SIGSEGV in in_opentelemetry plugin when receiving OTLP/gRPC log with KeyValue missing a Value

[mkuzdzal]

Bug Report

Describe the bug
Fluent Bit crashes with a segmentation fault (SIGSEGV) when a log sent via OTLP/gRPC contains a KeyValue with no value set.

To Reproduce
Send the following log using OpenTelemetry gRPC (protos directory contains OTLP .proto files):

grpcurl -plaintext \
-import-path './protos' \
-proto './protos/opentelemetry/proto/collector/v1/logs/v1/logs_service.proto' \
-d '{
  "resourceLogs": [
    {
      "scopeLogs": [
        {
          "scope": {},
          "logRecords": [
            {
              "timeUnixNano": "1722904465173450100",
              "body": {
                "kvlistValue": {
                  "values": [
                    {
                      "key": "key_1"
                    }
                  ]
                }
              }
            }
          ]
        }
      ]
    }
  ]
}' \
localhost:4317 opentelemetry.proto.collector.logs.v1.LogsService/Export

Fluent Bit crashes with the following stack trace:

Fluent Bit v4.0.5
* Copyright (C) 2015-2025 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

______ _                  _    ______ _ _             ___  _____
|  ___| |                | |   | ___ (_) |           /   ||  _  |
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __/ /| || |/' |
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / / /_| ||  /| |
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /\___  |\ |_/ /
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/     |_(_)___/


[2025/08/01 17:56:17] [ info] Configuration:
[2025/08/01 17:56:17] [ info]  flush time     | 1.000000 seconds
[2025/08/01 17:56:17] [ info]  grace          | 5 seconds
[2025/08/01 17:56:17] [ info]  daemon         | 0
[2025/08/01 17:56:17] [ info] ___________
[2025/08/01 17:56:17] [ info]  inputs:
[2025/08/01 17:56:17] [ info]      opentelemetry
[2025/08/01 17:56:17] [ info] ___________
[2025/08/01 17:56:17] [ info]  filters:
[2025/08/01 17:56:17] [ info] ___________
[2025/08/01 17:56:17] [ info]  outputs:
[2025/08/01 17:56:17] [ info]      stdout.0
[2025/08/01 17:56:17] [ info] ___________
[2025/08/01 17:56:17] [ info]  collectors:
[2025/08/01 17:56:17] [ info] [fluent bit] version=4.0.5, commit=d9be919eab, pid=1
[2025/08/01 17:56:17] [debug] [engine] coroutine stack size: 196608 bytes (192.0K)
[2025/08/01 17:56:17] [ info] [storage] ver=1.5.3, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2025/08/01 17:56:17] [ info] [simd    ] NEON
[2025/08/01 17:56:17] [ info] [cmetrics] version=1.0.4
[2025/08/01 17:56:17] [ info] [ctraces ] version=0.6.6
[2025/08/01 17:56:17] [ info] [input:opentelemetry:opentelemetry.0] initializing
[2025/08/01 17:56:17] [ info] [input:opentelemetry:opentelemetry.0] storage_strategy='memory' (memory only)
[2025/08/01 17:56:17] [debug] [opentelemetry:opentelemetry.0] created event channels: read=25 write=26
[2025/08/01 17:56:17] [debug] [downstream] listening on 0.0.0.0:4317
[2025/08/01 17:56:17] [ info] [input:opentelemetry:opentelemetry.0] listening on 0.0.0.0:4317
[2025/08/01 17:56:17] [debug] [stdout:stdout.0] created event channels: read=28 write=29
[2025/08/01 17:56:17] [ info] [sp] stream processor started
[2025/08/01 17:56:17] [ info] [engine] Shutdown Grace Period=5, Shutdown Input Grace Period=2
[2025/08/01 17:56:17] [ info] [output:stdout:stdout.0] worker #0 started
[2025/08/01 17:56:41] [engine] caught signal (SIGSEGV)
#0  0xacce547f2b6f      in  otel_pack_kvlist() at plugins/in_opentelemetry/opentelemetry_logs.c:106
#1  0xacce547f2b6f      in  otlp_pack_any_value() at plugins/in_opentelemetry/opentelemetry_logs.c:167
#2  0xacce547f3537      in  binary_payload_to_msgpack() at plugins/in_opentelemetry/opentelemetry_logs.c:530
#3  0xacce547f3537      in  opentelemetry_process_logs() at plugins/in_opentelemetry/opentelemetry_logs.c:639
#4  0xacce547f2047      in  opentelemetry_prot_handle_ng() at plugins/in_opentelemetry/opentelemetry_prot.c:1272
#5  0xacce548eadcf      in  flb_http_server_client_activity_event_handler() at src/http_server/flb_http_server.c:225
#6  0xacce5476a3b3      in  flb_engine_start() at src/flb_engine.c:1131
#7  0xacce54746de3      in  flb_lib_worker() at src/flb_lib.c:835
#8  0xf1bb81ecee9f      in  ???() at ???:0
#9  0xf1bb81f37b1b      in  ???() at ???:0
#10 0xffffffffffffffff  in  ???() at ???:0

Expected behavior
Logs should be successfully received and correctly handled.

Screenshots
N/A

Your Environment

• Version used: v4.0.5
• Configuration:

service:
  log_level: trace

pipeline:
  inputs:
    - name: opentelemetry
      port: 4317

  outputs:
    - name: stdout
      match: '*'

• Environment name and version (e.g. Kubernetes? What version?): Docker
• Server type and version: N/A
• Operating System and version: fluent/fluent-bit:4.0.5
• Filters and plugins: built-in OpenTelemetry input plugin