env_docker: make --cpus customizable

README.md

@@ -187,6 +187,23 @@ In the profiling UI choose a profile type, filter by instances (autocompleted) a
 
 For runnables that are both instrumented and profiled you can use [`e2eobs.AsObservable`](observable/observable.go).
 
+### Debugging flaky tests
+
+Sometimes tests might fail due to timing problems on highly CPU constrained systems such as GitHub actions. To facilitate fixing these issues, `e2e` supports limiting CPU time allocated to Docker containers through `E2E_DOCKER_CPUS` environment variable:
+
+```go mdox-exec="sed -n '285,288p' env_docker.go"
+	dockerCPUsEnv := os.Getenv(dockerCPUEnvName)
+	if dockerCPUsEnv != "" {
+		dockerCPUsParam = dockerCPUsEnv
+	}
+```
+
+You can set it either through command line parameters or `t.Setenv("E2E_DOCKER_CPUS", "...")`.
+
+Alternatively, you could pass `WithCPUs` through environment options so that some e2e test would have permanently reduced available CPU time.
+
+See what values you can pass to the `--cpus` flag on [Docker website](https://docs.docker.com/config/containers/resource_constraints/#configure-the-default-cfs-scheduler).
+
 ### Troubleshooting
 
 #### Can't create docker network

env.go

@@ -28,6 +28,13 @@ type environmentOptions struct {
 	name    string
 
 	volumes []string
+	cpus    string
+}
+
+func WithCPUs(cpus string) EnvironmentOption {
+	return func(o *environmentOptions) {
+		o.cpus = cpus
+	}
 }
 
 // WithLogger tells environment to use custom logger to default one (stdout).

env_docker.go

@@ -44,6 +44,7 @@ type DockerEnvironment struct {
 
 	hostAddr      string
 	dockerVolumes []string
+	cpus          string
 
 	registered map[string]struct{}
 	listeners  []EnvironmentListener
@@ -113,6 +114,7 @@ func New(opts ...EnvironmentOption) (_ *DockerEnvironment, err error) {
 		verbose:       e.verbose,
 		registered:    map[string]struct{}{},
 		dockerVolumes: e.volumes,
+		cpus:          e.cpus,
 	}
 
 	// Force a shutdown in order to cleanup from a spurious situation in case
@@ -269,12 +271,28 @@ func (e *DockerEnvironment) SharedDir() string {
 	return e.dir
 }
 
+const dockerCPUEnvName = "E2E_DOCKER_CPUS"
+
 func (e *DockerEnvironment) buildDockerRunArgs(name string, ports map[string]int, opts StartOptions) []string {
 	args := []string{"--rm", "--net=" + e.networkName, "--name=" + dockerNetworkContainerHost(e.networkName, name), "--hostname=" + name}
 
 	// Mount the docker env working directory into the container. It's shared across all containers to allow easier scenarios.
 	args = append(args, "-v", fmt.Sprintf("%s:%s:z", e.dir, e.dir))
 
+	// Allow reducing available CPU Time via environment variables or
+	// environment parameters. The latter takes precedence.
+	dockerCPUsParam := ""
+	dockerCPUsEnv := os.Getenv(dockerCPUEnvName)
+	if dockerCPUsEnv != "" {
+		dockerCPUsParam = dockerCPUsEnv
+	}
+	if e.cpus != "" {
+		dockerCPUsParam = e.cpus
+	}
+	if dockerCPUsParam != "" {
+		args = append(args, "--cpus", dockerCPUsParam)
+	}
+
 	for _, v := range e.dockerVolumes {
 		args = append(args, "-v", v)
 	}