ExternalProject: Verify when a fetch occurs during update test.

The performance feature of only performing a git fetch when needed
during the ExternalProject update step is verified during the test.
A fetch is identified by removing the FETCH_HEAD file and checking for
its reincarnation.

Tests/ExternalProjectUpdate/ExternalProjectUpdateTest.cmake

@@ -1,7 +1,15 @@
 # Set the ExternalProject GIT_TAG to desired_tag, and make sure the
-# resulting checked out version is resulting_sha and a rebuild.
-# This check's the viability of the ExternalProject UPDATE_COMMAND.
-macro(check_a_tag desired_tag resulting_sha)
+# resulting checked out version is resulting_sha and rebuild.
+# This check's the correct behavior of the ExternalProject UPDATE_COMMAND.
+# Also verify that a fetch only occurs when fetch_expected is 1.
+macro(check_a_tag desired_tag resulting_sha fetch_expected)
+  message( STATUS "Checking ExternalProjectUpdate to tag: ${desired_tag}" )
+
+  # Remove the FETCH_HEAD file, so we can check if it gets replaced with a 'git
+  # fetch'.
+  set( FETCH_HEAD_file ${ExternalProjectUpdate_BINARY_DIR}/CMakeExternals/Source/TutorialStep1-GIT/.git/FETCH_HEAD )
+  file( REMOVE ${FETCH_HEAD_file} )
+
   # Configure
   execute_process(COMMAND ${CMAKE_COMMAND}
     -G ${CMAKE_TEST_GENERATOR}
@@ -43,18 +51,26 @@ when
 was expected."
     )
   endif()
+
+  if( NOT EXISTS ${FETCH_HEAD_file} AND ${fetch_expected})
+    message( FATAL_ERROR "Fetch did NOT occur when it was expected.")
+  endif()
+  if( EXISTS ${FETCH_HEAD_file} AND NOT ${fetch_expected})
+    message( FATAL_ERROR "Fetch DID occur when it was not expected.")
+  endif()
 endmacro()
 
 find_package(Git)
 if(GIT_EXECUTABLE)
-  check_a_tag(origin/master 5842b503ba4113976d9bb28d57b5aee1ad2736b7)
-  check_a_tag(tag1          d1970730310fe8bc07e73f15dc570071f9f9654a)
+  check_a_tag(origin/master 5842b503ba4113976d9bb28d57b5aee1ad2736b7 1)
+  check_a_tag(tag1          d1970730310fe8bc07e73f15dc570071f9f9654a 1)
   # With the Git UPDATE_COMMAND performance patch, this will not required a
   # 'git fetch'
-  check_a_tag(tag1          d1970730310fe8bc07e73f15dc570071f9f9654a)
-  check_a_tag(tag2          5842b503ba4113976d9bb28d57b5aee1ad2736b7)
-  check_a_tag(d19707303     d1970730310fe8bc07e73f15dc570071f9f9654a)
-  check_a_tag(origin/master 5842b503ba4113976d9bb28d57b5aee1ad2736b7)
+  check_a_tag(tag1          d1970730310fe8bc07e73f15dc570071f9f9654a 0)
+  check_a_tag(tag2          5842b503ba4113976d9bb28d57b5aee1ad2736b7 1)
+  check_a_tag(d19707303     d1970730310fe8bc07e73f15dc570071f9f9654a 1)
+  check_a_tag(d19707303     d1970730310fe8bc07e73f15dc570071f9f9654a 0)
+  check_a_tag(origin/master 5842b503ba4113976d9bb28d57b5aee1ad2736b7 1)
   # This is a remote symbolic ref, so it will always trigger a 'git fetch'
-  check_a_tag(origin/master 5842b503ba4113976d9bb28d57b5aee1ad2736b7)
+  check_a_tag(origin/master 5842b503ba4113976d9bb28d57b5aee1ad2736b7 1)
 endif()