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

@@ -345,9 +345,15 @@ 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 {
@@ -362,9 +368,15 @@ 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);
+    })
+  );
+}