BUG: Using pointer that points to a destructed string's content

[Tamas Szakaly <sghctoma@gmail.com>]

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Dear notmuch developers,

The following line is from _notmuch_message_add_directory_terms in
lib/message.cc (line 652 in HEAD):

direntry = (*i).c_str ();

'i' is a Xapian::TermIterator, whose operator* returns a std::string by value.
This means that c_str() is called on a temporary, which is destructed after the
full expression (essentially the particular line in this case), so 'direntry'
will point to a destructed std::string's data.
(See https://gcc.gnu.org/onlinedocs/gcc/Temporaries.html)

One possible modification to correct this issue is using strdup:

direntry = strdup((*i).c_str ());

Note:
Despite the fact that it is wrong, it *generally* works, because delete[]-ing
the underlying character array in the destructor of std::string does not really
touch the memory content, and there is only a minor chance that this memory area
will be allocated again (e.g. from another thread). This caused me some headache
though with 'notmuch new' on FreeBSD 11-CURRENT, where jemalloc is configured so
that freed memory will be filled with 0x5a's.

Best regards,
sghctoma

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