docker logs --timestamps does not correctly handle long logs (>16kiB) that were split when stored

[jonaslb]

Description

Long logs get split into 16 kiB segments by the log driver (at least the json lines one).
When requesting --timestamps, we would expect to see a timestamp at the start of the log line, but the timestamp is also injected into the middle of the output for every 16k characters.

This was inspired from grafana/loki#12197, as I thought "surely the docker cli does this without mangling the logs" but sadly it seems not :( I guess that, when requesting timestamps, it is not enough to directly forward the byte streams - the timestamps should be stripped from frames that have not immediately followed a newline.

Reproduce

1. container=$(docker create python:latest python3 -c 'print("-" * 16400)')
2. docker start $container
3. docker logs --timestamps $container
4. Observe the presence of two timestamps, with the second one having been injected after 16384 characters.

Expected behavior

The CLI should show timestamps at the start of the line, but not in the middle of the lines.

docker version

Client: Docker Engine - Community
 Version:           24.0.7
 API version:       1.43
 Go version:        go1.20.10
 Git commit:        afdd53b
 Built:             Thu Oct 26 09:07:41 2023
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          24.0.7
  API version:      1.43 (minimum version 1.12)
  Go version:       go1.20.10
  Git commit:       311b9ff
  Built:            Thu Oct 26 09:07:41 2023
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.6.25
  GitCommit:        d8f198a4ed8892c764191ef7b3b06d8a2eeb5c7f
 runc:
  Version:          1.1.10
  GitCommit:        v1.1.10-0-g18a0cb0
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info

Client: Docker Engine - Community
 Version:    24.0.7
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.11.2
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.21.0
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 6
  Running: 4
  Paused: 0
  Stopped: 2
 Images: 42
 Server Version: 24.0.7
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: active
  NodeID: woua6888bdr2pa6qs2jwl7nnu
  Is Manager: true
  ClusterID: t5vjx0ip74vvgf58v3xg0pbks
  Managers: 1
  Nodes: 1
  Default Address Pool: 10.0.0.0/8
  SubnetSize: 24
  Data Path Port: 4789
  Orchestration:
   Task History Retention Limit: 5
  Raft:
   Snapshot Interval: 10000
   Number of Old Snapshots to Retain: 0
   Heartbeat Tick: 1
   Election Tick: 10
  Dispatcher:
   Heartbeat Period: 5 seconds
  CA Configuration:
   Expiry Duration: 3 months
   Force Rotate: 0
  Autolock Managers: false
  Root Rotation In Progress: false
  Node Address: 172.19.158.143
  Manager Addresses:
   172.19.158.143:2377
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: d8f198a4ed8892c764191ef7b3b06d8a2eeb5c7f
 runc version: v1.1.10-0-g18a0cb0
 init version: de40ad0
 Security Options:
  seccomp
   Profile: builtin
 Kernel Version: 5.15.146.1-microsoft-standard-WSL2
 Operating System: Ubuntu 23.10
 OSType: linux
 Architecture: x86_64
 CPUs: 12
 Total Memory: 15.62GiB
 Name: LPCP2158
 ID: 7CGJ:WOZO:CHP4:FBWC:PIJ4:JX4C:DQU7:I27X:IHY2:HCZQ:2CH3:BHIB
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: No blkio throttle.read_bps_device support
WARNING: No blkio throttle.write_bps_device support
WARNING: No blkio throttle.read_iops_device support
WARNING: No blkio throttle.write_iops_device support

Additional Info

No response