EBADF Error When Spawning Processes On MacOS In Watch Mode Environments
Introduction
When using tree-kill on macOS within applications that frequently restart, such as NestJS in watch mode, the module fails with a spawn EBADF error. This error occurs in the buildProcessTree function when attempting to spawn child processes to get the process tree. This issue appears to be due to file descriptor handling issues specific to macOS.
Environment
- OS: macOS 15.3.1
- Node.js version: v20.18.3
- tree-kill version: v1.2.2
Error Stack Trace
node:internal/child_process:420
throw new ErrnoException(err, 'spawn');
^
Error: spawn EBADF
at ChildProcess.spawn (node:internal/child_process:420:11)
at spawn (node:child_process:762:9)
at /Users/.../node_modules/tree-kill/index.js:33:18
at buildProcessTree (/Users/.../node_modules/tree-kill/index.js:90:14)
at module.exports (/Users/.../node_modules/tree-kill/index.js:32:9)
at /Users/.../node_modules/@nestjs/cli/actions/start.action.js:57:17
at Object.onWatchStatusChange (/Users/.../node_modules/@nestjs/cli/lib/compiler/watch-compiler.js:83:17)
at /Users/.../node_modules/typescript/lib/typescript.js:124995:32
at emitFilesAndReportErrors (/Users/.../node_modules/typescript/lib/typescript.js:124843:7)
at result.afterProgramCreate (/Users/.../node_modules/typescript/lib/typescript.js:124991:7) {
errno: -9,
code: 'EBADF',
syscall: 'spawn'
}
Issue Description
The issue appears to be related to how spawn handles file descriptors on macOS when used recursively in rapid succession. The current implementation uses spawn to build the process tree, which can lead to file descriptor exhaustion or race conditions when processes are frequently being killed and restarted. This issue is particularly noticeable in large monorepo projects, such as those using NestJS in watch mode.
Fix
A potential fix for this issue is to use execSync instead of spawn to build the process tree. This approach avoids the file descriptor handling issues associated with spawn and can help prevent the EBADF error.
function buildProcessTreeMacOS(parentPid, tree, pidsToProcess, cb) {
try {
let stdout = '';
try {
stdout = execSync('pgrep -P ' + parentPid, {
encoding: 'utf8',
timeout: 2000,
maxBuffer: 1024 * 1024
});
} catch (execError) {
if (execError.status !== 1) {
throw execError;
}
}
delete pidsToProcess[parentPid];
const childPids = stdout.trim().split('\n').filter(Boolean);
if (childPids.length === 0) {
if (Object.keys(pidsToProcess).length === 0) {
cb();
}
return;
}
childPids.forEach(function(pidStr) {
const pid = parseInt(pidStr, 10);
if (!isNaN(pid)) {
tree[parentPid].push(pid);
tree[pid] = [];
pidsToProcess[pid] = 1;
buildProcessTreeMacOS(pid, tree, pidsToProcess, cb);
}
});
} catch (err) {
delete pidsToProcess[parentPid];
if (Object.keys(pidsToProcess).length === 0) {
cb();
}
}
}
Conclusion
Q: What is the EBADF error and how does it relate to spawning processes on macOS?
A: The EBADF error is a Unix error code that stands for "Bad File Descriptor." It occurs when a process attempts to access a file descriptor that is invalid or has been closed. In the context of spawning processes on macOS, the EBADF error can occur when the spawn function is used to create a new process, and the file descriptor handling is not properly managed.
Q: What is the cause of the EBADF error in watch mode environments?
A: The cause of the EBADF error in watch mode environments is related to how the spawn function handles file descriptors on macOS. When used recursively in rapid succession, the spawn function can lead to file descriptor exhaustion or race conditions, resulting in the EBADF error.
Q: How can I prevent the EBADF error in watch mode environments?
A: To prevent the EBADF error in watch mode environments, you can use execSync instead of spawn to build the process tree. This approach avoids the file descriptor handling issues associated with spawn and can help prevent the EBADF error.
Q: What is the difference between spawn and execSync?
A: spawn is a function that creates a new process and returns a ChildProcess object. It allows for asynchronous execution of the child process. execSync, on the other hand, is a function that executes a command in the shell and returns the output as a string. It is a synchronous function that blocks the execution of the parent process until the child process completes.
Q: Can I use spawn with macOS and avoid the EBADF error?
A: While it is technically possible to use spawn with macOS and avoid the EBADF error, it is not recommended. The EBADF error is a known issue with spawn on macOS, and using execSync is a more reliable and efficient solution.
Q: How can I implement the fix using execSync?
A: To implement the fix using execSync, you can modify the buildProcessTree function to use execSync instead of spawn. Here is an example of how you can do this:
function buildProcessTreeMacOS(parentPid, tree, pidsToProcess, cb) {
try {
let stdout = '';
try {
stdout = execSync('pgrep -P ' + parentPid, {
encoding: 'utf8',
timeout: 2000,
maxBuffer: 1024 * 1024
});
} catch (execError) {
if (execError.status !== 1) {
throw execError;
}
}
delete pidsToProcess[parentPid];
const childPids = stdout.trim().split('\n').filter(Boolean);
if (childPids.length === 0) {
if (Object.keys(pidsToProcess).length === 0) {
cb();
}
return;
}
childPids.forEach(function(pidStr) {
const pid = parseInt(pidStr, 10);
if (!isNaN(pid)) {
tree[parentPid].push(pid);
tree[pid] = [];
pidsToProcess[pid] = 1;
buildProcessTreeMacOS(pid, tree, pidsToProcess, cb);
}
});
} catch (err) {
delete pidsToProcess[parentPid];
if (Object.keys(pidsToProcess).length === 0) {
cb();
}
}
}
Q: Can I submit a PR to the relevant project with the fix?
A: Yes, you can submit a PR to the relevant project with the fix. If you have implemented the fix using execSync and it works for you, you can share it with the community by submitting a PR to the project. This can help others who are experiencing the same issue and provide a more reliable solution for building the process tree.