Replace fuzzer::FuzzerDriver's INTERFACE marking with new LLVMRunFuzzerDriver.

[IanPudney]

Summary

This adds a new extern "C" function that serves the same purpose. This removes the need for external users to depend on internal headers in order to use this feature. It also standardizes the interface in a way that other fuzzing engines will be able to match.

[hctim]

Revert of the original changes was already done in 540b92147da0e9a2c2d096505eb0785ec79f2024 :) - a rebase should fix it up.

I'll let @kcc take this one.

[kcc]

Code LGTM, thanks!
Please add a section in docs/LibFuzzer.html.
I'd add it after "Startup initialization", something like "Using libFuzzer as a library".

I assume you will also need to introduce a .so build target.
Will you do it in a separate change?

[IanPudney]

Removed changes that were already made in a previous change, and adds docs.

[kcc]

LGTM.
Matt, please help land it

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

#endif // LIBFUZZER_EMSCRIPTEN
   if (Flags.verbosity)
     Printf("Done %zd runs in %zd second(s)\n", F->getTotalNumberOfRuns(),
            F->secondsSinceProcessStartUp());
   F->PrintFinalStats();
 
   exit(0);  // Don't let F destroy itself.
 }
 
+extern "C" ATTRIBUTE_INTERFACE int
+LLVMFuzzerRunDriver(int *argc, char ***argv,
+                    int (*UserCb)(const uint8_t *Data, size_t Size)) {
+  return FuzzerDriver(argc, argv, UserCb);
+}
+
 // Storage for global ExternalFunctions object.
 ExternalFunctions *EF = nullptr;
 
 }  // namespace fuzzer

llvm/docs/LibFuzzer.rst

 
 .. code-block:: c++
 
    extern "C" int LLVMFuzzerInitialize(int *argc, char ***argv) {
     ReadAndMaybeModify(argc, argv);
     return 0;
    }
 
+Using libFuzzer as a library
+----------------------------
+If the code being fuzzed must provide its own `main`, it's possible to
+invoke libFuzzer as a library. Be sure to pass ``-fsanitize=fuzzer-no-link``
+during compilation, and link your binary against the no-main version of
+libFuzzer. On Linux installations, this is typically located at:
+
+.. code-block:: bash
+
+  /usr/lib/<llvm-version>/lib/clang/<clang-version>/lib/linux/libclang_rt.fuzzer_no_main-<architecture>.a
+
+If building libFuzzer from source, this is located at the following path
+in the build output directory:
+
+.. code-block:: bash
+
+  lib/linux/libclang_rt.fuzzer_no_main-<architecture>.a
+
+From here, the code can do whatever setup it requires, and when it's ready
+to start fuzzing, it can call `LLVMFuzzerRunDriver`, passing in the program
+arguments and a callback. This callback is invoked just like
+`LLVMFuzzerTestOneInput`, and has the same signature.
+
+.. code-block:: c++
+
+  extern "C" int LLVMFuzzerRunDriver(int *argc, char ***argv,
+                    int (*UserCb)(const uint8_t *Data, size_t Size));
+
+
 
 Leaks
 -----
 
 Binaries built with AddressSanitizer_ or LeakSanitizer_ will try to detect
 memory leaks at the process shutdown.
 For in-process fuzzing this is inconvenient
 since the fuzzer needs to report a leak with a reproducer as soon as the leaky