DAOS libraries and the daos_init() call are
now done at filesystem registration time, not as part of each
function call in the filesystem API.  If the libraries are not
installed then the DFS filesystem will not be not registered,
and no calls into DFS functions will ever occur.  In this case
tensorflow will just report
    "File system scheme 'dfs' not implemented"
when a "dfs://" path is used.

A number of separate functions existed each of which was only
called once as part of DFS destruction, these were combined into
the DFS destructor for simplicity.  Similar recombinations were
done to simplify DFS construction.

Signed-off-by: Kevan Rehm <kevan.rehm@hpe.com>

Author: krehm

WORKSPACE

@@ -293,11 +293,11 @@ http_archive(
 http_archive(
     name = "daos",
     build_file = "//third_party:daos.BUILD",
-    sha256 = "6fb24cf72a1d07d9f9efcd738bba210e927b25526a5f01763ad39d0aabee5f76",
-    strip_prefix = "daos-1.3.106-tb",
+    sha256 = "9789a5a0065cfa4249105f1676b9eba89f68b54bc03083140549b7a8a8f615d3",
+    strip_prefix = "daos-2.0.2",
     urls = [
-        "https://storage.googleapis.com/mirror.tensorflow.org/github.com/daos-stack/daos/archive/refs/tags/v1.3.106-tb.tar.gz",
-        "https://github.com/daos-stack/daos/archive/refs/tags/v1.3.106-tb.tar.gz",
+        "https://storage.googleapis.com/mirror.tensorflow.org/github.com/daos-stack/daos/archive/refs/tags/v2.0.2.tar.gz",
+        "https://github.com/daos-stack/daos/archive/refs/tags/v2.0.2.tar.gz",
     ],
 )
 

tensorflow_io/core/filesystems/dfs/BUILD

@@ -15,15 +15,11 @@ cc_library(
         "dfs_utils.h",
     ],
     copts = tf_io_copts(),
-    linkopts = [
-        "-ldaos",
-        "-luuid",
-        "-ldfs",
-        "-lduns",
-    ],
     linkstatic = True,
     deps = [
         "//tensorflow_io/core/filesystems:filesystem_plugins_header",
+        "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/synchronization",
         "@daos",
     ],
     alwayslink = 1,