maintain envdecode inside ocis-pkg

ocis-pkg/config/envdecode/README.md

@@ -0,0 +1,88 @@
+`envdecode` is a Go package for populating structs from environment
+variables. It's basically a fork of https://github.com/joeshaw/envdecode,
+but changed to support multiple environment variables (precedence).
+
+`envdecode` uses struct tags to map environment variables to fields,
+allowing you you use any names you want for environment variables.
+`envdecode` will recurse into nested structs, including pointers to
+nested structs, but it will not allocate new pointers to structs.
+
+## API
+
+Full API docs are available on
+[godoc.org](https://godoc.org/github.com/owncloud/ocis/ocis-pkg/config/envdecode).
+
+Define a struct with `env` struct tags:
+
+```go
+type Config struct {
+    Hostname  string `env:"SERVER_HOSTNAME,default=localhost"`
+    Port      uint16 `env:"HTTP_PORT;SERVER_PORT,default=8080"`
+
+    AWS struct {
+        ID        string   `env:"AWS_ACCESS_KEY_ID"`
+        Secret    string   `env:"AWS_SECRET_ACCESS_KEY,required"`
+        SnsTopics []string `env:"AWS_SNS_TOPICS"`
+    }
+
+    Timeout time.Duration `env:"TIMEOUT,default=1m,strict"`
+}
+```
+
+Fields _must be exported_ (i.e. begin with a capital letter) in order
+for `envdecode` to work with them. An error will be returned if a
+struct with no exported fields is decoded (including one that contains
+no `env` tags at all).
+Default values may be provided by appending ",default=value" to the
+struct tag. Required values may be marked by appending ",required" to the
+struct tag. Strict values may be marked by appending ",strict" which will
+return an error on Decode if there is an error while parsing.
+
+Then call `envdecode.Decode`:
+
+```go
+var cfg Config
+err := envdecode.Decode(&cfg)
+```
+
+If you want all fields to act `strict`, you may use `envdecode.StrictDecode`:
+
+```go
+var cfg Config
+err := envdecode.StrictDecode(&cfg)
+```
+
+All parse errors will fail fast and return an error in this mode.
+
+## Supported types
+
+- Structs (and pointer to structs)
+- Slices of below defined types, separated by semicolon
+- `bool`
+- `float32`, `float64`
+- `int`, `int8`, `int16`, `int32`, `int64`
+- `uint`, `uint8`, `uint16`, `uint32`, `uint64`
+- `string`
+- `time.Duration`, using the [`time.ParseDuration()` format](http://golang.org/pkg/time/#ParseDuration)
+- `*url.URL`, using [`url.Parse()`](https://godoc.org/net/url#Parse)
+- Types those implement a `Decoder` interface
+
+## Custom `Decoder`
+
+If you want a field to be decoded with custom behavior, you may implement the interface `Decoder` for the filed type.
+
+```go
+type Config struct {
+  IPAddr IP `env:"IP_ADDR"`
+}
+
+type IP net.IP
+
+// Decode implements the interface `envdecode.Decoder`
+func (i *IP) Decode(repl string) error {
+  *i = net.ParseIP(repl)
+  return nil
+}
+```
+
+`Decoder` is the interface implemented by an object that can decode an environment variable string representation of itself.