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src,lib: support path in v8.writeHeapSnapshot output #58959

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e833a33
src,lib: support path in `v8.writeHeapSnapshot` output
juanarbol Jul 5, 2025
0055948
src: avoid duplicating namespaces in node_internals.h
juanarbol Jul 5, 2025
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5 changes: 5 additions & 0 deletions doc/api/v8.md
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Original file line number Diff line number Diff line change
Expand Up @@ -504,6 +504,9 @@ disk unless [`v8.stopCoverage()`][] is invoked before the process exits.
<!-- YAML
added: v11.13.0
changes:
- version: REPLACEME
pr-url: https://github.com/nodejs/node/pull/58959
description: Support path option.
- version: v19.1.0
pr-url: https://github.com/nodejs/node/pull/44989
description: Support options to configure the heap snapshot.
Expand All @@ -526,6 +529,8 @@ changes:
**Default:** `false`.
* `exposeNumericValues` {boolean} If true, expose numeric values in
artificial fields. **Default:** `false`.
* `path` {string|Buffer|URL} If provided, specifies the file system location
where the snapshot will be written. **Default:** `undefined`
* Returns: {string} The filename where the snapshot was saved.

Generates a snapshot of the current V8 heap and writes it to a JSON
Expand Down
10 changes: 8 additions & 2 deletions lib/v8.js
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Expand Up @@ -77,16 +77,22 @@ const { getOptionValue } = require('internal/options');
* @param {string} [filename]
* @param {{
* exposeInternals?: boolean,
* exposeNumericValues?: boolean
* exposeNumericValues?: boolean,
* path?: string,
* }} [options]
* @returns {string}
*/
function writeHeapSnapshot(filename, options) {
if (filename !== undefined) {
filename = getValidatedPath(filename);
}

if (options?.path !== undefined) {
options.path = getValidatedPath(options.path);
}

const optionArray = getHeapSnapshotOptions(options);
return triggerHeapSnapshot(filename, optionArray);
return triggerHeapSnapshot(filename, optionArray, options?.path);
}

/**
Expand Down
54 changes: 38 additions & 16 deletions src/heap_utils.cc
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Expand Up @@ -448,30 +448,52 @@ void CreateHeapSnapshotStream(const FunctionCallbackInfo<Value>& args) {
void TriggerHeapSnapshot(const FunctionCallbackInfo<Value>& args) {
Environment* env = Environment::GetCurrent(args);
Isolate* isolate = args.GetIsolate();
CHECK_EQ(args.Length(), 2);
CHECK_EQ(args.Length(), 3);
Local<Value> filename_v = args[0];
auto options = GetHeapSnapshotOptions(args[1]);
bool has_path = !args[2]->IsUndefined() && !args[2]->IsNull();
std::string final_filename;
BufferValue path_v(isolate, args[2]);
ToNamespacedPath(env, &path_v);

if (filename_v->IsUndefined()) {
DiagnosticFilename name(env, "Heap", "heapsnapshot");
THROW_IF_INSUFFICIENT_PERMISSIONS(
env,
permission::PermissionScope::kFileSystemWrite,
Environment::GetCwd(env->exec_path()));
if (WriteSnapshot(env, *name, options).IsNothing()) return;
if (String::NewFromUtf8(isolate, *name).ToLocal(&filename_v)) {
args.GetReturnValue().Set(filename_v);
if (!has_path) {
THROW_IF_INSUFFICIENT_PERMISSIONS(
env,
permission::PermissionScope::kFileSystemWrite,
Environment::GetCwd(env->exec_path()));
final_filename = *name;
} else {
THROW_IF_INSUFFICIENT_PERMISSIONS(
env,
permission::PermissionScope::kFileSystemWrite,
path_v.ToStringView());
final_filename = path_v.ToString() + "/" +
*name; // NOLINT(readability/pointer_notation)
}
} else {
BufferValue filename(isolate, filename_v);
CHECK_NOT_NULL(*filename);

if (has_path) {
final_filename = path_v.ToString() + "/" + filename.ToString();
THROW_IF_INSUFFICIENT_PERMISSIONS(
env, permission::PermissionScope::kFileSystemWrite, final_filename);
} else {
ToNamespacedPath(env, &filename);
THROW_IF_INSUFFICIENT_PERMISSIONS(
env,
permission::PermissionScope::kFileSystemWrite,
filename.ToStringView());
final_filename = filename.ToString();
}
return;
}

BufferValue path(isolate, filename_v);
CHECK_NOT_NULL(*path);
ToNamespacedPath(env, &path);
THROW_IF_INSUFFICIENT_PERMISSIONS(
env, permission::PermissionScope::kFileSystemWrite, path.ToStringView());
if (WriteSnapshot(env, *path, options).IsNothing()) return;
return args.GetReturnValue().Set(filename_v);
if (WriteSnapshot(env, final_filename.c_str(), options).IsNothing()) return;

args.GetReturnValue().Set(
String::NewFromUtf8(isolate, final_filename.c_str()).ToLocalChecked());
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@jasnell jasnell Jul 19, 2025 •
edited
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Unfortunately this is going to be a problem because it assumes the final_filename is utf8 encoded. Consider the following case:

const path = Buffer.from([0xe6]);  // latin 1 character
fs.mkdirSync(path);
const res = v8.writeHeapSnapshot(undefined, { path });
fs.readfileSync(res);  // fails! path doesn't exist

The reason fs.readfileSync would fail here is because the returned res will have interpreted the 0xe6 character incorrectly and will have replaced it with the unicode replacement char. The snapshot will have been written to disk correctly, but the returned path is wrong.

