doc: fix default maxBuffer size

Correctly document the default maxBuffer size for execSync,
execFileSync, and spawnSync. It is 200 * 1024, not Infinity.
Add tests to verify behaviour is as documented.

doc/api/child_process.md

@@ -721,7 +721,7 @@ changes:
     process will be killed. **Default:** `'SIGTERM'`.
   * `maxBuffer` {number} Largest amount of data in bytes allowed on stdout or
     stderr. If exceeded, the child process is terminated. See caveat at
-    [`maxBuffer` and Unicode][]. **Default:** `200 * 1024`.
+    [`maxBuffer` and Unicode][]. **Default:** `Infinity`.
   * `encoding` {string} The encoding used for all stdio inputs and outputs.
     **Default:** `'buffer'`.
   * `windowsHide` {boolean} Hide the subprocess console window that would
@@ -788,7 +788,7 @@ changes:
   * `maxBuffer` {number} Largest amount of data in bytes allowed on stdout or
     stderr. If exceeded, the child process is terminated and any output is
     truncated. See caveat at [`maxBuffer` and Unicode][].
-    **Default:** `200 * 1024`.
+    **Default:** `Infinity`.
   * `encoding` {string} The encoding used for all stdio inputs and outputs.
     **Default:** `'buffer'`.
   * `windowsHide` {boolean} Hide the subprocess console window that would
@@ -852,7 +852,7 @@ changes:
   * `maxBuffer` {number} Largest amount of data in bytes allowed on stdout or
     stderr. If exceeded, the child process is terminated and any output is
     truncated. See caveat at [`maxBuffer` and Unicode][].
-    **Default:** `200 * 1024`.
+    **Default:** `Infinity`.
   * `encoding` {string} The encoding used for all stdio inputs and outputs.
     **Default:** `'buffer'`.
   * `shell` {boolean|string} If `true`, runs `command` inside of a shell. Uses

test/parallel/test-child-process-exec-maxBuffer.js

@@ -10,6 +10,29 @@ function runChecks(err, stdio, streamName, expected) {
   assert.deepStrictEqual(stdio[streamName], expected);
 }
 
+// default value
+{
+  const cmd = `"${process.execPath}" -e "console.log('a'.repeat(200 * 1024))"`;
+
+  cp.exec(cmd, common.mustCall((err) => {
+    assert(err instanceof RangeError);
+    assert.strictEqual(err.message, 'stdout maxBuffer length exceeded');
+    assert.strictEqual(err.code, 'ERR_CHILD_PROCESS_STDIO_MAXBUFFER');
+  }));
+}
+
+// default value
+{
+  // eslint-disable-next-line max-len
+  const cmd = `${process.execPath} -e "console.log('a'.repeat(200 * 1024 - 1))"`;
+
+  cp.exec(cmd, common.mustCall((err, stdout, stderr) => {
+    assert.ifError(err);
+    assert.strictEqual(stdout.trim(), 'a'.repeat(200 * 1024 - 1));
+    assert.strictEqual(stderr, '');
+  }));
+}
+
 {
   const cmd = `"${process.execPath}" -e "console.log('hello world');"`;
   const options = { maxBuffer: Infinity };

test/parallel/test-child-process-execfile-maxBuffer.js

@@ -0,0 +1,94 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const { execFile } = require('child_process');
+
+function checkFactory(streamName) {
+  return common.mustCall((err) => {
+    assert(err instanceof RangeError);
+    assert.strictEqual(err.message, `${streamName} maxBuffer length exceeded`);
+    assert.strictEqual(err.code, 'ERR_CHILD_PROCESS_STDIO_MAXBUFFER');
+  });
+}
+
+// default value
+{
+  execFile(
+    process.execPath,
+    ['-e', "console.log('a'.repeat(200 * 1024))"],
+    checkFactory('stdout')
+  );
+}
+
+// default value
+{
+  execFile(
+    process.execPath,
+    ['-e', "console.log('a'.repeat(200 * 1024 - 1))"],
+    common.mustCall((err, stdout, stderr) => {
+      assert.ifError(err);
+      assert.strictEqual(stdout.trim(), 'a'.repeat(200 * 1024 - 1));
+      assert.strictEqual(stderr, '');
+    })
+  );
+}
+
+{
+  const options = { maxBuffer: Infinity };
+
+  execFile(
+    process.execPath,
+    ['-e', "console.log('hello world');"],
+    options,
+    common.mustCall((err, stdout, stderr) => {
+      assert.ifError(err);
+      assert.strictEqual(stdout.trim(), 'hello world');
+      assert.strictEqual(stderr, '');
+    })
+  );
+}
+
+{
+  execFile('echo', ['hello world'], { maxBuffer: 5 }, checkFactory('stdout'));
+}
+
+const unicode = '中文测试'; // length = 4, byte length = 12
+
+{
+  execFile(
+    process.execPath,
+    ['-e', `console.log('${unicode}');`],
+    { maxBuffer: 10 },
+    checkFactory('stdout'));
+}
+
+{
+  execFile(
+    process.execPath,
+    ['-e', `console.error('${unicode}');"`],
+    { maxBuffer: 10 },
+    checkFactory('stderr')
+  );
+}
+
+{
+  const child = execFile(
+    process.execPath,
+    ['-e', `console.log('${unicode}');`],
+    { encoding: null, maxBuffer: 10 },
+    checkFactory('stdout')
+  );
+
+  child.stdout.setEncoding('utf-8');
+}
+
+{
+  const child = execFile(
+    process.execPath,
+    ['-e', `console.error('${unicode}');`],
+    { encoding: null, maxBuffer: 10 },
+    checkFactory('stderr')
+  );
+
+  child.stderr.setEncoding('utf-8');
+}

