Add ParseVarArgs

This allows a parsing of generic keyword <n> arg0..argN format for
arguments.

rmutil/util.c

@@ -243,4 +243,35 @@ int RedisModule_TryGetValue(RedisModuleKey *key, const RedisModuleType *type, vo
   } else {
     return RMUTIL_VALUE_MISMATCH;
   }
-}
+}
+
+RedisModuleString **RMUtil_ParseVarArgs(RedisModuleString **argv, int argc, int offset,
+                                        const char *keyword, size_t *nargs) {
+  if (offset > argc) {
+    return NULL;
+  }
+
+  argv += offset;
+  argc -= offset;
+
+  int ix = RMUtil_ArgIndex(keyword, argv, argc);
+  if (ix < 0) {
+    return NULL;
+  } else if (ix >= argc - 1) {
+    *nargs = RMUTIL_VARARGS_BADARG;
+    return argv;
+  }
+
+  argv += (ix + 1);
+  argc -= (ix + 1);
+
+  long long n = 0;
+  RMUtil_ParseArgs(argv, argc, 0, "l", &n);
+  if (n > argc - 1 || n < 0) {
+    *nargs = RMUTIL_VARARGS_BADARG;
+    return argv;
+  }
+
+  *nargs = n;
+  return argv + 1;
+}

rmutil/util.h

@@ -61,6 +61,17 @@ int RMUtil_ParseArgsAfter(const char *token, RedisModuleString **argv, int argc,
 
 int rmutil_vparseArgs(RedisModuleString **argv, int argc, int offset, const char *fmt, va_list ap);
 
+#define RMUTIL_VARARGS_BADARG ((size_t)-1)
+/**
+ * Parse arguments in the form of KEYWORD {len} {arg} .. {arg}_len.
+ * If keyword is present, returns the position within `argv` containing the arguments.
+ * Returns NULL if the keyword is not found.
+ * If a parse error has occurred, `nargs` is set to RMUTIL_VARARGS_BADARG, but
+ * the return value is not NULL.
+ */
+RedisModuleString **RMUtil_ParseVarArgs(RedisModuleString **argv, int argc, int offset,
+                                        const char *keyword, size_t *nargs);
+
 // A single key/value entry in a redis info map
 typedef struct {
   const char *key;