wslua: fix wslua file test failure

Lua 5.2.4 built with -DLUA_USE_APICHECK detected a stack issue:

    tshark: lapi.c:175: lua_settop: Assertion `(-(idx+1) <= (L->top - (func + 1))) && "invalid new top"' failed.

Function File_read always assumes that File_read_line pushes a value on
the stack (which clearly did not happen). On read failure, it would then
pop the stack (tripping the assertion) to push nil.

The other user (File_lines) is also affected by this change, but the Lua
5.2.4 documentation says that it should also return nil on EOF, fitting
this implementation.

Change-Id: I9cc8a5319523b2b56f4ae4735bbdbc1196387386
Reviewed-on: https://code.wireshark.org/review/18016
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>

1 files changed, 10 insertions, 0 deletions

diff --git a/epan/wslua/wslua_file.c b/epan/wslua/wslua_file.c
index 51fd712b0e..cc11d07a1c 100644
--- a/epan/wslua/wslua_file.c
+++ b/epan/wslua/wslua_file.c
@@ -171,6 +171,10 @@ static int File_read_number (lua_State *L, FILE_T ft) {
     }
 }
 
+/**
+ * Attempts to read one line from the file. The actual data read is pushed on
+ * the stack (or nil on EOF).
+ */
 static int File_read_line(lua_State *L, FILE_T ft) {
     static gchar linebuff[MAX_LINE_LENGTH];
     gint64 pos_before = file_tell(ft);
@@ -179,6 +183,8 @@ static int File_read_line(lua_State *L, FILE_T ft) {
     if (file_gets(linebuff, MAX_LINE_LENGTH, ft) == NULL) {
         /* No characters found, or error */
         /* *err = file_error(ft, err_info); */
+        /* io.lines() and file:read() requires nil on EOF */
+        lua_pushnil(L);
         return 0;
     }
 
@@ -212,6 +218,10 @@ static int File_read_line(lua_State *L, FILE_T ft) {
 #define lua_rawlen lua_objlen
 #endif
 
+/**
+ * Reads some data and returns the number of bytes read.
+ * The actual data (possibly an empty string) is pushed on the Lua stack.
+ */
 static int File_read_chars(lua_State *L, FILE_T ft, size_t n) {
     size_t rlen;  /* how much to read */
     size_t nr;  /* number of chars actually read */