[cfe-dev] Tool for easy test seam injection in legacy C code

[Ivan Koster]

Hello all,

I want to create a tool that facilitates easy test seam injection in legacy
C code.
The reason for this is many colleagues (including myself) finding it very
tedious and maintenance intensive to create stubs / mocks for our legacy C
code base. This causes us to write less unittests than we want to. We
currently use a routing mechanism with #defines in a .h file that reroutes
functions to stubs/mocks and is included when we compile for unittesting.

I have been researching several ways of creating test seams, and have
settled on a method which leverages function pointers. A welcome help was
the article at
http://meekrosoft.wordpress.com/2012/07/20/test-seams-in-c-function-pointers-vs-preprocessor-hash-defines-vs-link-time-substitution/
But to make it truly easy, I need the compiler to do it for me. This is
were clang/llvm comes in.

What I want is the following. Given the following 2 files:

// interface.h
__attribute__((easy_test_seam))
int foo(void);

// implementation.c
int foo(void)
{
    return 2;
}


I actually want it to be compiled as:

// interface.h
extern int (*foo)(void);

// implementation.c
int foo_orig(void)
{
    return 2;
}
int (*foo)(void) = foo_orig;


In a later stage we could also add standard stub generation, maybe an
attribute like:

__attribute__((easy_test_seam(5)))
int foo(void);

Which would also create a standard stub that returns 5;

I see this method as a huge relief. It's maintenance friendly, we only have
to add the attribute to the legacy interfaces. We will only act on the
attribute if we compile for unittesting. Since we then won't have changed
production code it also won't mess up static analysis of production code
and won't fool IDE's jump to definition functionality.

The last few days I have been researching how to do this.

I have seen many suggest source-to-source transformation using the clang
Rewriter class. This is not my preferred choice, I don't have the need to
edit the *.c and *.h files, I just want them compiled differently. I would
also have to extend our build system to clone my *.c and *.h files, run
them through the source-to-source transform tool, and then compile the
resulting .c and .h files. This is not insurmountable, but not trivial
either.

I've seen some suggestions against changing the clang AST, because it's
supposed to be immutable and is hard.

I've also seen some suggestions to do LLVM IR transform passes, but I
haven't dived deep enough in LLVM IR to determine if this can work or not.

I'm very new to clang and LLVM, can you guys help me determine which of the
three ways would be the way to go? Or maybe there is another way?

Thanks in advance!

Regards,

Ivan
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