util: limit inspection output size to 128 MB

The maximum hard limit that `util.inspect()` could theoretically handle
is the maximum string size. That is ~2 ** 28 on 32 bit systems and
~2 ** 30 on 64 bit systems.

Due to the recursive algorithm a complex object could easily exceed
that limit without throwing an error right away and therefore
crashing the application by exceeding the heap limit.

`util.inspect()` is fast enough to compute 128 MB of data below one
second on an Intel(R) Core(TM) i7-5600U CPU. This hard limit allows
to inspect arbitrary big objects from now on without crashing the
application or blocking the event loop significantly.

PR-URL: nodejs#22756
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>

doc/api/util.md

@@ -360,6 +360,10 @@ stream.write('With ES6');
 <!-- YAML
 added: v0.3.0
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/22756
+    description: The inspection output is now limited to about 128 MB. Data
+                 above that size will not be fully inspected.
   - version: v10.6.0
     pr-url: https://github.com/nodejs/node/pull/20725
     description: Inspecting linked lists and similar objects is now possible
@@ -408,11 +412,11 @@ changes:
     TODO(BridgeAR): Deprecate `maxArrayLength` and replace it with
                     `maxEntries`.
     -->
-  * `maxArrayLength` {number} Specifies the maximum number of `Array`,
+  * `maxArrayLength` {integer} Specifies the maximum number of `Array`,
     [`TypedArray`][], [`WeakMap`][] and [`WeakSet`][] elements to include when
     formatting. Set to `null` or `Infinity` to show all elements. Set to `0` or
     negative to show no elements. **Default:** `100`.
-  * `breakLength` {number} The length at which an object's keys are split
+  * `breakLength` {integer} The length at which an object's keys are split
     across multiple lines. Set to `Infinity` to format an object as a single
     line. **Default:** `60` for legacy compatibility.
   * `compact` {boolean} Setting this to `false` changes the default indentation
@@ -532,9 +536,10 @@ console.log(inspect(weakSet, { showHidden: true }));
 ```
 
 Please note that `util.inspect()` is a synchronous method that is mainly
-intended as a debugging tool. Some input values can have a significant
-performance overhead that can block the event loop. Use this function
-with care and never in a hot code path.
+intended as a debugging tool. Its maximum output length is limited to
+approximately 128 MB and input values that result in output bigger than that
+will not be inspected fully. Such values can have a significant performance
+overhead that can block the event loop for a significant amount of time.
 
 ### Customizing `util.inspect` colors
 

lib/util.js

@@ -406,24 +406,27 @@ function inspect(value, opts) {
     maxArrayLength: inspectDefaultOptions.maxArrayLength,
     breakLength: inspectDefaultOptions.breakLength,
     indentationLvl: 0,
-    compact: inspectDefaultOptions.compact
+    compact: inspectDefaultOptions.compact,
+    budget: {}
   };
-  // Legacy...
-  if (arguments.length > 2) {
-    if (arguments[2] !== undefined) {
-      ctx.depth = arguments[2];
-    }
-    if (arguments.length > 3 && arguments[3] !== undefined) {
-      ctx.colors = arguments[3];
+  if (arguments.length > 1) {
+    // Legacy...
+    if (arguments.length > 2) {
+      if (arguments[2] !== undefined) {
+        ctx.depth = arguments[2];
+      }
+      if (arguments.length > 3 && arguments[3] !== undefined) {
+        ctx.colors = arguments[3];
+      }
     }
-  }
-  // Set user-specified options
-  if (typeof opts === 'boolean') {
-    ctx.showHidden = opts;
-  } else if (opts) {
-    const optKeys = Object.keys(opts);
-    for (var i = 0; i < optKeys.length; i++) {
-      ctx[optKeys[i]] = opts[optKeys[i]];
+    // Set user-specified options
+    if (typeof opts === 'boolean') {
+      ctx.showHidden = opts;
+    } else if (opts) {
+      const optKeys = Object.keys(opts);
+      for (var i = 0; i < optKeys.length; i++) {
+        ctx[optKeys[i]] = opts[optKeys[i]];
+      }
     }
   }
   if (ctx.colors) ctx.stylize = stylizeWithColor;
@@ -623,14 +626,19 @@ function noPrototypeIterator(ctx, value, recurseTimes) {
