mercurial: move Python sources to $SRC and build in build.sh (#2078)

This will clean up the Makefile on the hg side considerably, and will
also fix the coverage build. Fixes #2076.

Bonus: I did some extra work in build.sh so that incremental rebuilds
of the fuzzers won't needlessly recompile $OUT/sanpy, shortening
development time on new fuzzers and avoiding issues caused by a $OUT
that was built for a different fuzz configuration.

projects/mercurial/Dockerfile

@@ -18,7 +18,7 @@ FROM gcr.io/oss-fuzz-base/base-builder
 MAINTAINER security@mercurial-scm.org
 RUN apt-get update && apt-get install -y make autoconf automake libtool \
   python-dev mercurial curl
-RUN cd / && curl https://www.python.org/ftp/python/2.7.15/Python-2.7.15.tgz | tar xzf -
+RUN cd $SRC && curl https://www.python.org/ftp/python/2.7.15/Python-2.7.15.tgz | tar xzf -
 RUN hg clone https://www.mercurial-scm.org/repo/hg mercurial
 WORKDIR mercurial
 COPY build.sh $SRC/

projects/mercurial/build.sh

@@ -15,8 +15,7 @@
 #
 ################################################################################
 
-pushd /Python-2.7.15/
-ls
+pushd $SRC/Python-2.7.15/
 patch -p1 <<'EOF'
 Index: v2_7_unstable/Python/pymath.c
 ===================================================================
@@ -46,8 +45,19 @@ Index: v2_7_unstable/Modules/_ctypes/callproc.c
      avalues = (void **)alloca(sizeof(void *) * argcount);
      atypes = (ffi_type **)alloca(sizeof(ffi_type *) * argcount);
 EOF
+
+if [ -e $OUT/sanpy/cflags -a "$(cat $OUT/sanpy/cflags)" = "${CFLAGS}" ] ; then
+    echo 'Python cflags unchanged, no need to rebuild'
+else
+    rm -rf $OUT/sanpy
+    ASAN_OPTIONS=detect_leaks=0 ./configure --without-pymalloc \
+                --prefix=$OUT/sanpy CFLAGS="${CFLAGS}" LINKCC="${CXX}" \
+                LDFLAGS="${CXXFLAGS}"
+    grep -v HAVE_GETC_UNLOCKED < pyconfig.h > tmp && mv tmp pyconfig.h
+    ASAN_OPTIONS=detect_leaks=0 make && make install
+    echo "${CFLAGS}" > $OUT/sanpy/cflags
+fi
 popd
 
 cd contrib/fuzz
-export PYLDFLAGS=$(echo $CFLAGS | xargs -n 1 echo | egrep -- '-fsanitize=(memory|address)')
-make oss-fuzz
+make clean oss-fuzz