Ensure error message for failed slices stay within the slice

.werft/build.ts

@@ -1,6 +1,6 @@
 import * as fs from "fs";
 import { SpanStatusCode } from "@opentelemetry/api";
-import { Werft } from "./util/werft";
+import { FailedSliceError, Werft } from "./util/werft";
 import { reportBuildFailureInSlack } from "./util/slack";
 import * as Tracing from "./observability/tracing";
 import * as VM from "./vm/vm";
@@ -27,7 +27,12 @@ Tracing.initialize()
             message: err,
         });
 
-        console.log("Error", err);
+        if (err instanceof FailedSliceError) {
+            // This error was produced using werft.fail which means that we
+            // already handled it "gracefully"
+        } else {
+            console.log("Error", err);
+        }
 
         if (context.Repository.ref === "refs/heads/main") {
             reportBuildFailureInSlack(context, err).catch((error: Error) => {

.werft/jobs/build/validate-changes.ts

@@ -4,17 +4,12 @@ import { JobConfig } from "./job-config";
 
 export async function validateChanges(werft: Werft, config: JobConfig) {
     werft.phase("validate-changes", "validating changes");
-    try {
-        await Promise.all([
-            branchNameCheck(werft, config),
-            preCommitCheck(werft),
-            typecheckWerftJobs(werft),
-            leewayVet(werft),
-        ]);
-    } catch (err) {
-        werft.fail("validate-changes", err);
-    }
-    werft.done("validate-changes");
+    await Promise.all([
+        branchNameCheck(werft, config),
+        preCommitCheck(werft),
+        typecheckWerftJobs(werft),
+        leewayVet(werft),
+    ]);
 }
 
 // Branch names cannot be longer than 45 characters.

.werft/util/werft.ts

@@ -3,6 +3,14 @@ import { exec } from "./shell";
 
 let werft: Werft;
 
+export class FailedSliceError extends Error {
+    constructor(message: string) {
+        super(message);
+        this.name = "FailedSliceError";
+        Object.setPrototypeOf(this, FailedSliceError.prototype);
+    }
+}
+
 /**
  * For backwards compatibility with existing code we expose a global Werft instance
  */
@@ -75,7 +83,7 @@ export class Werft {
     /**
      * Use this when you intend to fail the werft job
      */
-    public fail(slice, err) {
+    public fail(slice: string, err: string | Error) {
         const span = this.sliceSpans[slice];
 
         if (span) {
@@ -92,12 +100,21 @@ export class Werft {
             }
             span.setStatus({
                 code: SpanStatusCode.ERROR,
-                message: err,
+                message: err.toString(),
             });
         });
 
-        console.log(`[${slice}|FAIL] ${err}`);
-        throw err;
+        // In case the error message is a multi-line string we want to ensure that we contain
+        // the error message within the slice (otherwise they'll be moved to the "default" slice of the phase)
+        err.toString()
+            .split("\n")
+            .forEach((line: string) => console.log(`[${slice}] ${line}`));
+
+        // The UI shows the last log of the slice which might not make a lot of sense
+        // for multi-line error messages, so instead we tell the user to expand the slice.
+        console.log(`[${slice}] Failed. Expand to see why`);
+        console.log(`[${slice}|FAIL]`);
+        throw new FailedSliceError(slice);
     }
 
     /**