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

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
./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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