child_process: truncate output when maxBuffer is exceeded

Preserves truncated output for `child_process.exec()` when `maxBuffer`
is exceeded.

This is particularly useful for commands which have indistinguishable
error codes for what output they produce.

PR-URL: #24951
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/child_process.md

@@ -147,8 +147,9 @@ changes:
  `'/bin/sh'` on UNIX, `process.env.ComSpec` on Windows.
  * `timeout` {number} **Default:** `0`
  * `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`.
+ stderr. If exceeded, the child process is terminated and any output is
+ truncated. See caveat at [`maxBuffer` and Unicode][].
+ **Default:** `200 * 1024`.
  * `killSignal` {string|integer} **Default:** `'SIGTERM'`
  * `uid` {number} Sets the user identity of the process (see setuid(2)).
  * `gid` {number} Sets the group identity of the process (see setgid(2)).
@@ -245,8 +246,9 @@ changes:
  * `encoding` {string} **Default:** `'utf8'`
  * `timeout` {number} **Default:** `0`
  * `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`.
+ stderr. If exceeded, the child process is terminated and any output is
+ truncated. See caveat at [`maxBuffer` and Unicode][].
+ **Default:** `200 * 1024`.
  * `killSignal` {string|integer} **Default:** `'SIGTERM'`
  * `uid` {number} Sets the user identity of the process (see setuid(2)).
  * `gid` {number} Sets the group identity of the process (see setgid(2)).
@@ -779,8 +781,9 @@ changes:
  * `killSignal` {string|integer} The signal value to be used when the spawned
  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`.
+ stderr. If exceeded, the child process is terminated and any output is
+ truncated. See caveat at [`maxBuffer` and Unicode][].
+ **Default:** `200 * 1024`.
  * `encoding` {string} The encoding used for all stdio inputs and outputs.
  **Default:** `'buffer'`.
  * `windowsHide` {boolean} Hide the subprocess console window that would
@@ -842,8 +845,9 @@ changes:
  * `killSignal` {string|integer} The signal value to be used when the spawned
  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`.
+ stderr. If exceeded, the child process is terminated and any output is
+ truncated. See caveat at [`maxBuffer` and Unicode][].
+ **Default:** `200 * 1024`.
  * `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

lib/child_process.js

@@ -343,9 +343,15 @@ exports.execFile = function execFile(file /* , args, options, callback */) {
 
  child.stdout.on('data', function onChildStdout(chunk) {
  var encoding = child.stdout._readableState.encoding;
- stdoutLen += encoding ? Buffer.byteLength(chunk, encoding) : chunk.length;
+ const length = encoding ?
+ Buffer.byteLength(chunk, encoding) :
+ chunk.length;
+ stdoutLen += length;
 
  if (stdoutLen > options.maxBuffer) {
+ const truncatedLen = options.maxBuffer - (stdoutLen - length);
+ _stdout.push(chunk.slice(0, truncatedLen));
+
  ex = new ERR_CHILD_PROCESS_STDIO_MAXBUFFER('stdout');
  kill();
  } else {
@@ -360,9 +366,15 @@ exports.execFile = function execFile(file /* , args, options, callback */) {
 
  child.stderr.on('data', function onChildStderr(chunk) {
  var encoding = child.stderr._readableState.encoding;
- stderrLen += encoding ? Buffer.byteLength(chunk, encoding) : chunk.length;
+ const length = encoding ?
+ Buffer.byteLength(chunk, encoding) :
+ chunk.length;
+ stderrLen += length;
 
  if (stderrLen > options.maxBuffer) {
+ const truncatedLen = options.maxBuffer - (stderrLen - length);
+ _stderr.push(chunk.slice(0, truncatedLen));
+
  ex = new ERR_CHILD_PROCESS_STDIO_MAXBUFFER('stderr');
  kill();
  } else {

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

@@ -22,13 +22,13 @@ function runChecks(err, stdio, streamName, expected) {
 }
 
 {
- const cmd = 'echo "hello world"';
+ const cmd = 'echo hello world';
 
  cp.exec(
  cmd,
  { maxBuffer: 5 },
  common.mustCall((err, stdout, stderr) => {
- runChecks(err, { stdout, stderr }, 'stdout', '');
+ runChecks(err, { stdout, stderr }, 'stdout', 'hello');
  })
  );
 }
@@ -42,7 +42,7 @@ const unicode = '中文测试'; // length = 4, byte length = 12
  cmd,
  { maxBuffer: 10 },
  common.mustCall((err, stdout, stderr) => {
- runChecks(err, { stdout, stderr }, 'stdout', '');
+ runChecks(err, { stdout, stderr }, 'stdout', '中文测试\n');
  })
  );
 }
@@ -54,7 +54,7 @@ const unicode = '中文测试'; // length = 4, byte length = 12
  cmd,
  { maxBuffer: 3 },
  common.mustCall((err, stdout, stderr) => {
- runChecks(err, { stdout, stderr }, 'stderr', '');
+ runChecks(err, { stdout, stderr }, 'stderr', '中文测');
  })
  );
 }
@@ -66,7 +66,7 @@ const unicode = '中文测试'; // length = 4, byte length = 12
  cmd,
  { encoding: null, maxBuffer: 10 },
  common.mustCall((err, stdout, stderr) => {
- runChecks(err, { stdout, stderr }, 'stdout', '');
+ runChecks(err, { stdout, stderr }, 'stdout', '中文测试\n');
  })
  );
 
@@ -80,9 +80,22 @@ const unicode = '中文测试'; // length = 4, byte length = 12
  cmd,
  { encoding: null, maxBuffer: 3 },
  common.mustCall((err, stdout, stderr) => {
- runChecks(err, { stdout, stderr }, 'stderr', '');
+ runChecks(err, { stdout, stderr }, 'stderr', '中文测');
  })
  );
 
  child.stderr.setEncoding('utf-8');
 }
+
+{
+ const cmd = `"${process.execPath}" -e "console.error('${unicode}');"`;
+
+ cp.exec(
+ cmd,
+ { encoding: null, maxBuffer: 5 },
+ common.mustCall((err, stdout, stderr) => {
+ const buf = Buffer.from(unicode).slice(0, 5);
+ runChecks(err, { stdout, stderr }, 'stderr', buf);
+ })
+ );
+}