psql: when tab-completing, use quotes on file names that need them

psql backslash commands that deal with file or directory names require
quotes around those that have spaces, single quotes, or backslashes.
However, tab-completing such names does not provide said quotes, and is
thus almost useless with them.

This patch fixes the problem by having a wrapper function around
rl_filename_completion_function that dequotes on input and quotes on
output.  This eases dealing with such names.

Author: Noah Misch

src/bin/psql/stringutils.c

@@ -272,3 +272,72 @@ strip_quotes(char *source, char quote, char escape, int encoding)
 
 	*dst = '\0';
 }
+
+
+/*
+ * quote_if_needed
+ *
+ * Opposite of strip_quotes().  If "source" denotes itself literally without
+ * quoting or escaping, returns NULL.  Otherwise, returns a malloc'd copy with
+ * quoting and escaping applied:
+ *
+ * source -			string to parse
+ * entails_quote -	any of these present?  need outer quotes
+ * quote -			doubled within string, affixed to both ends
+ * escape -			doubled within string
+ * encoding -		the active character-set encoding
+ *
+ * Do not use this as a substitute for PQescapeStringConn().  Use it for
+ * strings to be parsed by strtokx() or psql_scan_slash_option().
+ */
+char *
+quote_if_needed(const char *source, const char *entails_quote,
+				char quote, char escape, int encoding)
+{
+	const char *src;
+	char	   *ret;
+	char	   *dst;
+	bool		need_quotes = false;
+
+	psql_assert(source);
+	psql_assert(quote);
+
+	src = source;
+	dst = ret = pg_malloc(2 * strlen(src) + 3);	/* excess */
+
+	*dst++ = quote;
+
+	while (*src)
+	{
+		char		c = *src;
+		int			i;
+
+		if (c == quote)
+		{
+			need_quotes = true;
+			*dst++ = quote;
+		}
+		else if (c == escape)
+		{
+			need_quotes = true;
+			*dst++ = escape;
+		}
+		else if (strchr(entails_quote, c))
+			need_quotes = true;
+
+		i = PQmblen(src, encoding);
+		while (i--)
+			*dst++ = *src++;
+	}
+
+	*dst++ = quote;
+	*dst = '\0';
+
+	if (!need_quotes)
+	{
+		free(ret);
+		ret = NULL;
+	}
+
+	return ret;
+}