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a793564b25
Moving all strings to the errors package wasn't a good idea after all.
Our custom implementation of Go errors predates everything that's nice
and good about working with errors in Go. Take as an example what we
have to do to get an error message:
```go
func GetErrorMessage(err error) string {
switch err.(type) {
case errcode.Error:
e, _ := err.(errcode.Error)
return e.Message
case errcode.ErrorCode:
ec, _ := err.(errcode.ErrorCode)
return ec.Message()
default:
return err.Error()
}
}
```
This goes against every good practice for Go development. The language already provides a simple, intuitive and standard way to get error messages, that is calling the `Error()` method from an error. Reinventing the error interface is a mistake.
Our custom implementation also makes very hard to reason about errors, another nice thing about Go. I found several (>10) error declarations that we don't use anywhere. This is a clear sign about how little we know about the errors we return. I also found several error usages where the number of arguments was different than the parameters declared in the error, another clear example of how difficult is to reason about errors.
Moreover, our custom implementation didn't really make easier for people to return custom HTTP status code depending on the errors. Again, it's hard to reason about when to set custom codes and how. Take an example what we have to do to extract the message and status code from an error before returning a response from the API:
```go
switch err.(type) {
case errcode.ErrorCode:
daError, _ := err.(errcode.ErrorCode)
statusCode = daError.Descriptor().HTTPStatusCode
errMsg = daError.Message()
case errcode.Error:
// For reference, if you're looking for a particular error
// then you can do something like :
// import ( derr "github.com/docker/docker/errors" )
// if daError.ErrorCode() == derr.ErrorCodeNoSuchContainer { ... }
daError, _ := err.(errcode.Error)
statusCode = daError.ErrorCode().Descriptor().HTTPStatusCode
errMsg = daError.Message
default:
// This part of will be removed once we've
// converted everything over to use the errcode package
// FIXME: this is brittle and should not be necessary.
// If we need to differentiate between different possible error types,
// we should create appropriate error types with clearly defined meaning
errStr := strings.ToLower(err.Error())
for keyword, status := range map[string]int{
"not found": http.StatusNotFound,
"no such": http.StatusNotFound,
"bad parameter": http.StatusBadRequest,
"conflict": http.StatusConflict,
"impossible": http.StatusNotAcceptable,
"wrong login/password": http.StatusUnauthorized,
"hasn't been activated": http.StatusForbidden,
} {
if strings.Contains(errStr, keyword) {
statusCode = status
break
}
}
}
```
You can notice two things in that code:
1. We have to explain how errors work, because our implementation goes against how easy to use Go errors are.
2. At no moment we arrived to remove that `switch` statement that was the original reason to use our custom implementation.
This change removes all our status errors from the errors package and puts them back in their specific contexts.
IT puts the messages back with their contexts. That way, we know right away when errors used and how to generate their messages.
It uses custom interfaces to reason about errors. Errors that need to response with a custom status code MUST implementent this simple interface:
```go
type errorWithStatus interface {
HTTPErrorStatusCode() int
}
```
This interface is very straightforward to implement. It also preserves Go errors real behavior, getting the message is as simple as using the `Error()` method.
I included helper functions to generate errors that use custom status code in `errors/errors.go`.
By doing this, we remove the hard dependency we have eeverywhere to our custom errors package. Yes, you can use it as a helper to generate error, but it's still very easy to generate errors without it.
Please, read this fantastic blog post about errors in Go: http://dave.cheney.net/2014/12/24/inspecting-errors
Signed-off-by: David Calavera <david.calavera@gmail.com>
104 lines
3 KiB
Go
104 lines
3 KiB
Go
package httputils
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import (
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"encoding/json"
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"fmt"
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"io"
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"net/http"
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"strings"
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"golang.org/x/net/context"
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"github.com/docker/docker/api"
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"github.com/docker/docker/pkg/version"
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)
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// APIVersionKey is the client's requested API version.
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const APIVersionKey = "api-version"
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// APIFunc is an adapter to allow the use of ordinary functions as Docker API endpoints.
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// Any function that has the appropriate signature can be register as a API endpoint (e.g. getVersion).
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type APIFunc func(ctx context.Context, w http.ResponseWriter, r *http.Request, vars map[string]string) error
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// HijackConnection interrupts the http response writer to get the
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// underlying connection and operate with it.
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func HijackConnection(w http.ResponseWriter) (io.ReadCloser, io.Writer, error) {
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conn, _, err := w.(http.Hijacker).Hijack()
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if err != nil {
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return nil, nil, err
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}
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// Flush the options to make sure the client sets the raw mode
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conn.Write([]byte{})
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return conn, conn, nil
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}
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// CloseStreams ensures that a list for http streams are properly closed.
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func CloseStreams(streams ...interface{}) {
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for _, stream := range streams {
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if tcpc, ok := stream.(interface {
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CloseWrite() error
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}); ok {
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tcpc.CloseWrite()
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} else if closer, ok := stream.(io.Closer); ok {
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closer.Close()
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}
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}
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}
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// CheckForJSON makes sure that the request's Content-Type is application/json.
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func CheckForJSON(r *http.Request) error {
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ct := r.Header.Get("Content-Type")
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// No Content-Type header is ok as long as there's no Body
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if ct == "" {
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if r.Body == nil || r.ContentLength == 0 {
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return nil
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}
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}
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// Otherwise it better be json
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if api.MatchesContentType(ct, "application/json") {
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return nil
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}
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return fmt.Errorf("Content-Type specified (%s) must be 'application/json'", ct)
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}
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// ParseForm ensures the request form is parsed even with invalid content types.
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// If we don't do this, POST method without Content-type (even with empty body) will fail.
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func ParseForm(r *http.Request) error {
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if r == nil {
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return nil
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}
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if err := r.ParseForm(); err != nil && !strings.HasPrefix(err.Error(), "mime:") {
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return err
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}
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return nil
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}
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// ParseMultipartForm ensure the request form is parsed, even with invalid content types.
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func ParseMultipartForm(r *http.Request) error {
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if err := r.ParseMultipartForm(4096); err != nil && !strings.HasPrefix(err.Error(), "mime:") {
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return err
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}
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return nil
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}
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// WriteJSON writes the value v to the http response stream as json with standard json encoding.
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func WriteJSON(w http.ResponseWriter, code int, v interface{}) error {
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w.Header().Set("Content-Type", "application/json")
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w.WriteHeader(code)
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return json.NewEncoder(w).Encode(v)
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}
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// VersionFromContext returns an API version from the context using APIVersionKey.
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// It panics if the context value does not have version.Version type.
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func VersionFromContext(ctx context.Context) (ver version.Version) {
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if ctx == nil {
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return
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}
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val := ctx.Value(APIVersionKey)
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if val == nil {
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return
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}
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return val.(version.Version)
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}
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