It was a false positive, but it can't hurt to "fix" it.
Original warning:
CID 13685 (#1 of 1): Buffer not null terminated (BUFFER_SIZE)
6. buffer_size: Calling strncpy with a source string whose length (4 chars)
is greater than or equal to the size argument (4) will fail to
null-terminate b0p->b0_version.
Coverity detected a memory leak caused by not free'ing the value returned by
get_expr_line_src (basically vim_strsave(expr_line)). Replaced the copying
with direct manipulation of expr_line, since that also happens in other
parts of the codebase.
NOTE: I'm aware that this has different behaviour than vim_strnsave, namely
vim_strnsave always allocates `len` bytes, even if the string is shorter. I
don't see how that behaviour is helpful here though.
Also constified the arguments. The double casts for the `xstrdup` are ugly
but `vim_strsave` doesn't take `const` arguments for now so I couldn't keep
that.
- The data member of String's can now be passed directly to functions
expecting C strings, as we now guarantee that they are NUL-terminated.
This obviates the need to use xstrndup and free, simplifying code and
enhancing performance.
- Use xmemdupz instead of xstrndup for converting String's into C strings.
It's faster because it doesn't calculate strlen(string.data) (which is
unnecesary as that information is already provided in string.size anyway).
- Use cstr_to_string to convert from C strings to String, it is both shorter
and faster than the usual strlen/xstrndup combo, which calls strlen twice.
cstr_to_string internally calls strlen and then xmemdupz.
I believe we can now mostly assume that all encountered String's data
members are safe to pass into functions that accept C strings. That should
simplify interop with C string code.
