Port the clang-fuzzer code to the Makefile.

Still doesn't work because Prometheus client_golang broke everything for
non-modules in
github.com/prometheus/client_golang/commit/ee1078a03c073762e2660d305738bfdafbf69ba6

You can't resolve xxhash/v2 in the fuzzer bcause it doesn't support modules
yet, dvyukov/go-fuzz#195

.circleci/config.yml

@@ -145,13 +145,10 @@ jobs:
           wget -q -O fuzzit https://github.com/fuzzitdev/fuzzit/releases/latest/download/fuzzit_Linux_x86_64
           chmod a+x fuzzit
       - run: |
-          make install_deps
-          cd internal/vm
-          go-fuzz-build -libfuzzer -o fuzzer.a .
-          clang-9 -fsanitize=fuzzer fuzzer.a -o fuzzer
+          make clang-fuzzer
       - run: |
           ./fuzzit auth $FUZZIT_API_KEY
-          ./fuzzit create job --type fuzzing --branch ${CIRCLE_BRANCH} --revision ${CIRCLE_SHA1} mtail/internal-vm internal/vm/fuzzer
+          ./fuzzit create job --type fuzzing --branch ${CIRCLE_BRANCH} --revision ${CIRCLE_SHA1} mtail/internal-vm clang-fuzzer
 
   fuzz-regression:
     working_directory: /go/src/github.com/google/mtail
@@ -170,12 +167,9 @@ jobs:
           wget -q -O fuzzit https://github.com/fuzzitdev/fuzzit/releases/latest/download/fuzzit_Linux_x86_64
           chmod a+x fuzzit
       - run: |
-          make install_deps
-          cd internal/vm
-          go-fuzz-build -libfuzzer -o fuzzer.a .
-          clang-9 -fsanitize=fuzzer fuzzer.a -o fuzzer
+          make clang-fuzzer
       - run: |
-          ./fuzzit create job --type local-regression --branch ${CIRCLE_BRANCH} --revision ${CIRCLE_SHA1} mtail/internal-vm internal/vm/fuzzer
+          ./fuzzit create job --type local-regression --branch ${CIRCLE_BRANCH} --revision ${CIRCLE_SHA1} mtail/internal-vm clang-fuzzer
 
 # tag-filter ensures that jobs run for version-tagged changes as well as
 # regular commits.  Use for all normal build and test and stats (coverage)

Makefile

@@ -223,6 +223,10 @@ fuzz: vm-fuzz.zip | $(GOFUZZ)
 	cp examples/*.mtail workdir/corpus
 	$(GOFUZZ) -bin=vm-fuzz.zip -workdir=workdir
 
+clang-fuzzer: | $(GOFUZZBUILD)
+	GO111MODULE=off $(GOFUZZBUILD) -libfuzzer -o fuzzer.a github.com/google/mtail/internal/vm
+	clang-9 -fsanitize=fuzzer fuzzer.a -o clang-fuzzer
+
 ###
 ## dependency section
 #