How can I get Container Logs using Golang? (Error) - docker

Question

I am trying to code a Docker Monitoring software in Golang.
my Code looks as followed:
package main
import (
"bytes"
"context"
"fmt"
"time"
"github.com/docker/docker/api/types"
"github.com/docker/docker/client"
)
func main() {
ctx := context.Background()
cli, err := client.NewClientWithOpts(client.FromEnv)
if err != nil {
panic(err)
}
containers, err := cli.ContainerList(ctx, types.ContainerListOptions{})
if err != nil {
panic(err)
}
for _, container := range containers {
out, err := cli.ContainerLogs(ctx, container.ID, types.ContainerLogsOptions{
ShowStderr: true,
ShowStdout: true,
Timestamps: false,
Follow: true,
Tail: "40"})
if err != nil {
panic(err)
}
fmt.Println("The \"" + container.Image + "\" container, with the ID \"" + container.ID + "\" logged: ")
fmt.Println()
buf := new(bytes.Buffer)
fmt.Println(buf.ReadFrom(out))
fmt.Println(buf.String())
}
time.Sleep(time.Second * 3)
}
The problem is that the execution of the above code stops on the fmt.Println(buf.ReadFrom(out)) statement. The code used to work, but it suddenly just doesn't anymore. Either it stops without an error, or it returns an empty String.
The client I am trying to collect the logs from is also coded by myself, and it looks like follows:
package main
import (
"log"
"time"
)
func main() {
for i := 0; i > -1; i++ {
log.Output(1, "Hello World logged!")
time.Sleep(time.Minute)
}
}
I already tried debugging and checking Variables, but I just can't get to the source of the Problem.

Answer 1

I am really not sure as I don't have any error logs to confirm my hypothesis.
But could it be the case that as the ContainerLogs returns a stream (io.ReadCloser), maybe the stream itself hasn't closed yet?
If this is a possibility, you can either do a dry run first to test out this theory by adding a timeout and logging it after every small duration ?
one possible way to do this is
select {
case <-time.After(5 * time.Second):
fmt.Println("Timeout exceeded while reading container logs")
case <-ctx.Done():
fmt.Println("Context cancelled while reading container logs")
case b := <-out:
if b != nil {
buf.Write(b)
}
}