Ensure empty error tables in scripts don't crash Valkey (#2229)

When calling the command `EVAL error{} 0`, Valkey crashes with the
following stack trace. This patch ensures we never leave the
`err_info.msg` field null when we fail to extract a proper error
message.

```
=== VALKEY BUG REPORT START: Cut & paste starting from here ===
2595901:M 18 Jun 2025 01:20:12.917 # valkey 8.1.2 crashed by signal: 11, si_code: 1
2595901:M 18 Jun 2025 01:20:12.917 # Accessing address: (nil)
2595901:M 18 Jun 2025 01:20:12.917 # Crashed running the instruction at: 0x726f8e57ed1d

------ STACK TRACE ------
EIP:
/usr/lib/libc.so.6(+0x16ed1d) [0x726f8e57ed1d]

2595905 bio_aof
/usr/lib/libc.so.6(+0x9de22) [0x726f8e4ade22]
/usr/lib/libc.so.6(+0x91fda) [0x726f8e4a1fda]
/usr/lib/libc.so.6(+0x9264c) [0x726f8e4a264c]
/usr/lib/libc.so.6(pthread_cond_wait+0x14e) [0x726f8e4a4d1e]
valkey-server *:6379(bioProcessBackgroundJobs+0x1b4) [0x6530abb46db4]
/usr/lib/libc.so.6(+0x957eb) [0x726f8e4a57eb]
/usr/lib/libc.so.6(+0x11918c) [0x726f8e52918c]

2595904 bio_close_file
/usr/lib/libc.so.6(+0x9de22) [0x726f8e4ade22]
/usr/lib/libc.so.6(+0x91fda) [0x726f8e4a1fda]
/usr/lib/libc.so.6(+0x9264c) [0x726f8e4a264c]
/usr/lib/libc.so.6(pthread_cond_wait+0x14e) [0x726f8e4a4d1e]
valkey-server *:6379(bioProcessBackgroundJobs+0x1b4) [0x6530abb46db4]
/usr/lib/libc.so.6(+0x957eb) [0x726f8e4a57eb]
/usr/lib/libc.so.6(+0x11918c) [0x726f8e52918c]

2595901 valkey-server *
/usr/lib/libc.so.6(+0x3def0) [0x726f8e44def0]
/usr/lib/libc.so.6(+0x16ed1d) [0x726f8e57ed1d]
valkey-server *:6379(sdscatfmt+0x894) [0x6530abaa24a4]
valkey-server *:6379(luaCallFunction+0x39a) [0x6530abbc66ea]
valkey-server *:6379(+0x1a0992) [0x6530abbc6992]
valkey-server *:6379(scriptingEngineCallFunction+0x98) [0x6530abbc1298]
valkey-server *:6379(+0x11ff55) [0x6530abb45f55]
valkey-server *:6379(call+0x174) [0x6530aba94454]
valkey-server *:6379(processCommand+0x93d) [0x6530aba958dd]
valkey-server *:6379(processCommandAndResetClient+0x21) [0x6530abaa9d11]
valkey-server *:6379(processInputBuffer+0xe3) [0x6530abaaee83]
valkey-server *:6379(readQueryFromClient+0x65) [0x6530abaaef55]
valkey-server *:6379(+0x18e31a) [0x6530abbb431a]
valkey-server *:6379(aeProcessEvents+0x24a) [0x6530aba790ca]
valkey-server *:6379(aeMain+0x2d) [0x6530aba7938d]
valkey-server *:6379(main+0x3f6) [0x6530aba6e7b6]
/usr/lib/libc.so.6(+0x276b5) [0x726f8e4376b5]
/usr/lib/libc.so.6(__libc_start_main+0x89) [0x726f8e437769]
valkey-server *:6379(_start+0x25) [0x6530aba70235]

2595906 bio_lazy_free
/usr/lib/libc.so.6(+0x9de22) [0x726f8e4ade22]
/usr/lib/libc.so.6(+0x91fda) [0x726f8e4a1fda]
/usr/lib/libc.so.6(+0x9264c) [0x726f8e4a264c]
/usr/lib/libc.so.6(pthread_cond_wait+0x14e) [0x726f8e4a4d1e]
valkey-server *:6379(bioProcessBackgroundJobs+0x1b4) [0x6530abb46db4]
/usr/lib/libc.so.6(+0x957eb) [0x726f8e4a57eb]
/usr/lib/libc.so.6(+0x11918c) [0x726f8e52918c]

4/4 expected stacktraces.

------ STACK TRACE DONE ------

------ REGISTERS ------
2595901:M 18 Jun 2025 01:20:12.920 # 
RAX:0000000000000000 RBX:0000726f8dd35663
RCX:0000000000000000 RDX:0000000000000000
RDI:0000000000000000 RSI:0000000000000010
RBP:00007ffc2b821a80 RSP:00007ffc2b821938
R8 :000000000000000c R9 :00006530abc111b8
R10:0000000000000001 R11:0000000000000003
R12:00006530abc49adc R13:00006530abc111b7
R14:0000000000000001 R15:0000000000000001
RIP:0000726f8e57ed1d EFL:0000000000010283
CSGSFS:002b000000000033
2595901:M 18 Jun 2025 01:20:12.921 * hide-user-data-from-log is on, skip logging stack content to avoid spilling user data.

------ INFO OUTPUT ------
# Server
redis_version:7.2.4
server_name:valkey
valkey_version:8.1.2
valkey_release_stage:ga
redis_git_sha1:00000000
redis_git_dirty:0
redis_build_id:38d65aa7b4148d2c
server_mode:standalone
os:Linux 6.14.6-arch1-1 x86_64
arch_bits:64
monotonic_clock:POSIX clock_gettime
multiplexing_api:epoll
gcc_version:15.1.1
process_id:2595901
process_supervised:no
run_id:a0b75f67a217a81142f17553028c010e86c1ee80
tcp_port:6379
server_time_usec:1750209612917634
uptime_in_seconds:16
uptime_in_days:0
hz:10
configured_hz:10
clients_hz:10
lru_clock:5379148
executable:/home/fusl/valkey-server
config_file:
io_threads_active:0
availability_zone:
listener0:name=tcp,bind=*,bind=-::*,port=6379

