fix: drop static buffer usage for cursor in req. (#88)

The cursor received from ES/SQL is being sent back to it when fetching
the next result set "page". When building the request, it's value was
converted from SQLWCHAR to mb-string in a static buffer, to find out
first the space required for it in the request.

Since the cursor value is a Base64 encoding string, the conversion can
simply be done by ASCII conversion, and thus the buffer is no longer
needed.

This fixes the case where the cursor value received from ES/SQL goes
over the buffer size (4KB), thus generating a failed query.

driver/queries.c

@@ -1831,9 +1831,8 @@ static SQLRETURN serialize_params(esodbc_stmt_st *stmt, char *dest,
 SQLRETURN TEST_API serialize_statement(esodbc_stmt_st *stmt, cstr_st *buff)
 {
 	SQLRETURN ret = SQL_SUCCESS;
-	size_t bodylen, pos, u8len, len;
+	size_t bodylen, pos, len;
 	char *body;
-	char u8curs[ESODBC_BODY_BUF_START_SIZE];
 	esodbc_dbc_st *dbc = stmt->hdr.dbc;
 	esodbc_desc_st *apd = stmt->apd;
 
@@ -1849,18 +1848,11 @@ SQLRETURN TEST_API serialize_statement(esodbc_stmt_st *stmt, cstr_st *buff)
 	bodylen = 1; /* { */
 	/* evaluate how long the stringified REST object will be */
 	if (stmt->rset.ecurs.cnt) { /* eval CURSOR object length */
-		/* convert cursor to C [mb]string. */
-		/* TODO: ansi_w2c() fits better for Base64 encoded cursors. */
-		u8len = WCS2U8(stmt->rset.ecurs.str, (int)stmt->rset.ecurs.cnt, u8curs,
-				sizeof(u8curs));
-		if (u8len <= 0) {
-			ERRH(stmt, "failed to convert cursor `" LWPDL "` to UTF8: %d.",
-				LWSTR(&stmt->rset.ecurs), WCS2U8_ERRNO());
-			RET_HDIAGS(stmt, SQL_STATE_24000);
-		}
-
+		/* assumptions: (1) the cursor is a Base64 encoded string and thus
+		 * (2) no JSON escaping needed.
+		 * (both assertions checked on copy, below). */
 		bodylen += sizeof(JSON_KEY_CURSOR) - 1; /* "cursor":  */
-		bodylen += json_escape(u8curs, u8len, NULL, 0);
+		bodylen += stmt->rset.ecurs.cnt;
 		bodylen += 2; /* 2x `"` for cursor value */
 	} else { /* eval QUERY object length */
 		bodylen += sizeof(JSON_KEY_QUERY) - 1;
@@ -1913,7 +1905,20 @@ SQLRETURN TEST_API serialize_statement(esodbc_stmt_st *stmt, cstr_st *buff)
 		memcpy(body + pos, JSON_KEY_CURSOR, sizeof(JSON_KEY_CURSOR) - 1);
 		pos += sizeof(JSON_KEY_CURSOR) - 1;
 		body[pos ++] = '"';
-		pos += json_escape(u8curs, u8len, body + pos, bodylen - pos);
+		if (ascii_w2c(stmt->rset.ecurs.str, body + pos,
+					stmt->rset.ecurs.cnt) <= 0) {
+			if (buff->cnt < bodylen) { /* has it been alloc'd? */
+				free(body);
+			}
+			ERRH(stmt, "failed to convert cursor `" LWPDL "` to ASCII.",
+				LWSTR(&stmt->rset.ecurs));
+			RET_HDIAGS(stmt, SQL_STATE_24000);
+		} else {
+			/* no character needs JSON escaping */
+			assert(stmt->rset.ecurs.cnt == json_escape(body + pos,
+						stmt->rset.ecurs.cnt, NULL, 0));
+			pos += stmt->rset.ecurs.cnt;
+		}
 		body[pos ++] = '"';
 	} else { /* copy QUERY object */
 		memcpy(body + pos, JSON_KEY_QUERY, sizeof(JSON_KEY_QUERY) - 1);