internal: give more useful panics when (mis)using context.Background

internal/api.go

@@ -227,6 +227,8 @@ type context struct {
 
 var contextKey = "holds a *context"
 
+// fromContext returns the App Engine context or nil if ctx is not
+// derived from an App Engine context.
 func fromContext(ctx netcontext.Context) *context {
 	c, _ := ctx.Value(&contextKey).(*context)
 	return c
@@ -468,7 +470,7 @@ func Call(ctx netcontext.Context, service, method string, in, out proto.Message)
 	c := fromContext(ctx)
 	if c == nil {
 		// Give a good error message rather than a panic lower down.
-		return errors.New("not an App Engine context")
+		return errNotAppEngineContext
 	}
 
 	// Apply transaction modifications if we're in a transaction.

internal/api_classic.go

@@ -22,14 +22,20 @@ import (
 
 var contextKey = "holds an appengine.Context"
 
+// fromContext returns the App Engine context or nil if ctx is not
+// derived from an App Engine context.
 func fromContext(ctx netcontext.Context) appengine.Context {
 	c, _ := ctx.Value(&contextKey).(appengine.Context)
 	return c
 }
 
 // This is only for classic App Engine adapters.
-func ClassicContextFromContext(ctx netcontext.Context) appengine.Context {
-	return fromContext(ctx)
+func ClassicContextFromContext(ctx netcontext.Context) (appengine.Context, error) {
+	c := fromContext(ctx)
+	if c == nil {
+		return nil, errNotAppEngineContext
+	}
+	return c, nil
 }
 
 func withContext(parent netcontext.Context, c appengine.Context) netcontext.Context {
@@ -98,7 +104,7 @@ func Call(ctx netcontext.Context, service, method string, in, out proto.Message)
 	c := fromContext(ctx)
 	if c == nil {
 		// Give a good error message rather than a panic lower down.
-		return errors.New("not an App Engine context")
+		return errNotAppEngineContext
 	}
 
 	// Apply transaction modifications if we're in a transaction.

internal/api_common.go

@@ -5,12 +5,15 @@
 package internal
 
 import (
+	"errors"
 	"os"
 
 	"github.com/golang/protobuf/proto"
 	netcontext "golang.org/x/net/context"
 )
 
+var errNotAppEngineContext = errors.New("not an App Engine context")
+
 type CallOverrideFunc func(ctx netcontext.Context, service, method string, in, out proto.Message) error
 
 var callOverrideKey = "holds []CallOverrideFunc"
@@ -79,7 +82,11 @@ func Logf(ctx netcontext.Context, level int64, format string, args ...interface{
 		f(level, format, args...)
 		return
 	}
-	logf(fromContext(ctx), level, format, args...)
+	c := fromContext(ctx)
+	if c == nil {
+		panic(errNotAppEngineContext)
+	}
+	logf(c, level, format, args...)
 }
 
 // NamespacedContext wraps a Context to support namespaces.

internal/identity_classic.go

@@ -13,15 +13,45 @@ import (
 )
 
 func DefaultVersionHostname(ctx netcontext.Context) string {
-	return appengine.DefaultVersionHostname(fromContext(ctx))
+	c := fromContext(ctx)
+	if c == nil {
+		panic(errNotAppEngineContext)
+	}
+	return appengine.DefaultVersionHostname(c)
 }
 
-func RequestID(ctx netcontext.Context) string  { return appengine.RequestID(fromContext(ctx)) }
-func Datacenter(_ netcontext.Context) string   { return appengine.Datacenter() }
-func ServerSoftware() string                   { return appengine.ServerSoftware() }
-func ModuleName(ctx netcontext.Context) string { return appengine.ModuleName(fromContext(ctx)) }
-func VersionID(ctx netcontext.Context) string  { return appengine.VersionID(fromContext(ctx)) }
-func InstanceID() string                       { return appengine.InstanceID() }
-func IsDevAppServer() bool                     { return appengine.IsDevAppServer() }
+func Datacenter(_ netcontext.Context) string { return appengine.Datacenter() }
+func ServerSoftware() string                 { return appengine.ServerSoftware() }
+func InstanceID() string                     { return appengine.InstanceID() }
+func IsDevAppServer() bool                   { return appengine.IsDevAppServer() }
 
-func fullyQualifiedAppID(ctx netcontext.Context) string { return fromContext(ctx).FullyQualifiedAppID() }
+func RequestID(ctx netcontext.Context) string {
+	c := fromContext(ctx)
+	if c == nil {
+		panic(errNotAppEngineContext)
+	}
+	return appengine.RequestID(c)
+}
+
+func ModuleName(ctx netcontext.Context) string {
+	c := fromContext(ctx)
+	if c == nil {
+		panic(errNotAppEngineContext)
+	}
+	return appengine.ModuleName(c)
+}
+func VersionID(ctx netcontext.Context) string {
+	c := fromContext(ctx)
+	if c == nil {
+		panic(errNotAppEngineContext)
+	}
+	return appengine.VersionID(c)
+}
+
+func fullyQualifiedAppID(ctx netcontext.Context) string {
+	c := fromContext(ctx)
+	if c == nil {
+		panic(errNotAppEngineContext)
+	}
+	return c.FullyQualifiedAppID()
+}

internal/identity_vm.go

@@ -23,7 +23,11 @@ const (
 )
 
 func ctxHeaders(ctx netcontext.Context) http.Header {
-	return fromContext(ctx).Request().Header
+	c := fromContext(ctx)
+	if c == nil {
+		return nil
+	}
+	return c.Request().Header
 }
 
 func DefaultVersionHostname(ctx netcontext.Context) string {

user/user_classic.go

@@ -15,7 +15,11 @@ import (
 )
 
 func Current(ctx context.Context) *User {
-	u := user.Current(internal.ClassicContextFromContext(ctx))
+	c, err := internal.ClassicContextFromContext(ctx)
+	if err != nil {
+		panic(err)
+	}
+	u := user.Current(c)
 	if u == nil {
 		return nil
 	}