# Clients
connected_clients:1
cluster_connections:0
maxclients:10000
client_recent_max_input_buffer:0
client_recent_max_output_buffer:0
blocked_clients:0
tracking_clients:0
pubsub_clients:0
watching_clients:0
clients_in_timeout_table:0
total_watched_keys:0
total_blocking_keys:0
total_blocking_keys_on_nokey:0
paused_reason:none
paused_actions:none
paused_timeout_milliseconds:0

# Memory
used_memory:911824
used_memory_human:890.45K
used_memory_rss:15323136
used_memory_rss_human:14.61M
used_memory_peak:911824
used_memory_peak_human:890.45K
used_memory_peak_perc:100.29%
used_memory_overhead:892232
used_memory_startup:891824
used_memory_dataset:19592
used_memory_dataset_perc:97.96%
allocator_allocated:1845952
allocator_active:1986560
allocator_resident:6672384
allocator_muzzy:0
total_system_memory:67323842560
total_system_memory_human:62.70G
used_memory_lua:34816
used_memory_vm_eval:34816
used_memory_lua_human:34.00K
used_memory_scripts_eval:184
number_of_cached_scripts:1
number_of_functions:0
number_of_libraries:0
used_memory_vm_functions:33792
used_memory_vm_total:68608
used_memory_vm_total_human:67.00K
used_memory_functions:224
used_memory_scripts:408
used_memory_scripts_human:408B
maxmemory:0
maxmemory_human:0B
maxmemory_policy:noeviction
allocator_frag_ratio:1.00
allocator_frag_bytes:0
allocator_rss_ratio:3.36
allocator_rss_bytes:4685824
rss_overhead_ratio:2.30
rss_overhead_bytes:8650752
mem_fragmentation_ratio:17.18
mem_fragmentation_bytes:14431168
mem_not_counted_for_evict:0
mem_replication_backlog:0
mem_total_replication_buffers:0
mem_clients_slaves:0
mem_clients_normal:0
mem_cluster_links:0
mem_aof_buffer:0
mem_allocator:jemalloc-5.3.0
mem_overhead_db_hashtable_rehashing:0
active_defrag_running:0
lazyfree_pending_objects:0
lazyfreed_objects:0

# Persistence
loading:0
async_loading:0
current_cow_peak:0
current_cow_size:0
current_cow_size_age:0
current_fork_perc:0.00
current_save_keys_processed:0
current_save_keys_total:0
rdb_changes_since_last_save:0
rdb_bgsave_in_progress:0
rdb_last_save_time:1750209596
rdb_last_bgsave_status:ok
rdb_last_bgsave_time_sec:-1
rdb_current_bgsave_time_sec:-1
rdb_saves:0
rdb_last_cow_size:0
rdb_last_load_keys_expired:0
rdb_last_load_keys_loaded:0
aof_enabled:0
aof_rewrite_in_progress:0
aof_rewrite_scheduled:0
aof_last_rewrite_time_sec:-1
aof_current_rewrite_time_sec:-1
aof_last_bgrewrite_status:ok
aof_rewrites:0
aof_rewrites_consecutive_failures:0
aof_last_write_status:ok
aof_last_cow_size:0
module_fork_in_progress:0
module_fork_last_cow_size:0

# Stats
total_connections_received:1
total_commands_processed:0
instantaneous_ops_per_sec:0
total_net_input_bytes:34
total_net_output_bytes:0
total_net_repl_input_bytes:0
total_net_repl_output_bytes:0
instantaneous_input_kbps:0.00
instantaneous_output_kbps:0.00
instantaneous_input_repl_kbps:0.00
instantaneous_output_repl_kbps:0.00
rejected_connections:0
sync_full:0
sync_partial_ok:0
sync_partial_err:0
expired_keys:0
expired_stale_perc:0.00
expired_time_cap_reached_count:0
expire_cycle_cpu_milliseconds:0
evicted_keys:0
evicted_clients:0
evicted_scripts:0
total_eviction_exceeded_time:0
current_eviction_exceeded_time:0
keyspace_hits:0
keyspace_misses:0
pubsub_channels:0
pubsub_patterns:0
pubsubshard_channels:0
latest_fork_usec:0
total_forks:0
migrate_cached_sockets:0
slave_expires_tracked_keys:0
active_defrag_hits:0
active_defrag_misses:0
active_defrag_key_hits:0
active_defrag_key_misses:0
total_active_defrag_time:0
current_active_defrag_time:0
tracking_total_keys:0
tracking_total_items:0
tracking_total_prefixes:0
unexpected_error_replies:0
total_error_replies:0
dump_payload_sanitizations:0
total_reads_processed:1
total_writes_processed:0
io_threaded_reads_processed:0
io_threaded_writes_processed:0
io_threaded_freed_objects:0
io_threaded_accept_processed:0
io_threaded_poll_processed:0
io_threaded_total_prefetch_batches:0
io_threaded_total_prefetch_entries:0
client_query_buffer_limit_disconnections:0
client_output_buffer_limit_disconnections:0
reply_buffer_shrinks:0
reply_buffer_expands:0
eventloop_cycles:170
eventloop_duration_sum:17739
eventloop_duration_cmd_sum:0
instantaneous_eventloop_cycles_per_sec:9
instantaneous_eventloop_duration_usec:99
acl_access_denied_auth:0
