fuzzer/c2: add roundtrip test

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/c2.c

@@ -1521,6 +1521,10 @@ static const char *c2_condition_to_str2(const c2_ptr_t ptr) {
 	return buf;
 }
 
+const char *c2_lptr_to_str(const c2_lptr_t *ptr) {
+	return c2_condition_to_str2(ptr->ptr);
+}
+
 static inline bool c2_int_op(const c2_l_t *leaf, size_t ntargets, const int64_t *targets) {
 	for (size_t i = 0; i < ntargets; ++i) {
 		long long tgt = targets[i];

src/c2.h

@@ -29,6 +29,7 @@ struct managed_win;
 
 typedef void (*c2_userdata_free)(void *);
 c2_lptr_t *c2_parse(c2_lptr_t **pcondlst, const char *pattern, void *data);
+const char *c2_lptr_to_str(const c2_lptr_t *ptr);
 
 c2_lptr_t *c2_free_lptr(c2_lptr_t *lp, c2_userdata_free f);
 

src/fuzzer/c2.c

@@ -2,6 +2,7 @@
 #include "c2.h"
 #include <stddef.h>
 #include <stdint.h>
+#include <stdio.h>
 #include "config.h"
 #include "log.h"
 
@@ -14,7 +15,37 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 		return 0;
 	}
 	c2_lptr_t *cond = c2_parse(NULL, (char *)data, NULL);
-	(void)cond;
-	(void)size;
-	return 0;        // Values other than 0 and -1 are reserved for future use.
-}
+	if (!cond) {
+		return 0;
+	}
+
+	// If we can parse it, we check if it roundtrips.
+	// Except when the input or the stringified condition has ':' at the second
+	// position, because that becomes a "legacy" pattern and is parsed differently.
+	char *str = strdup(c2_lptr_to_str(cond));
+	c2_free_lptr(cond, NULL);
+	if (str[1] == ':') {
+		free(str);
+		return 0;
+	}
+
+	c2_lptr_t *cond2 = c2_parse(NULL, str, NULL);
+	// The stringified condition could legitimately fail to parse, for example,
+	// "a=1 || b=2 || c=3 || ... ", when stringified, will be "((((((a=1 || b=2) ||
+	// c=3) || ...)", which will fail to parse because of the parenthese nest level
+	// limit.
+	if (cond2 == NULL) {
+		free(str);
+		return 0;
+	}
+
+	const char *str2 = c2_lptr_to_str(cond2);
+	c2_free_lptr(cond2, NULL);
+	if (strcmp(str, str2) != 0) {
+		fprintf(stderr, "Mismatch: %s != %s\n", str, str2);
+		abort();
+	}
+
+	free(str);
+	return 0;
+}