go.blog/context: an article about go.net/context.Context.

Blog demo (internal):
http://olivia.nyc.corp.google.com:8081/context

Server demo (internal):
http://olivia.nyc.corp.google.com:8080/search?q=golang&timeout=1s

LGTM=bcmills, adg, r
R=r, rsc, adg, bcmills, ken, adonovan, dsymonds, crawshaw, campoy, hakim, dneil
https://golang.org/cl/116820044

1 files changed, 38 insertions, 0 deletions

diff --git a/content/context/userip/userip.go b/content/context/userip/userip.go
new file mode 100644
index 0000000..3001ade
--- /dev/null
+++ b/content/context/userip/userip.go
@@ -0,0 +1,38 @@
+// Package userip provides functions for extracting a user IP address from a
+// request and associating it with a Context.
+package userip
+
+import (
+	"fmt"
+	"net"
+	"net/http"
+	"strings"
+
+	"code.google.com/p/go.net/context"
+)
+
+// FromRequest extracts the user IP address from req, if present.
+func FromRequest(req *http.Request) (net.IP, error) {
+	s := strings.SplitN(req.RemoteAddr, ":", 2)
+	userIP := net.ParseIP(s[0])
+	if userIP == nil {
+		return nil, fmt.Errorf("userip: %q is not IP:port", req.RemoteAddr)
+	}
+	return userIP, nil
+}
+
+// The address &key is the context key.
+var key struct{}
+
+// NewContext returns a new Context carrying userIP.
+func NewContext(ctx context.Context, userIP net.IP) context.Context {
+	return context.WithValue(ctx, &key, userIP)
+}
+
+// FromContext extracts the user IP address from ctx, if present.
+func FromContext(ctx context.Context) (net.IP, bool) {
+	// ctx.Value returns nil if ctx has no value for the key;
+	// the net.IP type assertion returns ok=false for nil.
+	userIP, ok := ctx.Value(&key).(net.IP)
+	return userIP, ok
+}