fuzzutil: make it possible to use absl when C++17 isn't supported

Differential Revision: https://phab.mercurial-scm.org/D3676

contrib/fuzz/Makefile

@@ -1,46 +1,51 b''
 fuzzutil.o: fuzzutil.cc fuzzutil.h
 	$$CXX $$CXXFLAGS -g -O1 -fsanitize=fuzzer-no-link,address \
 	  -std=c++17 \
 	  -I../../mercurial -c -o fuzzutil.o fuzzutil.cc
 
+fuzzutil-oss-fuzz.o: fuzzutil.cc fuzzutil.h
+	$$CXX $$CXXFLAGS -std=c++17 \
+	  -I../../mercurial -c -o fuzzutil-oss-fuzz.o fuzzutil.cc
+
 bdiff.o: ../../mercurial/bdiff.c
 	$$CC $$CFLAGS -fsanitize=fuzzer-no-link,address -c -o bdiff.o \
 	  ../../mercurial/bdiff.c
 
 bdiff: bdiff.cc bdiff.o fuzzutil.o
 	$$CXX $$CXXFLAGS -DHG_FUZZER_INCLUDE_MAIN=1 -g -O1 -fsanitize=fuzzer-no-link,address \
 	  -std=c++17 \
 	  -I../../mercurial bdiff.cc bdiff.o fuzzutil.o -o bdiff
 
 bdiff-oss-fuzz.o: ../../mercurial/bdiff.c
 	$$CC $$CFLAGS -c -o bdiff-oss-fuzz.o ../../mercurial/bdiff.c
 
-bdiff_fuzzer: bdiff.cc bdiff-oss-fuzz.o fuzzutil.o
+bdiff_fuzzer: bdiff.cc bdiff-oss-fuzz.o fuzzutil-oss-fuzz.o
 	$$CXX $$CXXFLAGS -std=c++17 -I../../mercurial bdiff.cc \
-	  bdiff-oss-fuzz.o fuzzutil.o -lFuzzingEngine -o $$OUT/bdiff_fuzzer
+	  bdiff-oss-fuzz.o fuzzutil-oss-fuzz.o -lFuzzingEngine -o \
+	  $$OUT/bdiff_fuzzer
 
 x%.o: ../../mercurial/thirdparty/xdiff/x%.c ../../mercurial/thirdparty/xdiff/*.h
 	$$CC -g -O1 -fsanitize=fuzzer-no-link,address -c \
 	  -o $@ \
 	  $<
 
 xdiff: xdiff.cc xdiffi.o xprepare.o xutils.o fuzzutil.o
 	$$CXX $$CXXFLAGS -DHG_FUZZER_INCLUDE_MAIN=1 -g -O1 -fsanitize=fuzzer-no-link,address \
 	  -I../../mercurial xdiff.cc \
 	  xdiffi.o xprepare.o xutils.o fuzzutil.o -o xdiff
 
 fuzz-x%.o: ../../mercurial/thirdparty/xdiff/x%.c ../../mercurial/thirdparty/xdiff/*.h
 	$$CC $$CFLAGS -c \
 	  -o $@ \
 	  $<
 
-xdiff_fuzzer: xdiff.cc fuzz-xdiffi.o fuzz-xprepare.o fuzz-xutils.o fuzzutil.o
+xdiff_fuzzer: xdiff.cc fuzz-xdiffi.o fuzz-xprepare.o fuzz-xutils.o fuzzutil-oss-fuzz.o
 	$$CXX $$CXXFLAGS -std=c++17 -I../../mercurial xdiff.cc \
-	  fuzz-xdiffi.o fuzz-xprepare.o fuzz-xutils.o fuzzutil.o \
+	  fuzz-xdiffi.o fuzz-xprepare.o fuzz-xutils.o fuzzutil-oss-fuzz.o \
 	  -lFuzzingEngine -o $$OUT/xdiff_fuzzer
 
 all: bdiff xdiff
 
 oss-fuzz: bdiff_fuzzer xdiff_fuzzer
 
 .PHONY: all oss-fuzz

contrib/fuzz/fuzzutil.cc

@@ -1,26 +1,26 b''
 #include "fuzzutil.h"
 
 #include <utility>
 
-std::optional<two_inputs> SplitInputs(const uint8_t *Data, size_t Size)
+contrib::optional<two_inputs> SplitInputs(const uint8_t *Data, size_t Size)
 {
 	if (!Size) {
-		return std::nullopt;
+		return contrib::nullopt;
 	}
 	// figure out a random point in [0, Size] to split our input.
 	size_t left_size = (Data[0] / 255.0) * (Size - 1);
 
 	// Copy inputs to new allocations so if bdiff over-reads
 	// AddressSanitizer can detect it.
 	std::unique_ptr<char[]> left(new char[left_size]);
 	memcpy(left.get(), Data + 1, left_size);
 	// right starts at the next byte after left ends
 	size_t right_size = Size - (left_size + 1);
 	std::unique_ptr<char[]> right(new char[right_size]);
 	memcpy(right.get(), Data + 1 + left_size, right_size);
 	LOG(2) << "inputs are  " << left_size << " and " << right_size
 	       << " bytes" << std::endl;
 	two_inputs result = {std::move(right), right_size, std::move(left),
 	                     left_size};
 	return result;
 }

contrib/fuzz/fuzzutil.h

@@ -1,24 +1,47 b''
 #ifndef CONTRIB_FUZZ_FUZZUTIL_H
 #define CONTRIB_FUZZ_FUZZUTIL_H
 #include <iostream>
 #include <memory>
+#include <stdint.h>
+
+/* Try and use std::optional, but failing that assume we'll have a
+ * workable https://abseil.io/ install on the include path to get
+ * their backport of std::optional. */
+#ifdef __has_include
+#if __has_include(<optional>) && __cplusplus >= 201703L
 #include <optional>
-#include <stdint.h>
+#define CONTRIB_FUZZ_HAVE_STD_OPTIONAL
+#endif
+#endif
+#ifdef CONTRIB_FUZZ_HAVE_STD_OPTIONAL
+namespace contrib
+{
+using std::nullopt;
+using std::optional;
+} /* namespace contrib */
+#else
+#include "third_party/absl/types/optional.h"
+namespace contrib
+{
+using absl::nullopt;
+using absl::optional;
+} /* namespace contrib */
+#endif
 
 /* set DEBUG to 1 for a few debugging prints, or 2 for a lot */
 #define DEBUG 0
 #define LOG(level)                                                             \
 	if (level <= DEBUG)                                                    \
 	std::cout
 
 struct two_inputs {
 	std::unique_ptr<char[]> right;
 	size_t right_size;
 	std::unique_ptr<char[]> left;
 	size_t left_size;
 };
 
 /* Split a non-zero-length input into two inputs. */
-std::optional<two_inputs> SplitInputs(const uint8_t *Data, size_t Size);
+contrib::optional<two_inputs> SplitInputs(const uint8_t *Data, size_t Size);
 
 #endif /* CONTRIB_FUZZ_FUZZUTIL_H */