Re: [PATCH v6] C++ify gdb/common/environ.c

[Simon Marchi <simon dot marchi at polymtl dot ca>]

On 2017-06-19 16:26, Pedro Alves wrote:
> Right, m_environ_vector.clear() is not necessary.
>
> Note that this move assignment (and likewise the move ctor) leaves the
> source vector empty, which violates the "there's always a NULL entry
> at the end" invariant.  That's OK if the only thing we want to support
> of moved-from gdb_environ objects is destroying them, but please do
> document that.
>
> Otherwise, people assuming the standard library's rule, may be
> confused/surprised, into thinking that this, e.g., should work:
>
> gdb_environ env1;
> env1.set ("VAR1", "value1");
> gdb_environ env2;
> env2 = std::move (env1);    // env1 has no NULL terminator after this.
> env1.set ("VAR1", "value2); // whoops.
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 17.6.5.15 Moved-from state of library types
>              [lib.types.movedfrom]
>
>     Objects of types defined in the C++ standard library may be moved
> from (12.8).
>     Move operations may be explicitly specified or implicitly 
> generated. Unless
>     otherwise specified, such moved-from objects shall be placed in a 
> valid
>     but unspecified state.
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That's a good point.  We should definitely not let the environ object 
get in an invalid state.

Whatever the rule we choose for the terminating NULL, there exists some 
valid vector states which result in invalid environ states.  For 
example, an environ whose vector contains { NULL, NULL } is not valid.  
Trying to set an env var in it would give { NULL, "FOO=BAR", NULL }, and 
that results in an unexpected environment array in the end.

Does that mean that after the vector move, we should make sure to leave 
the moved-from vector in a known state (i.e. clear it, and possible add 
a NULL), to make sure that we leave our environ object in a valid state?