Provide an interface for fuzzing sequence producer plugins

tests/fuzz/seq_prod_fuzz_example/Makefile

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+# Copyright (c) Yann Collet, Meta Platforms, Inc.
+# All rights reserved.
+#
+# This source code is licensed under both the BSD-style license (found in the
+# LICENSE file in the root directory of this source tree) and the GPLv2 (found
+# in the COPYING file in the root directory of this source tree).
+# You may select, at your option, one of the above-listed licenses.
+
+CC = clang
+CFLAGS = -g -fno-omit-frame-pointer -fsanitize=undefined,address,fuzzer -I../ -I../../../lib/
+
+.PHONY: default
+default: example_seq_prod.o
+
+example_seq_prod.o: example_seq_prod.c
+	$(CC) -c $(CFLAGS) $^ -o $@

tests/fuzz/seq_prod_fuzz_example/README.md

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+# Fuzzing a Custom Sequence Producer Plugin
+This directory contains example code for using a custom sequence producer in the zstd fuzzers.
+
+You can build and run the code in this directory using these commands:
+```
+$ make corpora
+$ make -C seq_prod_fuzz_example/
+$ python3 ./fuzz.py build all --enable-fuzzer --enable-asan --enable-ubsan --cc clang --cxx clang++ --custom-seq-prod=seq_prod_fuzz_example/example_seq_prod.o
+$ python3 ./fuzz.py libfuzzer simple_round_trip
+```
+
+See `../fuzz_third_party_seq_prod.h` and `../README.md` for more information on zstd fuzzing.

tests/fuzz/seq_prod_fuzz_example/example_seq_prod.c

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+/*
+ * Copyright (c) Yann Collet, Meta Platforms, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ * You may select, at your option, one of the above-listed licenses.
+ */
+
+#include "fuzz_third_party_seq_prod.h"
+
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+
+_Thread_local size_t threadLocalState;
+
+size_t FUZZ_seqProdSetup(void) {
+    threadLocalState = 0;
+    return 0;
+}
+
+size_t FUZZ_seqProdTearDown(void) {
+    return 0;
+}
+
+void* FUZZ_createSeqProdState(void) {
+    return calloc(1, sizeof(size_t));
+}
+
+size_t FUZZ_freeSeqProdState(void* state) {
+    free(state);
+    return 0;
+}
+
+size_t FUZZ_thirdPartySeqProd(
+    void* sequenceProducerState,
+    ZSTD_Sequence* outSeqs, size_t outSeqsCapacity,
+    const void* src, size_t srcSize,
+    const void* dict, size_t dictSize,
+    int compressionLevel,
+    size_t windowSize
+) {
+    /* Try to catch unsafe use of the shared state */
+    size_t* const sharedStatePtr = (size_t*)sequenceProducerState;
+    assert(*sharedStatePtr == threadLocalState);
+    (*sharedStatePtr)++; threadLocalState++;
+
+    /* Check that fallback is enabled when FUZZ_THIRD_PARTY_SEQ_PROD is defined */
+    return ZSTD_SEQUENCE_PRODUCER_ERROR;
+}