huygn / ctxfix

ctxfix provides rudimentary rewriting to assist in migrating from golang.org/x/net/context to Go 1.7's context package

Introduction

Go 1.7 brought introduced the context package, which can be used instead of golang.org/x/net/context.

ctxfix assists in migration code to the new package, and was purpose built to solve one problem - it's not feature complete and will probably not work as intended on your base.

• It does not write correct Go code, it just attempts some common rewrites, this is only due to time constraints
• You will still need to run goimports to format the import paths correctly
• Backups of files are not taken, it's assumed your VCS will show you the changes made and allow reverting
• It's likely some decent sed will get you further than this tool

This tool was used to migrate to the new context package on a couple code bases, but only with the combination of hand editing.

Although I'd like this tool to help everyone, it's unlikely to do so, and I'm unlikely to support further changes as it's already done its job for me. It's simply made available for others in case it helps them. Discussions welcome, but this won't likely be a supported tool.

What it does

• Finds all .go files in the local directory, parse using go/ast (no type checking)
• Rewrite (in place) import of golang.org/x/net/context to context
• Find function declarations containing context.Context and *http.Request, remove context.Context
• Within those functions, changes the local variable for context.Context (probably ctx) and rewrite to *http.Request's variable with its context (e.g. r.Context())

You'll still need to:

• Handle setting of the context values, this only deals with reading of the values
• Run goimports to reorder the context import (in hindsight the rewrite is redundant if we're running goimports anyway)
• There's a countless edge cases that this tool didn't need to handle, which you may need to

Example

Example of the changes made, here's an application which stores the User-Agent inside context, following the pattern outlined: https://blog.golang.org/error-handling-and-go#TOC_3. as seen in testdata/in.go:

package main

import (
        "fmt"
        "net/http"
        "strings"

        "golang.org/x/net/context"
)

func main() {
        http.Handle("/", httpHandler(homeHandler))
        http.ListenAndServe(":3000", nil)
}

type httpHandler func(context.Context, http.ResponseWriter, *http.Request) (int, error)

func (h httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
        ctx := context.Background()

        ua := r.Header.Get("User-Agent")
        ctx = context.WithValue(ctx, "ua", ua)

        code, err := h(ctx, w, r)
        if err != nil {
                http.Error(w, http.StatusText(code), code)
        }
}

func homeHandler(ctx context.Context, w http.ResponseWriter, r *http.Request) (int, error) {
        fmt.Fprint(w, "Hello: ", ctx.Value("ua"))
        return http.StatusOK, nil
}

ctxfix will adjust the import and homeHandler signature and body correctly, as seen in testdata/ctxfix.go:

-func homeHandler(ctx context.Context, w http.ResponseWriter, r *http.Request) (int, error) {
-       fmt.Fprint(w, "Hello: ", ctx.Value("ua"))
+func homeHandler(w http.ResponseWriter, r *http.Request) (int, error) {
+       fmt.Fprint(w, "Hello: ", r.Context().Value("ua"))

But you are expected to rewrite the httpHandler signature and ServeHTTP method:

-type httpHandler func(context.Context, http.ResponseWriter, *http.Request) (int, error)
+type httpHandler func(http.ResponseWriter, *http.Request) (int, error)

 func (h httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-       ctx := context.Background()
-
        ua := r.Header.Get("User-Agent")
-       ctx = context.WithValue(ctx, "ua", ua)
+       ctx := context.WithValue(r.Context(), "ua", ua)
+       r = r.WithContext(ctx)

-       code, err := h(ctx, w, r)
+       code, err := h(w, r)
        if err != nil {
                http.Error(w, http.StatusText(code), code)
        }
 }

For after ctxfix and hand editing, as seen in testdata/final.go:

package main

import (
        "fmt"
        "net/http"

        "context"
)

func main() {
        http.Handle("/", httpHandler(homeHandler))
        http.ListenAndServe(":3000", nil)
}

type httpHandler func(http.ResponseWriter, *http.Request) (int, error)

func (h httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
        ua := r.Header.Get("User-Agent")
        ctx := context.WithValue(r.Context(), "ua", ua)
        r = r.WithContext(ctx)

        code, err := h(w, r)
        if err != nil {
                http.Error(w, http.StatusText(code), code)
        }
}

func homeHandler(w http.ResponseWriter, r *http.Request) (int, error) {
        fmt.Fprint(w, "Hello: ", r.Context().Value("ua"))
        return http.StatusOK, nil
}