enum.c: Make ast_get_txt() actually do something.

The ast_get_txt() API function (and by extension, the TXTCIDNAME
dialplan function) were broken in
65b8381550 such that we would never
actually make a DNS TXT query as described.

This patch restores the documented behavior.

ASTERISK-19460 #close
Reported by: George Joseph

Change-Id: I1b19aea711488cb1ecd63843cddce05010e39376

main/enum.c

@@ -931,37 +931,88 @@ int ast_get_enum(struct ast_channel *chan, const char *number, char *dst, int ds
 	return ret;
 }
 
+/*! \internal
+ * \brief Format a phone number as a domain in an ENUM-adjacent way.
+ *
+ * Creates a domain name suitable for query by:
+ *
+ * 1. Removing non-digits
+ * 2. Adding a '.' between adjacent digits
+ * 3. Reversing the string
+ * 4. Appending the specified suffix (or e164.arpa if none is specified)
+ */
+static char *format_numeric_domain(const char *number, const char *suffix)
+{
+	char *buffer, *dst;
+	size_t suffix_length;
+	size_t number_length = strlen(number);
+	const char *src = number + number_length - 1;
+
+	if (!suffix) {
+		suffix = "e164.arpa";
+	}
+
+	suffix_length = strlen(suffix);
+
+	dst = buffer = ast_malloc(
+		(number_length * 2) /* We need 2 bytes per input digit */
+		+ suffix_length     /* ... plus however long the suffix is */
+		+ 1                 /* ... plus room for the '.' separator */
+		+ 1                 /* ... and room for the \0 byte at the end */);
+
+	if (buffer) {
+		while (src >= number) {
+			if (isdigit(*src)) {
+				*dst++ = *src;
+				*dst++ = '.';
+			}
+			src--;
+		}
+
+		/* The length arguments below make sure that the \0 byte is copied into
+		   the final string */
+		if (*suffix == '.') {
+			memcpy(dst, &suffix[1], suffix_length);
+		} else {
+			memcpy(dst, suffix, suffix_length + 1);
+		}
+	}
+
+	return buffer;
+}
+
 int ast_get_txt(struct ast_channel *chan, const char *number, char *txt, int txtlen, char *suffix)
 {
 	struct txt_context context;
-	char tmp[259 + 512];
-	int pos = strlen(number) - 1;
-	int newpos = 0;
-	int ret = -1;
+	char *domain;
+	int ret;
+	int autoservice = 0;
 
 	ast_debug(4, "ast_get_txt: Number = '%s', suffix = '%s'\n", number, suffix);
 
-	if (pos > 128) {
-		pos = 128;
+	domain = format_numeric_domain(number, suffix);
+	if (!domain) {
+		return -1;
 	}
 
-	while (pos >= 0) {
-		if (isdigit(number[pos])) {
-			tmp[newpos++] = number[pos];
-			tmp[newpos++] = '.';
-		}
-		pos--;
+	if (chan) {
+		/* DNS might take a while, so service the channel while we're blocked */
+		autoservice = !ast_autoservice_start(chan);
 	}
 
-	ast_copy_string(&tmp[newpos], suffix, sizeof(tmp) - newpos);
-
-	if (ret < 0) {
-		ast_debug(2, "No such number found in ENUM: %s (%s)\n", tmp, strerror(errno));
-		ret = 0;
-	} else {
+	ret = ast_search_dns(&context, domain, C_IN, T_TXT, txt_callback);
+	if (ret > 0) {
 		ast_copy_string(txt, context.txt, txtlen);
+	} else {
+		ast_debug(2, "No such number found in ENUM: %s (%s)\n", domain, strerror(errno));
 	}
-	return ret;
+
+	if (autoservice) {
+		ast_autoservice_stop(chan);
+	}
+
+	ast_free(domain);
+	return 0;
 }
 
 /*! \brief Initialize the ENUM support subsystem */