WinSTL Registry library mods and fixes, part 3: exception-safety

In reviewing the implementation of the WinSTL Registry Library's winstl::basic_reg_value class - as described in part 1 and part 2 of this series of posts - I've also spotted a defect in exception-safety.

Consider the (chopped-down) definition of the basic_reg_value class:

template < . . . >
class basic_reg_value
{
. . .
private:
hkey_type    m_hkey; // The parent key of the value
string_type  m_name; // The name of the value
. . . // other members
};

The m_hkey member is obtained via winstl::reg_traits<>::reg_dup_key(). It is the basic_reg_value class itself that provides the RAII. Consequently, if any exception occur during its constructor, the release of m_hkey will not be carried out.

Since the m_name is a string class instance, its constructor can throw. Consequently, basic_reg_value is not exception safe.

Thankfully, the fix is very simple. Simply reverse the order of declaration of the two members. If m_name's constructor throws, that'll happen before the key duplication takes place. If the key duplication throws an exception, the (fully constructed) m_name's destructor will be invoked. Q.E.D.

WinSTL Registry library mods and fixes, part 2: race conditions

As discussed at great length in section 33.3 of Extended STL, volume 1, the Windows Registry API is one that is prone to race conditions, due to the fact that separate processes may make independent changes to the registry contents without any control over each other.

The recently discovered defect in the WinSTL Registry Library's winstl::basic_reg_value class, gave me cause to consider the implementation in detail again. It's been a long time since I've done that, and with the understanding of the registry race-conditions I gained while researching and writing Extended STL, I saw immediately the possibility of such a race accounting for the reported fault.

Consider again the implementation of winstl::basic_reg_value<>::value_sz() method. Assume that prior to the invocation of winstl::reg_traits<>::reg_query_info that the registry-value's value was non-zero size. The call commences. Meanwhile, another process overwrites the registry-value, with a zero size. reg_query_info returns, and indicates that the data size is zero. Without a further check on the data size, the same fault will be experienced. Naturally, the fix for the non-race defect will fix the race one as well. Which is nice.

WinSTL Registry library mods and fixes, part 1: empty values

An STLSoft user recently posted a possible defect in the implementation of the WinSTL Registry Library's winstl::basic_reg_value class, reporting that a registry value (of type REG_SZ) yields a data value of size 0, leading to a crash.

Upon first examination, I thought this was a result of the fragility of the Windows Registry with respect to race conditions, as I'll discuss in a follow-up post.

However, closer examination reveals it to be a true defect. The precise circumstances in which this occur are as follows:

• the registry-value whose value is being elicited - as a string (REG_SZ) or as an array of strings (REG_MULTI_SZ) has zero size, and
• it has one or more peer registry-values whose values are of non-zero size

This precise set of circumstances causes the defect to fault. The reason lies in a call to winstl::reg_traits<>::reg_query_info at the start of the winstl::basic_reg_value<>::value_sz() method. This is used to determine the maximum size of the value of any if the current key's registry-values. This is useful to be able to provide a buffer of the appropriate size to the subsequent call to winstl::reg_traits<>::reg_query_value(), which actually retrieves the value in question.

The problem occurs when the value's size is 0. The last block in the method decrements this - to account for the space for the nul-terminator added earlier - and then explicitly sets the nul-terminator. (I actually forget why it does this, but I do recall that it must be done this way.)

Anyway, when the value's size is 0, decrementing it gives a very large number, and so the next statement results in an access-violation. Yuck!

STLSoft 1.9.83 will contain the fix for this, which is simply to test again that the data size is non-0.

More 1.9.82 ...

It also includes a new method in winstl::reg_traits::reg_delete_tree(), which takes a key handle and a sub-key name, and deletes the sub-key and any/all its descendent keys, as in:

HKEY k = . . .

LONG res = winstl::reg_traits<char>::reg_delete_tree(k, "sub-key");

Use with care, because there's no un-delete!