 // corrected by setting `ctx.indentationLvL += diff` and then to decrease the
 // value afterwards again.
 function formatValue(ctx, value, recurseTimes) {
-  // Primitive types cannot have properties
+  // Primitive types cannot have properties.
   if (typeof value !== 'object' && typeof value !== 'function') {
     return formatPrimitive(ctx.stylize, value, ctx);
   }
   if (value === null) {
     return ctx.stylize('null', 'null');
   }
 
+  if (ctx.stop !== undefined) {
+    const name = getConstructorName(value) || value[Symbol.toStringTag];
+    return ctx.stylize(`[${name || 'Object'}]`, 'special');
+  }
+
   if (ctx.showProxy) {
     const proxy = getProxyDetails(value);
     if (proxy !== undefined) {
@@ -639,11 +647,11 @@ function formatValue(ctx, value, recurseTimes) {
   }
 
   // Provide a hook for user-specified inspect functions.
-  // Check that value is an object with an inspect function on it
+  // Check that value is an object with an inspect function on it.
   if (ctx.customInspect) {
     const maybeCustom = value[customInspectSymbol];
     if (typeof maybeCustom === 'function' &&
-        // Filter out the util module, its inspect function is special
+        // Filter out the util module, its inspect function is special.
         maybeCustom !== exports.inspect &&
         // Also filter out any prototype objects using the circular check.
         !(value.constructor && value.constructor.prototype === value)) {
@@ -685,7 +693,7 @@ function formatRaw(ctx, value, recurseTimes) {
 
   let extrasType = kObjectType;
 
-  // Iterators and the rest are split to reduce checks
+  // Iterators and the rest are split to reduce checks.
   if (value[Symbol.iterator]) {
     noIterator = false;
     if (Array.isArray(value)) {
@@ -766,7 +774,7 @@ function formatRaw(ctx, value, recurseTimes) {
       }
       base = dateToISOString(value);
     } else if (isError(value)) {
-      // Make error with message first say the error
+      // Make error with message first say the error.
       base = formatError(value);
       // Wrap the error in brackets in case it has no stack trace.
       const stackStart = base.indexOf('\n    at');
@@ -885,7 +893,21 @@ function formatRaw(ctx, value, recurseTimes) {
   }
   ctx.seen.pop();
 
-  return reduceToSingleString(ctx, output, base, braces);
+  const res = reduceToSingleString(ctx, output, base, braces);
+  const budget = ctx.budget[ctx.indentationLvl] || 0;
+  const newLength = budget + res.length;
+  ctx.budget[ctx.indentationLvl] = newLength;
+  // If any indentationLvl exceeds this limit, limit further inspecting to the
+  // minimum. Otherwise the recursive algorithm might continue inspecting the
+  // object even though the maximum string size (~2 ** 28 on 32 bit systems and
+  // ~2 ** 30 on 64 bit systems) exceeded. The actual output is not limited at
+  // exactly 2 ** 27 but a bit higher. This depends on the object shape.
+  // This limit also makes sure that huge objects don't block the event loop
+  // significantly.
+  if (newLength > 2 ** 27) {
+    ctx.stop = true;
+  }
+  return res;
 }
 
 function handleMaxCallStackSize(ctx, err, constructor, tag) {
@@ -1057,8 +1079,9 @@ function formatTypedArray(ctx, value, recurseTimes) {
     formatBigInt;
   for (var i = 0; i < maxLength; ++i)
     output[i] = elementFormatter(ctx.stylize, value[i]);
-  if (remaining > 0)
+  if (remaining > 0) {
     output[i] = `... ${remaining} more item${remaining > 1 ? 's' : ''}`;
+  }
   if (ctx.showHidden) {
     // .buffer goes last, it's not a primitive like the others.
     ctx.indentationLvl += 2;

test/parallel/test-util-inspect-long-running.js

@@ -0,0 +1,20 @@
+'use strict';
+
+require('../common');
+
+// Test that huge objects don't crash due to exceeding the maximum heap size.
+
+const util = require('util');
+
+// Create a difficult to stringify object. Without the artificial limitation
+// this would crash or throw an maximum string size error.
+let last = {};
+const obj = last;
+
+for (let i = 0; i < 1000; i++) {
+  last.next = { circular: obj, last, obj: { a: 1, b: 2, c: true } };
+  last = last.next;
+  obj[i] = last;
+}
+
+util.inspect(obj, { depth: Infinity });