fix(scp): work around incomplete triple backslashes for remote paths

Author: akinomyoga

completions/ssh

@@ -541,7 +541,27 @@ _comp_xfunc_scp_compgen_remote_files()
     local REPLY=$cur
     if [[ ! $_less_escaping ]]; then
         # unescape (3 backslashes to 1 for chars we escaped)
-        REPLY=$(command sed -e 's/\\\\\\\('"$_comp_cmd_scp__path_esc"'\)/\\\1/g' <<<"$REPLY")
+        #
+        # In the following while-loop, we essentially do the following:
+        #
+        #     REPLY=$(command sed -e 's/\\\\\\\('"$_comp_cmd_scp__path_esc"'\|$\)/\\\1/g' <<<"$REPLY")
+        #
+        # We implement this by the Bash built-in features because POSIX BRE
+        # does not support \|.  POSIX sed newly standardized the "-E" flag to
+        # use POSIX ERE in POSIX.1-2024, but older implementations of sed are
+        # still expected to remain in the market.  Also, we can avoid the fork
+        # cost by implementing this using built-in features.
+        #
+        # Note: We need to store \\\\\\ in a variable to work around "shopt -s
+        # compat31".
+        local _tail=$REPLY _regex_triple_backslashes='\\\\\\('$_comp_cmd_scp__path_esc'|$)(.*)$'
+        REPLY=
+        while [[ $_tail && $_tail =~ $_regex_triple_backslashes ]]; do
+            # shellcheck disable=SC1003
+            REPLY=${_tail::${#_tail}-${#BASH_REMATCH}}'\'${BASH_REMATCH[1]}
+            _tail=${BASH_REMATCH[2]}
+        done
+        REPLY+=$_tail
     fi
     _comp_dequote_incomplete "$REPLY"
     local cur_val=${REPLY-}

test/t/test_scp.py

@@ -133,6 +133,15 @@ def test_remote_path_with_backslash(self, bash):
             [r"abc\ def.txt", r"abc\\\ def.txt"]
         ) or completion == sorted([r"abc\\\ def.txt", r"abc\\\\\\\ def.txt"])
 
+    def test_remote_path_with_backslash_2(self, bash):
+        assert_bash_exec(
+            bash, r"ssh() { [[ $1 == abc ]]; printf '%s\n' 'abc OK'; }"
+        )
+        completion = assert_complete(bash, "scp remote_host:abc\\\\\\")
+        assert_bash_exec(bash, "unset -f ssh")
+
+        assert completion == "OK"
+
     def test_xfunc_remote_files(self, live_pwd, bash):
         def prefix_paths(prefix, paths):
             return [f"{prefix}{path}" for path in paths]