acl_access_denied_cmd:0
acl_access_denied_key:0
acl_access_denied_channel:0

# Replication
role:master
connected_slaves:0
replicas_waiting_psync:0
master_failover_state:no-failover
master_replid:d35a0bb7979f490a60174bb363524431d7eb2428
master_replid2:0000000000000000000000000000000000000000
master_repl_offset:0
second_repl_offset:-1
repl_backlog_active:0
repl_backlog_size:10485760
repl_backlog_first_byte_offset:0
repl_backlog_histlen:0

# CPU
used_cpu_sys:0.012543
used_cpu_user:0.016853
used_cpu_sys_children:0.000000
used_cpu_user_children:0.000000
used_cpu_sys_main_thread:0.012440
used_cpu_user_main_thread:0.016714

# Modules

# Commandstats

# Errorstats

# Latencystats

# Cluster
cluster_enabled:0

# Keyspace

------ CLIENT LIST OUTPUT ------
id=2 addr=127.0.0.1:41372 laddr=127.0.0.1:6379 fd=10 name=*redacted* age=0 idle=0 flags=N capa= db=0 sub=0 psub=0 ssub=0 multi=-1 watch=0 qbuf=0 qbuf-free=0 argv-mem=12 multi-mem=0 rbs=16384 rbp=16384 obl=0 oll=0 omem=0 tot-mem=17060 events=r cmd=eval user=*redacted* redir=-1 resp=2 lib-name= lib-ver= tot-net-in=34 tot-net-out=0 tot-cmds=0

------ CURRENT CLIENT INFO ------
id=2 addr=127.0.0.1:41372 laddr=127.0.0.1:6379 fd=10 name=*redacted* age=0 idle=0 flags=N capa= db=0 sub=0 psub=0 ssub=0 multi=-1 watch=0 qbuf=0 qbuf-free=0 argv-mem=12 multi-mem=0 rbs=16384 rbp=16384 obl=0 oll=0 omem=0 tot-mem=17060 events=r cmd=eval user=*redacted* redir=-1 resp=2 lib-name= lib-ver= tot-net-in=34 tot-net-out=0 tot-cmds=0
argc: 3
argv[0]: "eval"
argv[1]: 7 bytes
argv[2]: 1 bytes

------ EXECUTING CLIENT INFO ------
id=2 addr=127.0.0.1:41372 laddr=127.0.0.1:6379 fd=10 name=*redacted* age=0 idle=0 flags=N capa= db=0 sub=0 psub=0 ssub=0 multi=-1 watch=0 qbuf=0 qbuf-free=0 argv-mem=12 multi-mem=0 rbs=16384 rbp=16384 obl=0 oll=0 omem=0 tot-mem=17060 events=r cmd=eval user=*redacted* redir=-1 resp=2 lib-name= lib-ver= tot-net-in=34 tot-net-out=0 tot-cmds=0
argc: 3
argv[0]: "eval"
argv[1]: 7 bytes
argv[2]: 1 bytes

------ MODULES INFO OUTPUT ------

------ CONFIG DEBUG OUTPUT ------
repl-diskless-load disabled
debug-context ""
sanitize-dump-payload no
lazyfree-lazy-user-del yes
lazyfree-lazy-server-del yes
import-mode no
lazyfree-lazy-user-flush yes
list-compress-depth 0
dual-channel-replication-enabled no
repl-diskless-sync yes
activedefrag no
lazyfree-lazy-expire yes
io-threads 1
replica-read-only yes
client-query-buffer-limit 1gb
slave-read-only yes
lazyfree-lazy-eviction yes
proto-max-bulk-len 512mb

------ FAST MEMORY TEST ------
2595901:M 18 Jun 2025 01:20:12.921 # Bio worker thread #0 terminated
2595901:M 18 Jun 2025 01:20:12.921 # Bio worker thread #1 terminated
2595901:M 18 Jun 2025 01:20:12.921 # Bio worker thread #2 terminated
*** Preparing to test memory region 6530abce2000 (212992 bytes)
*** Preparing to test memory region 726f8af7f000 (2621440 bytes)
*** Preparing to test memory region 726f8b200000 (8388608 bytes)
*** Preparing to test memory region 726f8ba00000 (4194304 bytes)
*** Preparing to test memory region 726f8bffe000 (8388608 bytes)
*** Preparing to test memory region 726f8c7ff000 (8388608 bytes)
*** Preparing to test memory region 726f8d000000 (8388608 bytes)
*** Preparing to test memory region 726f8dc00000 (4194304 bytes)
*** Preparing to test memory region 726f8e290000 (16384 bytes)
*** Preparing to test memory region 726f8e3d2000 (20480 bytes)
*** Preparing to test memory region 726f8e5f8000 (32768 bytes)
*** Preparing to test memory region 726f8eb58000 (12288 bytes)
*** Preparing to test memory region 726f8eb5c000 (16384 bytes)
*** Preparing to test memory region 726f8ed63000 (4096 bytes)
*** Preparing to test memory region 726f8eef2000 (397312 bytes)
*** Preparing to test memory region 726f8efc7000 (4096 bytes)
.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O.O
Fast memory test PASSED, however your memory can still be broken. Please run a memory test for several hours if possible.

------ DUMPING CODE AROUND EIP ------
Symbol: (null) (base: (nil))
Module: /usr/lib/libc.so.6 (base 0x726f8e410000)
$ xxd -r -p /tmp/dump.hex /tmp/dump.bin
$ objdump --adjust-vma=(nil) -D -b binary -m i386:x86-64 /tmp/dump.bin
------

=== VALKEY BUG REPORT END. Make sure to include from START to END. ===
```

