use generic default container annotations for kubectl

Signed-off-by: pacoxu <paco.xu@daocloud.io>

keps/sig-cli/20201216-kubectl-default-container.md

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+---
+title: Default Container for Kubectl logs and exec
+authors:
+  - "@pacoxu"
+  - "@mfojtik"
+owning-sig: sig-cli
+participating-sigs:
+reviewers:
+  - "@dougsland"
+  - "@eddiezane"
+  - "@soltysh"
+  - "@howardjohn"
+
+approvers:
+  - ""
+
+editor: TBD
+creation-date: 2020-12-16
+last-updated: 2020-12-24
+status: provisional
+see-also:
+replaces:
+superseded-by:
+---
+
+# default container behavior
+
+## Table of Contents
+
+<!-- toc -->
+- [Summary](#summary)
+- [Motivation](#motivation)
+  - [Goals](#goals)
+  - [Non-Goals](#non-goals)
+- [Proposal](#proposal)
+  - [Implementation Details](#implementation-details)
+  - [Risks and Mitigations](#risks-and-mitigations)
+- [Graduation Criteria](#graduation-criteria)
+- [Alternatives](#alternatives)
+<!-- /toc -->
+
+## Summary
+
+Many Kubectl commands make requests to specific containers, `kubectl logs` and `kubectl exec` are most commonly
+used.
+
+All commands that need to specify container name:
+
+- kubectl attach - Attach to a running container
+- kubectl cp - Copy files and directories to and from containers.
+- kubectl exec - Execute a command in a container
+
+These three commands above are similar. If omitted, the first container in the pod will be chosen.
+
+
+- kubectl logs - Print the logs for a container in a pod
+
+`kubectl logs` is a little different. Not default value for it. 
+However it support to logs all containers with `--all-containers`.
+
+https://github.com/kubernetes/kubernetes/pull/87809 already add an annotation named
+`kubectl.kubernetes.io/default-logs-container` for log to use as default.
+
+We don't have a general default container name attribute for pod, and this would change pod spec and not acceptable.
+Why not use `kubectl.kubernetes.io/default-container` as the default container name annotation for all four commands
+above?
+
+
+**Note:** No server-side changes are required for this, all Request and Response template expansion is performed on
+the client side.
+
+## Motivation
+
+
+As the default log annotation is added, the motivation of it is like below
+> Kubectl provides a number of commands to simplify working with Kubernetes by making requests to
+> Resources and SubResources.  These requests are mostly static, with fields filled in by user
+> supplied flags.  Today the commands are compiled into the client, which as the following challenges:
+
+Sidecars are populor now.
+Alternatively, a way to generic describe the default container for anything that happens to need a "default", which could also be used by external tooling.
+
+Issues https://github.com/kubernetes/kubernetes/issues/96986 opened by @howardjohn 
+> However, it gets worse because aside from a warning, this couples the default exec container to the container ordering. 
+> The container ordering also happens to have an impact on container startup ordering. We have started offering an option 
+> to inject our sidecar as the first container (previously, it was the last one), which has resulted in users running 
+> kubectl exec and getting the "wrong" container.
+
+
+### Goals
+
+
+- Provide a way to know which is the default container in pod for cli: kubectl
+
+
+### Non-Goals
+
+- If the cli is not kubectl, we don't determine which is the default container.
+
+
+## Proposal
+
+A single and generic annotation for all above commands like `kubectl.kubernetes.io/default-container` is a good choice to avoid needing many new annotations in the future.
+
+As logs and exec are frurently used by users, we start from these two, but there would be more in the future.
+
+### Implementation Details
+
+**Publishing Data:**
+
+Beta:  default container annotation
+
+- in discussion, https://github.com/kubernetes/kubernetes/issues/95293
+- (WIP, will drop this design and use the generic annotation later)  Users might specify the `kubectl.kubernetes.io/default-container` annotation in a Pod to preselect container for kubectl exec and all kubectl commands.
+WIP in https://github.com/kubernetes/kubernetes/pull/97099
+
+
+**Data Command Structure:**
+
+
+**Example Command:**
+
+
+
+### Risks and Mitigations
+
+- It is not compatible with `kubectl.kubernetes.io/default-logs-container` for kubectl log.
+
+## Graduation Criteria
+
+- a general way to get default container in pod
+
+## Alternatives
+
+