Document blobserver.Loader methods, fix GetHandlerType to not force-load, diagnose loops better.

Change-Id: I03e989267e80ad610d3877c97fbf8adc8a88bb60

Makefile

@@ -2,7 +2,7 @@ all:
 	go install --tags=with_sqlite ./pkg/... ./server/... ./cmd/... ./third_party/...
 
 presubmit:
-	SKIP_DEP_TESTS=1 go test -short ./pkg/... ./server/camlistored ./cmd/... && echo PASS
+	SKIP_DEP_TESTS=1 go test --tags=with_sqlite -short ./pkg/... ./server/camlistored ./cmd/... && echo PASS
 
 embeds:
 	go install ./pkg/fileembed/genfileembed/ && genfileembed ./server/camlistored/ui

pkg/blobserver/registry.go

@@ -32,8 +32,15 @@ type FindHandlerByTyper interface {
 	// returns its prefix and handler if it's loaded.  If it's not
 	// loaded, the error will be ErrHandlerTypeNotFound.
 	//
-	// This is used by handler constructors to find siblings (such as the "ui" type handler)
+	// This is used by handlers to find siblings (such as the "ui" type handler)
 	// which might have more knowledge about the configuration for discovery, etc.
+	//
+	// Note that if this is called during handler construction
+	// time, only the prefix may be returned with a nil handler
+	// and nil err.  Unlike GetHandler and GetStorage, this does
+	// not cause the prefix to load immediately. At runtime (after
+	// construction of all handlers), then prefix and handler will
+	// both be non-nil when err is nil.
 	FindHandlerByType(handlerType string) (prefix string, handler interface{}, err error)
 }
 
@@ -43,12 +50,19 @@ type Loader interface {
 	// MyPrefix returns the prefix of the handler currently being constructed.
 	MyPrefix() string
 
-	GetStorage(prefix string) (Storage, error)
+	// GetHandlerType returns the handler's configured type, but does
+	// not force it to start being loaded yet.
 	GetHandlerType(prefix string) string // returns "" if unknown
 
-	// Returns either a Storage or an http.Handler
+	// GetHandler returns either a Storage or an http.Handler.
+	// It forces the handler to be loaded and returns an error if
+	// a cycle is created.
 	GetHandler(prefix string) (interface{}, error)
 
+	// GetStorage is like GetHandler but requires that the Handler be
+	// a storage Handler.
+	GetStorage(prefix string) (Storage, error)
+
 	// If we're loading configuration in response to a web request
 	// (as we do with App Engine), then this returns a request and
 	// true.

pkg/serverconfig/serverconfig.go

@@ -26,6 +26,7 @@ import (
 	"log"
 	"net/http"
 	"os"
+	"runtime"
 	"strconv"
 	"strings"
 
@@ -225,7 +226,6 @@ func (hl *handlerLoader) GetHandler(prefix string) (interface{}, error) {
 }
 
 func (hl *handlerLoader) GetHandlerType(prefix string) string {
-	hl.setupHandler(prefix)
 	return hl.configType(prefix)
 }
 
@@ -247,7 +247,9 @@ func (hl *handlerLoader) setupHandler(prefix string) {
 		return
 	}
 	if h.settingUp {
-		exitFailure("loop in configuration graph; %q tried to load itself indirectly", prefix)
+		buf := make([]byte, 1024)
+		buf = buf[:runtime.Stack(buf, false)]
+		exitFailure("loop in configuration graph; %q tried to load itself indirectly. Stack:\n%s", prefix, buf)
 	}
 	h.settingUp = true
 	defer func() {