Get previous commit for people

sourcetool/cmd/checklevelprov.go

@@ -57,8 +57,17 @@ func doCheckLevelProv(checkLevelProvArgs CheckLevelProvArgs) {
 		ver_options.ExpectedSan = checkLevelProvArgs.expectedSan
 	}
 
+	prevCommit := checkLevelProvArgs.prevCommit
+	var err error
+	if prevCommit == "" {
+		prevCommit, err = gh_connection.GetPriorCommit(ctx, checkLevelProvArgs.commit)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+
 	pa := attest.NewProvenanceAttestor(gh_connection, ver_options)
-	prov, err := pa.CreateSourceProvenance(ctx, checkLevelProvArgs.prevBundlePath, checkLevelProvArgs.commit, checkLevelProvArgs.prevCommit)
+	prov, err := pa.CreateSourceProvenance(ctx, checkLevelProvArgs.prevBundlePath, checkLevelProvArgs.commit, prevCommit)
 	if err != nil {
 		log.Fatal(err)
 	}

sourcetool/pkg/gh_control/connection.go

@@ -46,3 +46,23 @@ func (ghc *GitHubConnection) GetLatestCommit(ctx context.Context) (string, error
 	}
 	return *branch.Commit.SHA, nil
 }
+
+// Gets the previous commit to 'sha' if it has one.
+// If there are more than one parents this fails with an error.
+// (This tool generally operates in an environment of linear history)
+func (ghc *GitHubConnection) GetPriorCommit(ctx context.Context, sha string) (string, error) {
+	commit, _, err := ghc.Client.Git.GetCommit(ctx, ghc.Owner, ghc.Repo, sha)
+	if err != nil {
+		return "", fmt.Errorf("cannot get commit data for %s: %w", sha, err)
+	}
+
+	if len(commit.Parents) == 0 {
+		return "", fmt.Errorf("there is no commit earlier than %s, that isn't yet supported", sha)
+	}
+
+	if len(commit.Parents) > 1 {
+		return "", fmt.Errorf("commit %s has more than one parent (%v), which is not supported", sha, commit.Parents)
+	}
+
+	return *commit.Parents[0].SHA, nil
+}