util: add support for error subclassing and null prototype

lib/internal/util/inspect.js

@@ -643,7 +643,7 @@ function formatRaw(ctx, value, recurseTimes) {
       base = dateToISOString(value);
     } else if (isError(value)) {
       // Make error with message first say the error.
-      base = formatError(value);
+      base = formatError(value, getPrefix(constructor, '', 'Error'));
       // Wrap the error in brackets in case it has no stack trace.
       const stackStart = base.indexOf('\n    at');
       if (stackStart === -1) {
@@ -652,7 +652,8 @@ function formatRaw(ctx, value, recurseTimes) {
       // The message and the stack have to be indented as well!
       if (ctx.indentationLvl !== 0) {
         const indentation = ' '.repeat(ctx.indentationLvl);
-        base = formatError(value).replace(/\n/g, `\n${indentation}`);
+        base = formatError(value, getPrefix(constructor, '', 'Error'))
+                                  .replace(/\n/g, `\n${indentation}`);
       }
       if (keys.length === 0)
         return base;
@@ -861,8 +862,32 @@ function formatPrimitive(fn, value, ctx) {
   return fn(value.toString(), 'symbol');
 }
 
-function formatError(value) {
-  return value.stack || errorToString(value);
+function formatError(value, constructorName) {
+
+  if (value.stack) {
+    let stack = value.stack;
+    if (!stack.startsWith(constructorName) &&
+        !(stack.startsWith('file:') && !constructorName.startsWith('file'))) {
+      const stackStart = value.stack.indexOf('\n    at');
+      const errorAndMessage = value.stack.slice(0, stackStart);
+      const messageStart = errorAndMessage.indexOf(':');
+      if (stack && stackStart !== -1) {
+        stack = stack.slice(stackStart, stack.length);
+        if (messageStart !== -1) {
+          const message = errorAndMessage.slice(messageStart + 2,
+                                                stackStart.length);
+          // Reconstruct the stack
+          stack = constructorName.trim() + ': ' + message.trim() + stack;
+        } else {
+          stack = constructorName.trim() + stack;
+        }
+      }
+
+      return stack;
+    }
+    return stack;
+  }
+  return errorToString(value);
 }
 
 function formatNamespaceObject(ctx, value, recurseTimes, keys) {

test/parallel/test-util-inspect.js

@@ -1772,3 +1772,37 @@ assert.strictEqual(
   });
   assert.strictEqual(util.inspect(obj), '[Set: null prototype] { 1, 2 }');
 }
+
+// error subclassing and null prototype checks
+{
+  const error = new Error('My Message');
+  assert.ok(util.inspect(error).includes('Error: My Message'));
+
+  Object.setPrototypeOf(error, null);
+  assert.ok(util.inspect(error)
+                 .includes('[Error: null prototype]: My Message'));
+}
+
+{
+  class MyCustomError extends Error {}
+  const customErr = new MyCustomError('custom message');
+  assert.ok(util.inspect(customErr)
+                 .includes('MyCustomError: custom message'));
+
+  Object.setPrototypeOf(customErr, null);
+  assert.ok(util.inspect(customErr)
+                 .includes('[Error: null prototype]: custom message'));
+}
+
+// Check custom errors with name `File`
+// works
+{
+  class FileCustomError extends Error {}
+  const customErr = new FileCustomError();
+  assert.ok(util.inspect(customErr)
+            .includes('FileCustomError'));
+
+  Object.setPrototypeOf(customErr, null);
+  assert.ok(util.inspect(customErr)
+    .includes('[Error: null prototype]'));
+}