util: always visualize cause property in errors during inspection

While inspecting errors, always visualize the cause. That property
is non-enumerable by default while being useful in general for
debugging.

Duplicated stack frames are hidden.

Signed-off-by: Ruben Bridgewater <ruben@bridgewater.de>

lib/internal/util/inspect.js

@@ -1162,25 +1162,57 @@ function getFunctionBase(value, constructor, tag) {
   return base;
 }
 
-function formatError(err, constructor, tag, ctx, keys) {
-  const name = err.name != null ? String(err.name) : 'Error';
-  let len = name.length;
-  let stack = err.stack ? String(err.stack) : ErrorPrototypeToString(err);
+function identicalSequenceRange(a, b) {
+  for (let i = 0; i < a.length - 3; i++) {
+    // Find the first entry of b that matches the current entry of a.
+    const pos = b.indexOf(a[i]);
+    if (pos !== -1) {
+      const rest = b.length - pos;
+      if (rest > 3) {
+        let len = 1;
+        const maxLen = Math.min(a.length - i, rest);
+        // Count the number of consecutive entries.
+        while (maxLen > len && a[i + len] === b[pos + len]) {
+          len++;
+        }
+        if (len > 3) {
+          return { len, offset: i };
+        }
+      }
+    }
+  }
 
-  // Do not "duplicate" error properties that are already included in the output
-  // otherwise.
-  if (!ctx.showHidden && keys.length !== 0) {
-    for (const name of ['name', 'message', 'stack']) {
-      const index = keys.indexOf(name);
-      // Only hide the property in case it's part of the original stack
-      if (index !== -1 && stack.includes(err[name])) {
-        keys.splice(index, 1);
+  return { len: 0, offset: 0 };
+}
+
+function getStackString(error) {
+  return error.stack ? String(error.stack) : ErrorPrototypeToString(error);
+}
+
+function getStackFrames(ctx, err, stack) {
+  const frames = stack.split('\n');
+
+  // Remove stack frames identical to frames in cause.
+  if (err.cause) {
+    const causeStack = getStackString(err.cause)
+    const causeStackStart = causeStack.indexOf('\n    at');
+    if (causeStackStart !== -1) {
+      const causeFrames = causeStack.slice(causeStackStart + 1).split('\n');
+      const { len, offset } = identicalSequenceRange(frames, causeFrames);
+      if (len > 0) {
+        frames.splice(offset + 1, len - 2,
+          ctx.stylize(`    ... ${len - 2} lines matching cause stack trace ...`, 'undefined'));
       }
     }
   }
+  return frames;
+}
 
+function improveStack(stack, constructor, name, tag) {
   // A stack trace may contain arbitrary data. Only manipulate the output
   // for "regular errors" (errors that "look normal") for now.
+  let len = name.length;
+
   if (constructor === null ||
       (name.endsWith('Error') &&
       stack.startsWith(name) &&
@@ -1206,6 +1238,33 @@ function formatError(err, constructor, tag, ctx, keys) {
       }
     }
   }
+  return stack;
+}
+
+function removeDuplicateErrorKeys(ctx, keys, err, stack) {
+  if (!ctx.showHidden && keys.length !== 0) {
+    for (const name of ['name', 'message', 'stack']) {
+      const index = keys.indexOf(name);
+      // Only hide the property in case it's part of the original stack
+      if (index !== -1 && stack.includes(err[name])) {
+        keys.splice(index, 1);
+      }
+    }
+  }
+}
+
+function formatError(err, constructor, tag, ctx, keys) {
+  const name = err.name != null ? String(err.name) : 'Error';
+  let stack = getStackString(err);
+
+  removeDuplicateErrorKeys(ctx, keys, err, stack);
+
+  if (err.cause && (keys.length === 0 || !keys.includes('cause'))) {
+    keys.push('cause')
+  }
+
+  stack = improveStack(stack, constructor, name, tag);
+
   // Ignore the error message if it's contained in the stack.
   let pos = (err.message && stack.indexOf(err.message)) || -1;
   if (pos !== -1)
@@ -1217,7 +1276,7 @@ function formatError(err, constructor, tag, ctx, keys) {
   } else if (ctx.colors) {
     // Highlight userland code and node modules.
     let newStack = stack.slice(0, stackStart);
-    const lines = stack.slice(stackStart + 1).split('\n');
+    const lines = getStackFrames(ctx, err, stack.slice(stackStart + 1));
     for (const line of lines) {
       const core = line.match(coreModuleRegExp);
       if (core !== null && NativeModule.exists(core[1])) {

test/message/util-inspect-error-cause.js

@@ -0,0 +1,21 @@
+'use strict';
+
+require('../common');
+
+class FoobarError extends Error {
+  status = 'Feeling good';
+}
+
+const cause1 = new TypeError('Inner error');
+const cause2 = new FoobarError('Individual message', { cause: cause1 });
+cause2.extraProperties = 'Yes!';
+const cause3 = new Error('Stack causes', { cause: cause2 });
+
+process.nextTick(() => {
+  const error = new RangeError('New Stack Frames', { cause: cause2 });
+  const error2 = new RangeError('New Stack Frames', { cause: cause3 });
+
+  console.log(error);
+  console.log(cause3);
+  console.log(error2);
+});

test/message/util-inspect-error-cause.out

@@ -0,0 +1,68 @@
+RangeError: New Stack Frames
+    at *
+    at * {
+  [cause]: FoobarError: Individual message
+      at *
+      at *
+      ... 4 lines matching cause stack trace ...
+      at * {
+    status: 'Feeling good',
+    extraProperties: 'Yes!',
+    [cause]: TypeError: Inner error
+        at *
+        at *
+        at *
+        at *
+        at *
+        at *
+        at *
+  }
+}
+Error: Stack causes
+    at *
+    at *
+    ... 4 lines matching cause stack trace ...
+    at * {
+  [cause]: FoobarError: Individual message
+      at *
+      at *
+      ... 4 lines matching cause stack trace ...
+      at *
+    status: 'Feeling good',
+    extraProperties: 'Yes!',
+    [cause]: TypeError: Inner error
+        at *
+        at *
+        at *
+        at *
+        at *
+        at *
+        at *
+  }
+}
+RangeError: New Stack Frames
+    at *
+    at * {
+  [cause]: Error: Stack causes
+      at *
+      at *
+      ... 4 lines matching cause stack trace ...
+      at * {
+    [cause]: FoobarError: Individual message
+        at *
+        at *
+        ... 4 lines matching cause stack trace ...
+        at * {
+      status: 'Feeling good',
+      extraProperties: 'Yes!',
+      [cause]: TypeError: Inner error
+          at *
+          at *
+          at *
+          at *
+          at *
+          at *
+          at *
+    }
+  }
+}