Create filter plugin

The new plugin, `libXrdOssFilter.so`, allows the administrator to
only permit the opening / listings of files and directories that
match specified globs or prefixes.  This can be used to keep
some storage from being visible via XRootD (even with the appropriate
permissions).

For example, to hide all files and directories beginning with "."
from being exported, one could setup:

```
ofs.osslib ++ libXrdOssFilter.so
filter.glob /**
```

(This policy would be quite difficult to implement via the existing
authorization frameworks)

Author: bbockelm

CMakeLists.txt

@@ -69,6 +69,9 @@ endif()
 #
 add_definitions( -D_FILE_OFFSET_BITS=64 )
 
+######################
+##  libXrdOssS3     ##
+######################
 # On Linux, we hide all the symbols for the final libraries, exposing only what's needed for the XRootD
 # runtime loader.  So here we create the object library and will create a separate one for testing with
 # the symbols exposed.
@@ -80,6 +83,9 @@ target_link_libraries(XrdS3Obj ${XRootD_UTILS_LIBRARIES} ${XRootD_SERVER_LIBRARI
 add_library(XrdS3 MODULE "$<TARGET_OBJECTS:XrdS3Obj>")
 target_link_libraries(XrdS3 XrdS3Obj)
 
+######################
+##  libXrdOssHTTP   ##
+######################
 add_library(XrdHTTPServerObj OBJECT src/CurlUtil.cc src/HTTPFile.cc src/HTTPFileSystem.cc src/HTTPCommands.cc src/TokenFile.cc src/stl_string_utils.cc src/shortfile.cc src/logging.cc)
 set_target_properties(XrdHTTPServerObj PROPERTIES POSITION_INDEPENDENT_CODE ON)
 target_include_directories(XrdHTTPServerObj PRIVATE ${XRootD_INCLUDE_DIRS})
@@ -88,12 +94,26 @@ target_link_libraries(XrdHTTPServerObj ${XRootD_UTILS_LIBRARIES} ${XRootD_SERVER
 add_library(XrdHTTPServer MODULE "$<TARGET_OBJECTS:XrdHTTPServerObj>")
 target_link_libraries(XrdHTTPServer XrdHTTPServerObj)
 
-if(NOT APPLE)
-  set_target_properties(XrdS3 PROPERTIES OUTPUT_NAME "XrdS3-${XRootD_PLUGIN_VERSION}" SUFFIX ".so" LINK_FLAGS "-Wl,--version-script=${CMAKE_SOURCE_DIR}/configs/export-lib-symbols")
-  set_target_properties(XrdHTTPServer PROPERTIES OUTPUT_NAME "XrdHTTPServer-${XRootD_PLUGIN_VERSION}" SUFFIX ".so" LINK_FLAGS "-Wl,--version-script=${CMAKE_SOURCE_DIR}/configs/export-lib-symbols")
+######################
+## libXrdOssFilter  ##
+######################
+add_library( XrdOssFilterObj OBJECT src/Filter.cc src/logging.cc )
+set_target_properties( XrdOssFilterObj PROPERTIES POSITION_INDEPENDENT_CODE ON )
+target_include_directories( XrdOssFilterObj PRIVATE ${XRootD_INCLUDE_DIRS} )
+target_link_libraries( XrdOssFilterObj ${XRootD_UTILS_LIBRARIES} ${XRootD_SERVER_LIBRARIES} )
+
+add_library( XrdOssFilter MODULE "$<TARGET_OBJECTS:XrdOssFilterObj>" )
+target_link_libraries( XrdOssFilter XrdOssFilterObj )
+
+# Customize module's suffix and, on Linux, hide unnecessary symbols
+if( APPLE )
+  set_target_properties( XrdS3 PROPERTIES OUTPUT_NAME "XrdS3-${XRootD_PLUGIN_VERSION}" SUFFIX ".so" )
+  set_target_properties( XrdHTTPServer PROPERTIES OUTPUT_NAME "XrdHTTPServer-${XRootD_PLUGIN_VERSION}" SUFFIX ".so" )
+  set_target_properties( XrdOssFilter PROPERTIES OUTPUT_NAME "XrdOssFilter-${XRootD_PLUGIN_VERSION}" SUFFIX ".so" )
 else()
-  set_target_properties(XrdS3 PROPERTIES OUTPUT_NAME "XrdS3-${XRootD_PLUGIN_VERSION}" SUFFIX ".so")
-  set_target_properties(XrdHTTPServer PROPERTIES OUTPUT_NAME "XrdHTTPServer-${XRootD_PLUGIN_VERSION}" SUFFIX ".so")
+  set_target_properties( XrdS3 PROPERTIES OUTPUT_NAME "XrdS3-${XRootD_PLUGIN_VERSION}" SUFFIX ".so" LINK_FLAGS "-Wl,--version-script=${CMAKE_SOURCE_DIR}/configs/export-lib-symbols" )
+  set_target_properties( XrdHTTPServer PROPERTIES OUTPUT_NAME "XrdHTTPServer-${XRootD_PLUGIN_VERSION}" SUFFIX ".so" LINK_FLAGS "-Wl,--version-script=${CMAKE_SOURCE_DIR}/configs/export-lib-symbols" )
+  set_target_properties( XrdOssFilter PROPERTIES OUTPUT_NAME "XrdOssFilter-${XRootD_PLUGIN_VERSION}" SUFFIX ".so" LINK_FLAGS "-Wl,--version-script=${CMAKE_SOURCE_DIR}/configs/export-lib-symbols" )
 endif()
 
 SET(LIB_INSTALL_DIR "${CMAKE_INSTALL_PREFIX}/lib" CACHE PATH "Install path for libraries")
@@ -104,13 +124,20 @@ install(
 )
 
 if( BUILD_TESTING )
+  # Create shared libraries for testing from the existing objects
   add_library(XrdS3Testing SHARED "$<TARGET_OBJECTS:XrdS3Obj>")
   target_link_libraries(XrdS3Testing XrdS3Obj)
   target_include_directories(XrdS3Testing INTERFACE ${XRootD_INCLUDE_DIRS})
+
   add_library(XrdHTTPServerTesting SHARED "$<TARGET_OBJECTS:XrdHTTPServerObj>")
   target_link_libraries(XrdHTTPServerTesting XrdHTTPServerObj)
   target_include_directories(XrdHTTPServerTesting INTERFACE ${XRootD_INCLUDE_DIRS})
 
+  add_library( XrdOssFilterTesting SHARED "$<TARGET_OBJECTS:XrdOssFilterObj>" )
+  target_link_libraries( XrdOssFilterTesting XrdOssFilterObj )
+  target_include_directories( XrdOssFilterTesting INTERFACE ${XRootD_INCLUDE_DIRS} )
+
+  # Ensure that GTest is available
   if( NOT XROOTD_PLUGINS_EXTERNAL_GTEST )
     include( FetchContent )
     set( GTEST_URL "${CMAKE_CURRENT_SOURCE_DIR}/googletest-1.15.2.tar.gz" )
@@ -127,6 +154,7 @@ if( BUILD_TESTING )
   else()
     find_package(GTest REQUIRED)
   endif()
+
   enable_testing()
   add_subdirectory(test)
 endif()