BCP-21: Parse Profile Input

docs/rfcs/21-parse-profile-input.md

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+- Author: Prnyself <281239768@qq.com>
+- Start Date: 2021-08-24
+- RFC PR: [beyondstorage/beyond-ctl#21](https://github.com/beyondstorage/beyond-ctl/pull/21)
+- Tracking Issue: [beyondstorage/beyond-ctl#22](https://github.com/beyondstorage/beyond-ctl/issues/22)
+
+# BCP-21: Parse Profile Input
+
+## Background
+
+From [BCP-7], we have introduced profile support, and we can simplify our input with profile name, like:
+
+```
+beyondctl ls test-s3:dir_to_list
+```
+
+However, for local fs, all it need is to initialize a storager is the `work_dir`, so it introduces another inconvenience
+when using local fs.
+
+Moreover, shall we also support input connection string instead of adding profile before? Shall we support adding
+profile by environment variables?
+
+## Proposal
+
+So I propose to add following rules when parse input:
+
+- using `:` as the separator between `profile` and `key`, such as `test-s3:dir_to_list`.
+- try to find `profile` with given `name`
+    - if found, use `profile`'s connection string, error would return if connection string
+      invalid
+    - if not, a `profile not found` error would return
+- if no separator found, treat input as local fs path
+    - if the path is an absolute path, set `/` as `work_dir`, and the input as `key`
+    - if the path is a relative path, set `pwd` as `work_dir`, and the input as `key`
+
+### env support
+
+For now, we initialize `profile` list from config file, and modify config file when execute `add` or `remove` `profile`
+command. To support set profile by environment, we should specify the prefix of environment variables which will be used
+as profile key and value.
+
+For `BEYOND_CTL_PROFILE_xxxx = s3://xxxx`, a profile with name `xxxx` and connection string
+`s3://xxxx` will be merged into config profile, and can be used in input, without adding into config file.
+
+Moreover, to avoid writing environment variable profile into config file, we do not support using environment variables
+in `profile` commands.
+
+## Rationale
+
+### Why not support raw connection string from input?
+
+- First, the connection is usually too long to input.
+- Second, there would be some sensitive data such as sk or password in connection string, which is not appropriate to
+  show in input string.
+- Third, we add support for get profile from environment variable, so it is unnecessary for user to input raw
+  connection.
+
+### Why not support env vars in profile commands?
+
+It will introduce complex problems and situation into profiles. For example
+
+```
+BEYOND_CTL_PROFILE_key1 = s3://xxxx beyondctl add key1 fs:///xxx 
+```
+
+This will lead into a conflict between input and environment variable, which is meaningless.
+
+## Compatibility
+
+None.
+
+## Implementation
+
+1. Add input parser by rules listed in proposal
+2. Support reading environment variables with given prefix, and merge them into current `profile`
+
+[BCP-7]: https://github.com/beyondstorage/beyond-ctl/blob/master/docs/rfcs/7-add-profile-support.md