test/fuzz: fix datetime_strptime fuzzing test

Function `datetime_strptime` decodes string with datetime according to
specified format, it accepts a datetime struct, buffer with datetime and
string with format in arguments. Fuzzing test used static string
"iso8601" as a format and it blocked fuzzing test to cover functions
used by datetime_strptime under the hood. Fuzz introspector shows that
code coveraged by a test is quite low.

Patch updates the test to make it more effective: buffer with datetime
and format string are generated using FDP (Fuzzing Data Provider).

Test file extension was changed to .cc, because FuzzingDataProvider is
used and we need building it by C++ compiler.

Function `tnt_strptime` uses assert, that triggered by fuzzing tests.
Therefore it was replaced with to if..then.

1. https://storage.googleapis.com/oss-fuzz-introspector/tarantool/

Fixes #8490

NO_CHANGELOG=fuzzing test
NO_DOC=fuzzing test
NO_TEST=fuzzing test

(cherry picked from commit a1bd6e0b)

src/lib/tzcode/strptime.c

@@ -262,7 +262,8 @@ tnt_strptime(const char *__restrict buf, const char *__restrict fmt,
 				;
 
 			c = *ptr++;
-			assert(c == 'f');
+			if (c != 'f')
+				return NULL;
 			/* fallthru */
 		case 'f':
 			if (!is_digit((u_char)*buf))

test/fuzz/CMakeLists.txt

@@ -84,7 +84,7 @@ create_fuzz_test(PREFIX datetime_parse_full
 )
 
 create_fuzz_test(PREFIX datetime_strptime
-                 SOURCES datetime_strptime_fuzzer.c
+                 SOURCES datetime_strptime_fuzzer.cc
                  LIBRARIES core fuzzer_config
 )
 

test/fuzz/datetime_strptime_fuzzer.c → test/fuzz/datetime_strptime_fuzzer.cc

-#include <stdlib.h>
-#include <string.h>
+#include <fuzzer/FuzzedDataProvider.h>
+#include <stddef.h>
+
 #include "datetime.h"
-#include "trivia/util.h"
 
-void
+extern "C" void
 cord_on_yield(void) {}
 
-int
+extern "C" int
 LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
-	char *buf = xcalloc(size + 1, sizeof(char));
-	if (buf == NULL)
-		return 0;
-	memcpy(buf, data, size);
-	buf[size] = '\0';
+	FuzzedDataProvider fdp(data, size);
+
+	auto buf = fdp.ConsumeRandomLengthString();
+	auto fmt = fdp.ConsumeRandomLengthString();
+
 	struct datetime date_expected;
-	datetime_strptime(&date_expected, buf, "iso8601");
-	free(buf);
+	datetime_strptime(&date_expected, buf.c_str(), fmt.c_str());
 
 	return 0;
 }