secretsync: add concept of manifest variable

This is a file-type variable which contains the original mapping of
VARIABLE_NAME to VARIABLE_DATA.

This can be used to automatically repopulate a repository with
secrets that were originally taken from it and transmitted via
GitLab variables (i.e. out-of-band).

nix/pkgs/secretsync/default.nix

@@ -23,6 +23,8 @@ in secretsync // {
         gitlabEndpoint = "https://hg.lukegb.com";
         gitlabProject = "lukegb/depot";
         variablesToFile = {};
+        manifestVariable = "";
+        workingDir = "";
         logToStderr = true;
       } // baseConfig;
       args = {
@@ -31,9 +33,10 @@ in secretsync // {
         gitlab_project = config.gitlabProject;
         variable_to_file = lib.mapAttrsToList (name: value: "${name}=${value}") config.variablesToFile;
         logtostderr = config.logToStderr;
-      };
+      } // (if config.manifestVariable == "" then {} else { manifest_variable = config.manifestVariable; });
     in
     pkgs.writeShellScriptBin config.name ''
+      cd "${config.workingDir}"
       exec "${config.pkg}/bin/secretsync" ${lib.cli.toGNUCommandLineShell {} args}
     '';
 }

nix/pkgs/secretsync/secretsync.go

@@ -10,6 +10,7 @@ import (
 	"net/url"
 	"os"
 	"path"
+	"sort"
 	"strings"
 
 	log "github.com/golang/glog"
@@ -20,6 +21,7 @@ var (
 	gitlabAccessToken = flag.String("gitlab_access_token", os.Getenv("GITLAB_ACCESS_TOKEN"), "GitLab API Access Token")
 	gitlabEndpoint    = flag.String("gitlab_endpoint", "https://hg.lukegb.com", "GitLab API endpoint")
 	gitlabProject     = flag.String("gitlab_project", "lukegb/depot", "Project to sync secrets into")
+	manifestVariable  = flag.String("manifest_variable", "", "If non-empty, a variable to populate with the path to a [VARIABLE]=[FILENAME] manifest, separated by newlines.")
 
 	varToFileMap map[string]string
 )
@@ -206,33 +208,48 @@ func main() {
 		log.Exitf("gitlab.Variables: %v", err)
 	}
 
+	createOrReplaceVariable := func(varFuture ProjectVariable) error {
+		varCurrent, ok := vars[varFuture.Key]
+		switch {
+		case !ok:
+			log.Infof("%q needs to be created", varFuture.Key)
+			return gitlab.CreateVariable(ctx, *gitlabProject, varFuture)
+		case varCurrent.Value == varFuture.Value:
+			log.Infof("%q up-to-date", varFuture.Key)
+			return nil
+		default:
+			log.Infof("%q needs to be updated", varFuture.Key)
+			return gitlab.UpdateVariable(ctx, *gitlabProject, varFuture)
+		}
+	}
+
 	for varKey, varSource := range varToFileMap {
-		varFutureDataBytes, err := ioutil.ReadFile(varSource)
+		varFutureData, err := ioutil.ReadFile(varSource)
 		if err != nil {
 			log.Exitf("reading secret source file %q: %v", varSource, err)
 		}
-		varFutureData := string(varFutureDataBytes)
+		if err := createOrReplaceVariable(ProjectVariable{
-		varCurrent, ok := vars[varKey]
-		if ok && varCurrent.Value == varFutureData {
-			log.Infof("%q up-to-date", varKey)
-			continue
-		}
-		varFuture := ProjectVariable{
 			Key:          varKey,
 			VariableType: "file",
-			Value:        varFutureData,
+			Value:        string(varFutureData),
 			Protected:    true,
+		}); err != nil {
+			log.Errorf("createOrReplaceVariable(%q, %q): %v", *gitlabProject, varKey, err)
 		}
-		if !ok {
-			log.Infof("%q needs to be created", varKey)
-			if err := gitlab.CreateVariable(ctx, *gitlabProject, varFuture); err != nil {
-				log.Errorf("CreateVariable(%q, %q): %v", *gitlabProject, varKey, err)
-			}
-		} else {
-			log.Infof("%q needs to be updated", varKey)
-			if err := gitlab.UpdateVariable(ctx, *gitlabProject, varFuture); err != nil {
-				log.Errorf("UpdateVariable(%q, %q): %v", *gitlabProject, varKey, err)
 	}
+	if *manifestVariable != "" {
+		var manifestData []string
+		for k, v := range varToFileMap {
+			manifestData = append(manifestData, fmt.Sprintf("%s=%s", k, v))
+		}
+		sort.Strings(manifestData)
+		if err := createOrReplaceVariable(ProjectVariable{
+			Key:          *manifestVariable,
+			VariableType: "file",
+			Value:        strings.Join(manifestData, "\n") + "\n",
+			Protected:    true,
+		}); err != nil {
+			log.Errorf("createOrReplaceVariable(%q, %q): %v", *gitlabProject, *manifestVariable, err)
 		}
 	}
 }