RFC-79: Add virtual link support

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-Makefile.env
+Makefile.env
+
+# Jetbrain IDE
+.idea
+*.iml

rfcs/79-add-virtual-link-support.md

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+- Author: abyss-w <mad.hatter@foxmail.com>
+- Start Date: 2021-08-16
+- RFC PR: [beyondstorage/go-service-qingstor#79](https://github.com/beyondstorage/go-service-qingstor/pull/79)
+- Tracking Issue: [beyondstorage/go-service-qingstor#64](https://github.com/beyondstorage/go-service-qingstor/issues/64)
+
+# RFC-79: Add Virtual Link Support
+
+## Background
+
+As you can see from the [official documentation for qingstor](https://docs.qingcloud.com/qingstor/), it does not support symbolic links itself. However, qingstor supports [user-defined object metadata](https://docs.qingcloud.com/qingstor/api/common/metadata), and we can use this to simulate the implementation of symlink.
+
+## Proposal
+
+I propose to use user-defined object metadata to support virtual link.
+
+```go
+input := &service.PutObjectInput{
+    XQSMetadata: &map[string]string{
+        "x-qs-meta-bs-link-target": rt,
+    },
+}
+```
+
+- `PutObjectInput` is used to store the fields we need when calling `PutObjectWithContext` API to upload an object.
+- `XQSMetadata` is a map that stores user-defined metadata.
+  - `"x-qs-meta-bs-link-target"` is the name of user-defined metadata, the middle `bs` is used to avoid conflicts.
+  - `rt` is the symlink target, it is an absolute path.
+
+## Rationale
+
+### User-defined metadata
+
+As the [Object Metadata](https://docs.qingcloud.com/qingstor/api/common/metadata) says,there are two types of object metadata that can be changed by the user: standard HTTP headers and user-defined metadata. We can define our own metadata to store the fields we need when uploading an object. Note that the name of the user-defined metadata must start with `x-qs-meta`.
+
+### Drawbacks
+
+As qingstor itself does not support symlink, we can only simulate it. And the object created is not really a symlink object. When we call `stat`, we can only tell if it is a symlink by using user-defined metadata,
+
+```go
+if v, ok := metadata["x-qs-meta-bs-link-target"]; ok {
+	// The path is a symlink object 
+}
+```
+
+Calling `HeadObject` in `list` will increase the execution cost of `list` which we cannot afford. So we will relax the qingstor condition. We will not support getting the exact symlink object type in `list` when user has virtual link enabled, if the user wants to get the exact object mode they need to call `stat`.
+
+## Compatibility
+
+N/A
+
+## Implementation
+
+- Implement `virtual_link` in go-service-qingstor
+- Support `stat`
+- Setup linker tests
+