meson: support building fuzzers with libFuzzer

To support fuzzing via libFuzzer one has to pass a couple of compiler
options:

  - It is mandatory to enable the "fuzzer-no-link" sanitizer for
    coverage feedback.

  - It is recommended to enable at least one more sanitizer to catch
    issues, like the "address" sanitizer.

  - The fuzzing executables need to be linked with "-fsanitize=fuzzer"
    to wire up libFuzzer itself.

The first two items can already be achieved via the "-Db_sanitize="
option. But the last item cannot easily be achieved, as we can only
configure global link arguments.

Introduce a new "-Dfuzzers_link_args=" build option to plug this gap.
Add documentation so that users know how to set up libFuzzer.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

meson.build

@@ -161,6 +161,21 @@
 # These machine files can be passed to `meson setup` via the `--native-file`
 # option.
 #
+# Fuzzing
+# =======
+#
+# Meson supports building the fuzzing targets by setting `-Dfuzzers=true`. By
+# default, the targets will be built without libFuzzer and thus won't be usable
+# for fuzzing. You have to configure a couple of options to properly wire up
+# libFuzzer:
+#
+#   $ meson setup build-fuzzers \
+#       -Db_sanitize=address,fuzzer-no-link \
+#       -Dfuzzers=true \
+#       -Dfuzzers_link_args=-fsanitize=fuzzer
+#   $ meson compile -C build-fuzzers
+#   $ ./build-fuzzers/oss-fuzz/fuzz-config <args>
+#
 # Cross compilation
 # =================
 #

meson_options.txt

@@ -131,3 +131,5 @@ option('test_utf8_locale', type: 'string',
   description: 'Name of a UTF-8 locale used for testing.')
 option('fuzzers', type: 'boolean', value: false,
   description: 'Enable building fuzzers.')
+option('fuzzers_link_args', type: 'array', value: [],
+  description: 'Linker arguments used to link fuzzers. Use -fsanitize=fuzzer for fuzzing.')

oss-fuzz/meson.build

@@ -16,5 +16,6 @@ foreach fuzz_program : fuzz_programs
       fuzz_program,
     ],
     dependencies: [libgit_commonmain],
+    link_args: get_option('fuzzers_link_args'),
   )
 endforeach