[libvirt] [PATCH] Rewrite the way mockable functions are handled.

[Martin Kletzander]

On Wed, Jul 12, 2017 at 05:06:22PM +0100, Daniel P. Berrange wrote:
>Currently any mockable functions are marked with attributes
>noinline, noclone and weak. This prevents the compiler from
>optimizing away the impl of these functions.
>
>It has an unfortunate side effect with the libtool convenience
>libraries, if executables directly link to them. For example
>virlockd, virlogd both link to libvirt_util.la  When this is
>done, the linker does not consider weak symbols as being
>undefined, so it never copies them into the final executable.
>
>In this new approach, we stop annotating the headers entirely.
>Instead we create a weak function alias in the source.
>
>   int fooImpl(void) {
>      ..the real code..
>   }
>
>   int foo(void) __attribute__((noinline, noclone, weak, alias("fooImpl"))
>
>If any functions in the same file call "foo", this prevents the
>optimizer from inlining any part of fooImpl. When linking to the
>libtool convenience static library, we also get all the symbols
>present. Finally the test suite can just directly define a
>'foo' function in its source, removing the need to use LD_PRELOAD
>(though removal of LD_PRELOADS is left for a future patch).
>

What are you using for tag navigation?  With this macro-defined
functions I cannot easily jump to them (the main reason why I don't like
macros defining functions).  I would very much prefer if
ATTRIBUTE_MOCKABLE just took a parameter like this:

#define ATTRIBUTE_MOCKABLE(func) __attribute__((noinline, noclone, weak, alias(#func "Impl"))

(written by hand, don't take mu word for it working out of the box) and
the original function would be left untouched.


Also, this fails on OS X [1] and I don't really see why:

util/vircommand.c:988:1: error: only weak aliases are supported on darwin
VIR_MOCKABLE(void,
^
./internal.h:252:60: note: expanded from macro 'VIR_MOCKABLE'
      ret name(__VA_ARGS__) __attribute__((noinline, weak, __alias__(#name "Impl"))); \
                                                           ^
1 error generated.
make[3]: *** [util/libvirt_util_la-vircommand.lo] Error 1
make[3]: *** Waiting for unfinished jobs....

[1] https://travis-ci.org/nertpinx/libvirt/jobs/253136004
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