fail dedup command with explanation when LFS extensions configured

commands/command_dedup.go

@@ -33,6 +33,10 @@ func dedupTestCommand(*cobra.Command, []string) {
 		Exit("This system does not support deduplication. %s", err)
 	}
 
+	if len(cfg.Extensions()) > 0 {
+		Exit("This platform supports file de-duplication, however, Git LFS extensions are configured and therefore de-duplication can not be used.")
+	}
+
 	Print("OK: This platform and repository support file de-duplication.")
 }
 
@@ -49,6 +53,10 @@ func dedupCommand(cmd *cobra.Command, args []string) {
 		Exit("This system does not support deduplication.")
 	}
 
+	if len(cfg.Extensions()) > 0 {
+		Exit("This platform supports file de-duplication, however, Git LFS extensions are configured and therefore de-duplication can not be used.")
+	}
+
 	if dirty, err := git.IsWorkingCopyDirty(); err != nil {
 		ExitWithError(err)
 	} else if dirty {

docs/man/git-lfs-dedup.1.ronn

@@ -12,6 +12,11 @@ using the operating system's copy-on-write file creation functionality.
 
 If the operating system or file system don't support copy-on-write file creation, this command exits unsuccessfully.
 
+This command will also exit without success if any Git LFS extensions are
+configured, as these will typically be used to alter the file contents
+before they are written to the Git LFS storage directory, and therefore the
+working tree files should not be copy-on-write clones of the LFS object files.
+
 ## SEE ALSO
 
 Part of the git-lfs(1) suite.

t/t-dedup.sh

@@ -10,6 +10,21 @@ begin_test "dedup"
   git init $reponame
   cd $reponame
 
+  # Confirm Git LFS extensions prevent de-duplication
+  git config lfs.extension.foo.clean "foo-clean %f"
+  git config lfs.extension.foo.smudge "foo-smudge %f"
+  git config lfs.extension.foo.priority 0
+
+  result=$(git lfs dedup 2>&1) && true
+  if ( echo $result | grep "This system does not support deduplication." ); then
+    exit
+  fi
+  echo "$result" | grep 'This platform supports file de-duplication, however, Git LFS extensions are configured and therefore de-duplication can not be used.'
+
+  git config --unset lfs.extension.foo.clean
+  git config --unset lfs.extension.foo.smudge
+  git config --unset lfs.extension.foo.priority
+
   # Create a commit with some files tracked by git-lfs
   git lfs track *.dat
   echo "test data" > a.dat
@@ -25,9 +40,6 @@ begin_test "dedup"
 
   # DO
   result=$(git lfs dedup 2>&1) && true
-  if ( echo $result | grep "This system does not support deduplication." ); then
-    exit
-  fi
 
   # VERIFY: Expected
   #  Success: a.dat
@@ -46,11 +58,23 @@ begin_test "dedup test"
   git init $reponame
   cd $reponame
 
-  # DO
+  # Confirm Git LFS extensions prevent de-duplication
+  git config lfs.extension.foo.clean "foo-clean %f"
+  git config lfs.extension.foo.smudge "foo-smudge %f"
+  git config lfs.extension.foo.priority 0
+
   result=$(git lfs dedup --test 2>&1) && true
   if ( echo $result | grep "This system does not support deduplication." ); then
     exit
   fi
+  echo "$result" | grep 'This platform supports file de-duplication, however, Git LFS extensions are configured and therefore de-duplication can not be used.'
+
+  git config --unset lfs.extension.foo.clean
+  git config --unset lfs.extension.foo.smudge
+  git config --unset lfs.extension.foo.priority
+
+  # DO
+  result=$(git lfs dedup --test 2>&1) && true
 
   # Verify: This platform and repository support file de-duplication.
   echo "$result" | grep 'This platform and repository support file de-duplication.'