test/parallel/test-child-process-execfilesync-maxbuf.js

@@ -0,0 +1,46 @@
+'use strict';
+require('../common');
+
+// This test checks that the maxBuffer option for child_process.spawnSync()
+// works as expected.
+
+const assert = require('assert');
+const execFileSync = require('child_process').execFileSync;
+const msgOut = 'this is stdout';
+const msgOutBuf = Buffer.from(`${msgOut}\n`);
+
+const args = [
+  '-e',
+  `console.log("${msgOut}");`
+];
+
+// Verify that an error is returned if maxBuffer is surpassed.
+{
+  assert.throws(
+    () => execFileSync(process.execPath, args, { maxBuffer: 1 }),
+    (e) => {
+      assert.ok(e, 'maxBuffer should error');
+      assert.strictEqual(e.errno, 'ENOBUFS');
+      assert.deepStrictEqual(e.stdout, msgOutBuf);
+      return true;
+    }
+  );
+}
+
+// Verify that a maxBuffer size of Infinity works.
+{
+  const ret = execFileSync(process.execPath, args, { maxBuffer: Infinity });
+
+  assert.deepStrictEqual(ret, msgOutBuf);
+}
+
+// maxBuffer size is Infinity at default.
+{
+  const ret = execFileSync(
+    process.execPath,
+    ['-e', "console.log('a'.repeat(200 * 1024))"],
+    { encoding: 'utf-8' }
+  );
+
+  assert.ifError(ret.error);
+}

test/parallel/test-child-process-execsync-maxbuf.js

@@ -0,0 +1,37 @@
+'use strict';
+require('../common');
+
+// This test checks that the maxBuffer option for child_process.spawnSync()
+// works as expected.
+
+const assert = require('assert');
+const { execSync } = require('child_process');
+const msgOut = 'this is stdout';
+const msgOutBuf = Buffer.from(`${msgOut}\n`);
+
+const args = [
+  '-e',
+  `"console.log('${msgOut}')";`
+];
+
+// Verify that an error is returned if maxBuffer is surpassed.
+{
+  assert.throws(() => {
+    execSync(`"${process.execPath}" ${args.join(' ')}`, { maxBuffer: 1 });
+  }, (e) => {
+    assert.ok(e, 'maxBuffer should error');
+    assert.strictEqual(e.errno, 'ENOBUFS');
+    assert.deepStrictEqual(e.stdout, msgOutBuf);
+    return true;
+  });
+}
+
+// Verify that a maxBuffer size of Infinity works.
+{
+  const ret = execSync(
+    `"${process.execPath}" ${args.join(' ')}`,
+    { maxBuffer: Infinity }
+  );
+
+  assert.deepStrictEqual(ret, msgOutBuf);
+}

test/parallel/test-child-process-spawnsync-maxbuf.js

@@ -32,3 +32,11 @@ const args = [
   assert.ifError(ret.error);
   assert.deepStrictEqual(ret.stdout, msgOutBuf);
 }
+
+// maxBuffer size is Infinity at default.
+{
+  const args = ['-e', "console.log('a'.repeat(200 * 1024))"];
+  const ret = spawnSync(process.execPath, args, { encoding: 'utf-8' });
+
+  assert.ifError(ret.error);
+}