Add a replay_build.sh for ffmpeg. (#13427)

This is meant to be a manual override over what chronos
(https://github.com/google/oss-fuzz/tree/master/infra/experimental/chronos)
generates.

The idea is we'll snapshot the ffmpeg build container after running
compile into a new Docker image. This image will have build artifacts
saved so that subsequent `make` calls only trigger on changed files.

This makes rebuilding a specific target **a lot** faster (From 2600
seconds down to instantaneous).

This does change the `build.sh` -- merging the two ./configure calls
down to one in order to faciliate this.

Author: oliverchang

projects/ffmpeg/Dockerfile

@@ -41,4 +41,4 @@ RUN git clone --depth 1 https://gitlab.xiph.org/xiph/theora.git
 RUN git clone --depth 1 https://gitlab.xiph.org/xiph/vorbis.git
 RUN git clone --depth 1 https://gitlab.gnome.org/GNOME/libxml2.git
 
-COPY build.sh group_seed_corpus.py $SRC/
+COPY build.sh replay_build.sh group_seed_corpus.py $SRC/

projects/ffmpeg/build.sh

@@ -218,10 +218,15 @@ fi
         --disable-shared \
         --disable-doc \
         --disable-programs \
+        --enable-demuxers \
         $FFMPEG_BUILD_ARGS
 make clean
 make -j$(nproc) install
 
+if [[ -n ${CAPTURE_REPLAY_SCRIPT-} ]]; then
+  exit 0
+fi
+
 # Download test samples, will be used as seed corpus.
 # DISABLED.
 # TODO: implement a better way to maintain a minimized seed corpora
@@ -274,6 +279,8 @@ if [ -n "${OSS_FUZZ_CI-}" ]; then
       CONDITIONALS=(${CONDITIONALS[@]:0:2})
 fi
 
+# FIXME: currently, enc fuzzers are clobbering dec variants because they share the same
+# name.
 for c in $CONDITIONALS; do
       fuzzer_name=ffmpeg_AV_CODEC_ID_${c}_fuzzer
       symbol=$(echo $c | sed "s/.*/\L\0/")
@@ -315,7 +322,7 @@ fuzzer_name=ffmpeg_IO_DEMUXER_fuzzer
 make tools/target_io_dem_fuzzer
 mv tools/target_io_dem_fuzzer $OUT/${fuzzer_name}
 
-#Build fuzzers for individual demuxers
+# Reduce size of demuxer fuzzers by disabling various components.
 ./configure \
         --cc=$CC --cxx=$CXX --ld="$CXX $CXXFLAGS -std=c++11" \
         --extra-cflags="-I$FFMPEG_DEPS_PATH/include" \

projects/ffmpeg/replay_build.sh

@@ -0,0 +1,71 @@
+#!/bin/bash -eux
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+# Disable UBSan vptr since several targets built with -fno-rtti.
+export CFLAGS="$CFLAGS -fno-sanitize=vptr"
+export CXXFLAGS="$CXXFLAGS -fno-sanitize=vptr"
+
+if [[ "$CXXFLAGS" == *"-fsanitize=address"* ]]; then
+    export CXXFLAGS="$CXXFLAGS -fno-sanitize-address-use-odr-indicator"
+fi
+
+if [[ "$CFLAGS" == *"-fsanitize=address"* ]]; then
+    export CFLAGS="$CFLAGS -fno-sanitize-address-use-odr-indicator"
+fi
+
+if [[ "$ARCHITECTURE" == i386 ]]; then
+  export CFLAGS="$CFLAGS -m32"
+  export CXXFLAGS="$CXXFLAGS -m32"
+fi
+
+# The option `-fuse-ld=gold` can't be passed via `CFLAGS` or `CXXFLAGS` because
+# Meson injects `-Werror=ignored-optimization-argument` during compile tests.
+# Remove the `-fuse-ld=` and let Meson handle it.
+# https://github.com/mesonbuild/meson/issues/6377#issuecomment-575977919
+export MESON_CFLAGS="$CFLAGS"
+if [[ "$CFLAGS" == *"-fuse-ld=gold"* ]]; then
+    export MESON_CFLAGS="${CFLAGS//-fuse-ld=gold/}"
+    export CC_LD=gold
+fi
+export MESON_CXXFLAGS="$CXXFLAGS"
+if [[ "$CXXFLAGS" == *"-fuse-ld=gold"* ]]; then
+    export MESON_CXXFLAGS="${CXXFLAGS//-fuse-ld=gold/}"
+    export CXX_LD=gold
+fi
+
+cd $SRC/ffmpeg
+make -j$(nproc) install
+
+if [ "$#" -lt 1 ]; then
+  exit 0
+fi
+
+name=$(echo "$1" | sed -E 's/ffmpeg_(AV_CODEC_ID_)?(.*)_fuzzer/\L\2/' | sed 's/demuxer/dem/')
+make tools/target_${name}_fuzzer || true
+if [ -f tools/target_${name}_fuzzer ]; then
+  mv tools/target_${name}_fuzzer $OUT/$1
+fi
+
+make tools/target_dec_${name}_fuzzer || true
+if [ -f tools/target_dec_${name}_fuzzer ]; then
+  mv tools/target_dec_${name}_fuzzer $OUT/$1
+fi
+
+make tools/target_enc_${name}_fuzzer || true
+if [ -f tools/target_enc_${name}_fuzzer ]; then
+  mv tools/target_enc_${name}_fuzzer $OUT/$1
+fi