Redis module : calling dump command in module return nothing

Question

I develop a redis module that call the native redis DUMP command. But when i call dump command on existing key, nothing is return but if i replace the DUMP command by GET the value is returned

Here is the code :

#include "redismodule.h"

#include <stdio.h>
#include <stdlib.h>

int mydump_command(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
  REDISMODULE_NOT_USED(argc);
  RedisModule_AutoMemory(ctx);

  RedisModuleCallReply *rep = RedisModule_Call(ctx, "DUMP", "s", argv[1]);

  const char *value = RedisModule_CallReplyStringPtr(rep, NULL);

  RedisModule_Log(ctx, "warning", value);

  RedisModule_ReplyWithSimpleString(ctx, value);
  return REDISMODULE_OK;
}

int myget_command(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
  REDISMODULE_NOT_USED(argc);
  RedisModule_AutoMemory(ctx);

  RedisModuleCallReply *rep = RedisModule_Call(ctx, "GET", "s", argv[1]);

  const char *value = RedisModule_CallReplyStringPtr(rep, NULL);

  RedisModule_Log(ctx, "warning", value);

  RedisModule_ReplyWithSimpleString(ctx, value);
  return REDISMODULE_OK;
}

int RedisModule_OnLoad(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {

  if (RedisModule_Init(ctx, "module_test", 1, REDISMODULE_APIVER_1) == REDISMODULE_ERR) {
    return REDISMODULE_ERR;
  }

  if (RedisModule_CreateCommand(ctx, "mydump", mydump_command, "write deny-oom", 1, 1, 1) == REDISMODULE_ERR) {
    return REDISMODULE_ERR;
  }

  if (RedisModule_CreateCommand(ctx, "myget", myget_command, "write deny-oom", 1, 1, 1) == REDISMODULE_ERR) {
    return REDISMODULE_ERR;
  }

  return REDISMODULE_OK;
}

In redis-cli i get the following result :

127.0.0.1:6379> SET foo bar
OK
127.0.0.1:6379> GET foo
"bar"
127.0.0.1:6379> DUMP foo
"\x00\x03bar\b\x00_\x93\xa5\xdfG\x7fw/"
127.0.0.1:6379> mydump foo

127.0.0.1:6379> myget foo
bar
127.0.0.1:6379> 

Note that with my custom command, redis-cli don't disply double quotes.

Can someone help me know why calling DUMP command return nothing.

Answer 1

The problem in your code is that you're treating the reply as a simple string, whereas Redis' may be binary. While GET returns a simple string value (as you SET foo bar), DUMPs encoding is binary. The following works:

int mydump_command(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
  REDISMODULE_NOT_USED(argc);
  RedisModule_AutoMemory(ctx);

  RedisModuleCallReply *rep = RedisModule_Call(ctx, "DUMP", "s", argv[1]);

  size_t vlen;
  const char *value = RedisModule_CallReplyStringPtr(rep, &vlen);

  RedisModule_ReplyWithStringBuffer(ctx, value, vlen);
  return REDISMODULE_OK;
}