Print debugConfig if supplied; fix RawMessage use

Summary:
When we use debugConfig, it's nice to say what tweaks happened in the log.
To avoid golang/go#6528, needed to switch to using *json.RawMessage.

Test Plan: Existing unit doesn't cover it, sadly, but should be quite safe.

Reviewers: nate

Reviewed By: nate

Subscribers: changesbot, anupc

Differential Revision: https://tails.corp.dropbox.com/D147620

Author: kylec1

client/config.go

@@ -63,17 +63,17 @@ type Config struct {
 		// If this build is expected to generate a snapshot, this is the snapshot ID.
 		ID string
 	}
-	DebugConfig map[string]json.RawMessage `json:"debugConfig"`
+	DebugConfig map[string]*json.RawMessage `json:"debugConfig"`
 }
 
 // GetDebugConfig parses the debug config JSON at the given key to dest, returning whether the key
 // was present, and if it was, any error that occurred in trying to parse it to dest.
 func (c *Config) GetDebugConfig(key string, dest interface{}) (present bool, err error) {
 	data, ok := c.DebugConfig[key]
-	if !ok {
+	if !ok || data == nil {
 		return false, nil
 	}
-	e := json.Unmarshal([]byte(data), dest)
+	e := json.Unmarshal([]byte(*data), dest)
 	if e != nil {
 		e = fmt.Errorf("Malformed JSON in debug config key %q: %s", key, e)
 	}

engine/engine.go

@@ -1,6 +1,7 @@
 package engine
 
 import (
+	"encoding/json"
 	"errors"
 	"flag"
 	"fmt"
@@ -127,6 +128,14 @@ func (e *Engine) Run() (Result, error) {
 
 	e.clientLog.Printf("changes-client version: %s", version.GetVersion())
 	e.clientLog.Printf("Running jobstep %s for %s (%s)", e.config.JobstepID, e.config.Project.Name, e.config.Project.Slug)
+	if e.config.DebugConfig != nil {
+		if jsout, err := json.MarshalIndent(e.config.DebugConfig, "", "  "); err != nil {
+			// Should never happen, but no use crashing about it.
+			sentry.Error(err, map[string]string{})
+		} else {
+			e.clientLog.Printf("Debug config: %s", jsout)
+		}
+	}
 
 	if err := e.checkForSnapshotInconsistency(); err != nil {
 		sentry.Error(err, map[string]string{})