xdiff: don't attempt to use fuzzer inputs larger than 100k

This is the recommended approach from [0], and limiting the input was
suggested in https://github.com/google/oss-fuzz/issues/2076 when
discussing our broken coverage build.

0: https://github.com/google/oss-fuzz/blob/master/docs/new_project_guide.md#custom-libfuzzer-options-for-clusterfuzz

Differential Revision: https://phab.mercurial-scm.org/D5525

contrib/fuzz/xdiff.cc

@@ -1,58 +1,63
 /*
  * xdiff.cc - fuzzer harness for thirdparty/xdiff
  *
  * Copyright 2018, Google Inc.
  *
  * This software may be used and distributed according to the terms of
  * the GNU General Public License, incorporated herein by reference.
  */
 #include "thirdparty/xdiff/xdiff.h"
 #include <inttypes.h>
 #include <stdlib.h>
 
 #include "fuzzutil.h"
 
 extern "C" {
 
 int hunk_consumer(long a1, long a2, long b1, long b2, void *priv)
 {
 	// TODO: probably also test returning -1 from this when things break?
 	return 0;
 }
 
 int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size)
 {
+	// Don't allow fuzzer inputs larger than 100k, since we'll just bog
+	// down and not accomplish much.
+	if (Size > 100000) {
+		return 0;
+	}
 	auto maybe_inputs = SplitInputs(Data, Size);
 	if (!maybe_inputs) {
 		return 0;
 	}
 	auto inputs = std::move(maybe_inputs.value());
 	mmfile_t a, b;
 
 	a.ptr = inputs.left.get();
 	a.size = inputs.left_size;
 	b.ptr = inputs.right.get();
 	b.size = inputs.right_size;
 	xpparam_t xpp = {
 	    XDF_INDENT_HEURISTIC, /* flags */
 	};
 	xdemitconf_t xecfg = {
 	    XDL_EMIT_BDIFFHUNK, /* flags */
 	    hunk_consumer,      /* hunk_consume_func */
 	};
 	xdemitcb_t ecb = {
 	    NULL, /* priv */
 	};
 	xdl_diff(&a, &b, &xpp, &xecfg, &ecb);
 	return 0; // Non-zero return values are reserved for future use.
 }
 
 #ifdef HG_FUZZER_INCLUDE_MAIN
 int main(int argc, char **argv)
 {
 	const char data[] = "asdf";
 	return LLVMFuzzerTestOneInput((const uint8_t *)data, 4);
 }
 #endif
 
 } // extern "C"