This likely needs to return the resulting path as a Buffer if the path was given as a Buffer and allow the API to accept an encoding option that optionally decodes the result as a string. But, that gets a bit awkward.

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I love how picky this is, aaah, let me put the encoding option to make this a bit more consistent and leave utf8 as default encoding.

Nice catch, thanks! Anything else missing?

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Nothing that sticks out, but I should have time to take another pass through by end of day monday

}

void Initialize(Local<Object> target,
Expand Down
12 changes: 4 additions & 8 deletions src/node_internals.h
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Expand Up @@ -422,16 +422,17 @@ namespace heap {
v8::Maybe<void> WriteSnapshot(Environment* env,
const char* filename,
v8::HeapProfiler::HeapSnapshotOptions options);
}

namespace heap {

void DeleteHeapSnapshot(const v8::HeapSnapshot* snapshot);
using HeapSnapshotPointer =
DeleteFnPtr<const v8::HeapSnapshot, DeleteHeapSnapshot>;

BaseObjectPtr<AsyncWrap> CreateHeapSnapshotStream(
Environment* env, HeapSnapshotPointer&& snapshot);

v8::HeapProfiler::HeapSnapshotOptions GetHeapSnapshotOptions(
v8::Local<v8::Value> options);

} // namespace heap

node_module napi_module_to_node_module(const napi_module* mod);
Expand Down Expand Up @@ -460,11 +461,6 @@ std::ostream& operator<<(std::ostream& output,

bool linux_at_secure();

namespace heap {
v8::HeapProfiler::HeapSnapshotOptions GetHeapSnapshotOptions(
v8::Local<v8::Value> options);
} // namespace heap

enum encoding ParseEncoding(v8::Isolate* isolate,
v8::Local<v8::Value> encoding_v,
v8::Local<v8::Value> encoding_id,
Expand Down
12 changes: 12 additions & 0 deletions test/common/heap.js
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Expand Up @@ -316,6 +316,18 @@ function getHeapSnapshotOptionTests() {
],
}],
},
// This is the same as the previous case, but with a path option.
// The path option will be mutated by the test
// test-write-heapsnapshot-options.js
// Refs: https://github.com/nodejs/node/issues/58857
{
options: { exposeNumericValues: true, path: 'TBD' },
expected: [{
children: [
{ edge_name: 'numeric', node_name: 'smi number' },
],
}],
},
];
return {
fixtures: fixtures.path('klass-with-fields.js'),
Expand Down
24 changes: 22 additions & 2 deletions test/sequential/test-write-heapsnapshot-options.js
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Expand Up @@ -4,7 +4,9 @@

require('../common');

const assert = require('assert');
const fs = require('fs');
const tmpdir = require('../common/tmpdir');
const { getHeapSnapshotOptionTests, recordState } = require('../common/heap');

class ReadStream {
Expand All @@ -20,7 +22,12 @@ if (process.argv[2] === 'child') {
const { writeHeapSnapshot } = require('v8');
require(tests.fixtures);
const { options, expected } = tests.cases[parseInt(process.argv[3])];
const { path } = options;
// If path is set, writeHeapSnapshot will write to the tmpdir.
if (path) options.path = tmpdir.path;
const filename = writeHeapSnapshot(undefined, options);
// If path is set, the filename will be the provided path.
if (path) assert(filename.includes(tmpdir.path));
const snapshot = recordState(new ReadStream(filename));
console.log('Snapshot nodes', snapshot.snapshot.length);
console.log('Searching for', expected[0].children);
Expand All @@ -30,8 +37,6 @@ if (process.argv[2] === 'child') {
}

const { spawnSync } = require('child_process');
const assert = require('assert');
const tmpdir = require('../common/tmpdir');
tmpdir.refresh();

// Start child processes to prevent the heap from growing too big.
Expand All @@ -48,3 +53,18 @@ for (let i = 0; i < tests.cases.length; ++i) {
console.log('[STDOUT]', stdout);
assert.strictEqual(child.status, 0);
}

{
// Don't need to spawn child process for this test, it will fail inmediately.
// Refs: https://github.com/nodejs/node/issues/58857
for (const pathName of [null, true, false, {}, [], () => {}, 1, 0, NaN]) {
assert.throws(() => {
require('v8').writeHeapSnapshot(undefined, {
path: pathName
});
}, {
code: 'ERR_INVALID_ARG_TYPE',
name: 'TypeError'
});
}
}
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