fix: Keep existing trap when installing trap __ebctrap DEBUG

Before this change, any DEBUG trap installed before bash completion was
overwritten. This change adds to the existing trap, so if the trap
is:
 trap -- '__somefunc' DEBUG
the complete trap ends up being:
 trap -- '__somefunc; __ebctrap' DEBUG

fixes #78

Author: szermatt

bash-completion.el

@@ -1557,7 +1557,13 @@ Return the status code of the command, as a number."
                "  PROMPT_COMMAND=\"${__ebcp[1]}\";"
                "  unset __ebcp __ebcret;"
                " fi;"
-               "} ; trap __ebctrap DEBUG ; function __ebcpre {"
+               "} ; "
+               "if [[ \"$(trap -p DEBUG)\" =~ trap\\ --\\ \\'(.*)\\'\\ DEBUG ]]; then "
+               " trap \"${BASH_REMATCH[1]}; __ebctrap\" DEBUG; "
+               "else "
+               " trap __ebctrap DEBUG; "
+               "fi; "
+               "function __ebcpre {"
                "  __ebcor=${__ebcor:-$?}; "
                "  set +x; set +o emacs; set +o vi;"
                "  echo \"==emacs==bash=${BASH_VERSION}==.\";"

test/bash-completion-integration-test.el

@@ -960,4 +960,30 @@ $ ")))))
                           "101\n"
                           "$ ")))))
 
+(ert-deftest bash-completion-keep-existing-trap ()
+  (bash-completion_test-with-shell-harness
+   (concat ; .bashrc
+    "calls=0\n"
+    "function _calltrap {\n"
+    " calls=$((calls+1))\n"
+    "}\n"
+    "trap _calltrap DEBUG\n"
+    "PS1='\$ '")
+   nil
+   (bash-completion_test-send "n=$calls")
+   (bash-completion_test-send "tru" 'complete)
+   (bash-completion_test-send "fals" 'complete)
+   (bash-completion_test-send "[[ $calls -gt $n ]] && echo ok")
+   (bash-completion_test-send "trap -p DEBUG")
+   (should (equal (bash-completion_test-buffer-string)
+                  (concat
+                   "$ n=$calls\n"
+                   "$ true\n"
+                   "$ false\n"
+                   "$ [[ $calls -gt $n ]] && echo ok\n"
+                   "ok\n"
+                   "$ trap -p DEBUG\n"
+                   "trap -- '_calltrap; __ebctrap' DEBUG\n"
+                   "$ ")))))
+
 ;;; bash-completion-integration-test.el ends here