112 lines
4.2 KiB
Markdown
112 lines
4.2 KiB
Markdown
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# spawn-wrap
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Wrap all spawned Node.js child processes by adding environs and
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arguments ahead of the main JavaScript file argument.
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Any child processes launched by that child process will also be
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wrapped in a similar fashion.
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This is a bit of a brutal hack, designed primarily to support code
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coverage reporting in cases where tests or the system under test are
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loaded via child processes rather than via `require()`.
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It can also be handy if you want to run your own mock executable
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instead of some other thing when child procs call into it.
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[](https://travis-ci.org/istanbuljs/spawn-wrap)
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## USAGE
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```javascript
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var wrap = require('spawn-wrap')
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// wrap(wrapperArgs, environs)
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var unwrap = wrap(['/path/to/my/main.js', 'foo=bar'], { FOO: 1 })
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// later to undo the wrapping, you can call the returned function
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unwrap()
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```
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In this example, the `/path/to/my/main.js` file will be used as the
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"main" module, whenever any Node or io.js child process is started,
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whether via a call to `spawn` or `exec`, whether node is invoked
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directly as the command or as the result of a shebang `#!` lookup.
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In `/path/to/my/main.js`, you can do whatever instrumentation or
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environment manipulation you like. When you're done, and ready to run
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the "real" main.js file (ie, the one that was spawned in the first
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place), you can do this:
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```javascript
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// /path/to/my/main.js
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// process.argv[1] === 'foo=bar'
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// and process.env.FOO === '1'
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// my wrapping manipulations
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setupInstrumentationOrCoverageOrWhatever()
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process.on('exit', function (code) {
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storeCoverageInfoSynchronously()
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})
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// now run the instrumented and covered or whatever codes
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require('spawn-wrap').runMain()
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```
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## ENVIRONMENT VARIABLES
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Spawn-wrap responds to two environment variables, both of which are
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preserved through child processes.
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`SPAWN_WRAP_DEBUG=1` in the environment will make this module dump a
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lot of information to stderr.
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`SPAWN_WRAP_SHIM_ROOT` can be set to a path on the filesystem where
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the shim files are written in a `.node-spawn-wrap-<id>` folder. By
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default this is done in `$HOME`, but in some environments you may wish
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to point it at some other root. (For example, if `$HOME` is mounted
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as read-only in a virtual machine or container.)
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## CONTRACTS and CAVEATS
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The initial wrap call uses synchronous I/O. Probably you should not
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be using this script in any production environments anyway.
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Also, this will slow down child process execution by a lot, since
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we're adding a few layers of indirection.
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The contract which this library aims to uphold is:
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* Wrapped processes behave identical to their unwrapped counterparts
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for all intents and purposes. That means that the wrapper script
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propagates all signals and exit codes.
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* If you send a signal to the wrapper, the child gets the signal.
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* If the child exits with a numeric status code, then the wrapper
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exits with that code.
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* If the child dies with a signal, then the wrapper dies with the
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same signal.
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* If you execute any Node child process, in any of the various ways
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that such a thing can be done, it will be wrapped.
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* Children of wrapped processes are also wrapped.
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(Much of this made possible by
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[foreground-child](http://npm.im/foreground-child).)
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There are a few ways situations in which this contract cannot be
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adhered to, despite best efforts:
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1. In order to handle cases where `node` is invoked in a shell script,
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the `PATH` environment variable is modified such that the the shim
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will be run before the "real" node. However, since Windows does
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not allow executing shebang scripts like regular programs, a
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`node.cmd` file is required.
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2. Signal propagation through `dash` doesn't always work. So, if you
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use `child_process.exec()` on systems where `/bin/sh` is actually
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`dash`, then the process may exit with a status code > 128 rather
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than indicating that it received a signal.
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3. `cmd.exe` is even stranger with how it propagates and interprets
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unix signals. If you want your programs to be portable, then
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probably you wanna not rely on signals too much.
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4. It *is* possible to escape the wrapping, if you spawn a bash
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script, and that script modifies the `PATH`, and then calls a
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specific `node` binary explicitly.
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