Fix symlink following for gitattributes files

tests/test_init.bats

@@ -131,6 +131,51 @@ SETUP_SKIP_INIT_TRANSCRYPT=1
   [ ! "$(git config --get transcrypt.openssl-path)" = 'openssl' ]
 }
 
+@test "init: --uninstall works when core.attributesFile is a symlink" {
+  # transcrypt must edit the attributes file through the symlink. Editing with
+  # `sed -i` errors on BSD/macOS for symlinks ("in-place editing only works for
+  # regular files") and silently replaces the symlink with a regular file on
+  # GNU/Linux.
+  echo '"sensitive_file" filter=crypt diff=crypt merge=crypt' > attributes_target
+  ln -sf attributes_target attributes_link
+  git config core.attributesFile "$BATS_TEST_DIRNAME/attributes_link"
+
+  init_transcrypt
+  [ -L attributes_link ]
+
+  run ../transcrypt --uninstall --yes
+  [ "$status" -eq 0 ]
+
+  # The symlink must be preserved, not replaced by a regular file...
+  [ -L attributes_link ]
+  # ...and the crypt pattern removed from its target
+  run cat attributes_link
+  [[ "${output}" != *"filter=crypt"* ]]
+
+  rm -f attributes_link attributes_target
+}
+
+@test "init: --upgrade works when core.attributesFile is a symlink" {
+  # The upgrade path rewrites "diff=crypt" lines to add "merge=crypt", and must
+  # likewise edit through the symlink rather than over it.
+  echo '"sensitive_file" filter=crypt diff=crypt' > attributes_target
+  ln -sf attributes_target attributes_link
+  git config core.attributesFile "$BATS_TEST_DIRNAME/attributes_link"
+
+  init_transcrypt
+
+  run ../transcrypt --upgrade --yes
+  [ "$status" -eq 0 ]
+
+  # The symlink must be preserved, not replaced by a regular file...
+  [ -L attributes_link ]
+  # ...and the pattern upgraded in place to include "merge=crypt"
+  run cat attributes_link
+  [[ "${output}" = *"diff=crypt merge=crypt"* ]]
+
+  rm -f attributes_link attributes_target
+}
+
 @test "init: transcrypt.crypt-dir config setting is applied during init" {
   # Clear tmp crypt/ directory, in case junk was left there from prior test runs
   rm -fR /tmp/crypt/

transcrypt

@@ -143,6 +143,19 @@ die() {
 	exit "$st"
 }
 
+# Apply a sed script ($1) in-place to a file ($2), following symlinks so a
+# symlinked file is edited in place rather than being replaced. We avoid
+# `sed -i` because BSD/macOS sed refuses to edit symlinks ("in-place editing
+# only works for regular files") while GNU sed would replace the symlink with
+# a regular file, breaking the link. Writing back through `cat >` follows the
+# symlink and preserves it.
+sed_inplace() {
+	local script="$1" file="$2" tmp
+	tmp=$(mktemp)
+	sed "$script" "$file" >"$tmp" && cat "$tmp" >"$file"
+	rm -f "$tmp"
+}
+
 # return context name if $1 has format 'context=name-of-context' else empty string
 extract_context_name_from_name_value_arg() {
 	local before_last_equals=${1%=*}
@@ -956,14 +969,7 @@ uninstall_transcrypt() {
 		fi
 
 		# remove any defined crypt patterns in gitattributes
-		case $OSTYPE in
-		darwin*)
-			/usr/bin/sed -i '' "/filter=crypt/d" "$GIT_ATTRIBUTES"
-			;;
-		linux*)
-			sed -i "/filter=crypt/d" "$GIT_ATTRIBUTES"
-			;;
-		esac
+		sed_inplace "/filter=crypt/d" "$GIT_ATTRIBUTES"
 
 		if [[ ! $upgrade ]]; then
 			printf 'The transcrypt configuration has been completely removed from the repository.\n'
@@ -1037,14 +1043,7 @@ upgrade_transcrypt() {
 	echo "$ORIG_GITATTRIBUTES" >"$GIT_ATTRIBUTES"
 
 	# Update .gitattributes for transcrypt'ed files to include "merge=crypt" config
-	case $OSTYPE in
-	darwin*)
-		/usr/bin/sed -i '' 's/diff=crypt$/diff=crypt merge=crypt/' "$GIT_ATTRIBUTES"
-		;;
-	linux*)
-		sed -i 's/diff=crypt$/diff=crypt merge=crypt/' "$GIT_ATTRIBUTES"
-		;;
-	esac
+	sed_inplace 's/diff=crypt$/diff=crypt merge=crypt/' "$GIT_ATTRIBUTES"
 
 	printf 'Upgrade is complete\n'