---------

Signed-off-by: Fusl <[email protected]>
Signed-off-by: Binbin <[email protected]>
Co-authored-by: Binbin <[email protected]>

Author: Fusl

src/lua/script_lua.c

@@ -1668,6 +1668,11 @@ void luaExtractErrorInformation(lua_State *lua, errorInfo *err_info) {
         err_info->ignore_err_stats_update = lua_toboolean(lua, -1);
     }
     lua_pop(lua, 1);
+
+    if (err_info->msg == NULL) {
+        /* Ensure we never return a NULL msg. */
+        err_info->msg = sdsnew("ERR unknown error");
+    }
 }
 
 /* This is the core of our Lua debugger, called each time Lua is about

tests/unit/scripting.tcl

@@ -2464,4 +2464,21 @@ start_server {tags {"scripting"}} {
         # Using a null byte never seemed to work with functions, so
         # we don't have a test for that case.
     }
+
+    test {EVAL - explicit error() call handling} {
+        # error("simple string error")
+        assert_error {ERR user_script:1: simple string error script: *} {
+            r eval "error('simple string error')" 0
+        }
+
+        # error({"err": "ERR table error"})
+        assert_error {ERR table error script: *} {
+            r eval "error({err='ERR table error'})" 0
+        }
+
+        # error({})
+        assert_error {ERR unknown error script: *} {
+            r eval "error({})" 0
+        }
+    }
 }