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refactor(exec): move child process to source file #786

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Merged
merged 2 commits into from
Oct 19, 2017
Merged

refactor(exec): move child process to source file #786

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f37b723
refactor(exec): move child process to source file
nfischer Oct 12, 2017
057f442
Disallow requiring exec-child.js
nfischer Oct 19, 2017
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42 changes: 42 additions & 0 deletions src/exec-child.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,42 @@
var childProcess = require('child_process');
var fs = require('fs');

// Note: this will break if `paramFilePath` contains special characters ( '\n',
// '\t', etc.). Such characters are possible if $TMP gets modified. We already
// rely on tempdir() to work for other things, so this is an acceptable risk.
var paramFilePath = process.argv[2];

var serializedParams = fs.readFileSync(paramFilePath, 'utf8');
var params = JSON.parse(serializedParams);

var cmd = params.command;
var execOptions = params.execOptions;
var pipe = params.pipe;
var stdoutFile = params.stdoutFile;
var stderrFile = params.stderrFile;
var codeFile = params.codeFile;

var c = childProcess.exec(cmd, execOptions, function (err) {
if (!err) {
fs.writeFileSync(codeFile, '0');
} else if (err.code === undefined) {
fs.writeFileSync(codeFile, '1');
} else {
fs.writeFileSync(codeFile, err.code.toString());
}
});

var stdoutStream = fs.createWriteStream(stdoutFile);
var stderrStream = fs.createWriteStream(stderrFile);

c.stdout.pipe(stdoutStream, { end: false });
c.stderr.pipe(stderrStream, { end: false });
c.stdout.pipe(process.stdout);
c.stderr.pipe(process.stderr);

if (pipe) {
c.stdin.end(pipe);
}

c.stdout.on('end', stdoutStream.end);
c.stderr.on('end', stderrStream.end);
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If this file is require'd, it will actually do something. To prevent this, wrap the contents like this:

if (require.main === module) {
    // everything goes here
}

This way, the code is only executed when the file is the entrypoint, like when node exec-file.js is run.

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Seems reasonable. Do you think it's worth adding this to similar files (e.g. bin/shjs)?

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Yes, I think this would be a good idea for other executable-only files.

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Filed #789. I think it would be better to fail loudly in this case (with an exception). Take a look at my alternate suggestion, let me know if there's some issue with this.

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Failing loudly sounds like a good idea.

76 changes: 29 additions & 47 deletions src/exec.js
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Original file line number Diff line number Diff line change
Expand Up @@ -24,10 +24,10 @@ function execSync(cmd, opts, pipe) {
}

var tempDir = _tempDir();
var stdoutFile = path.resolve(tempDir + '/' + common.randomFileName());
var stderrFile = path.resolve(tempDir + '/' + common.randomFileName());
var codeFile = path.resolve(tempDir + '/' + common.randomFileName());
var scriptFile = path.resolve(tempDir + '/' + common.randomFileName());
var paramsFile = path.resolve(tempDir + '/' + common.randomFileName());
var stderrFile = path.resolve(tempDir + '/' + common.randomFileName());
var stdoutFile = path.resolve(tempDir + '/' + common.randomFileName());

opts = common.extend({
silent: common.config.silent,
Expand All @@ -37,47 +37,29 @@ function execSync(cmd, opts, pipe) {
encoding: 'utf8',
}, opts);

if (fs.existsSync(scriptFile)) common.unlinkSync(scriptFile);
if (fs.existsSync(stdoutFile)) common.unlinkSync(stdoutFile);
if (fs.existsSync(stderrFile)) common.unlinkSync(stderrFile);
if (fs.existsSync(codeFile)) common.unlinkSync(codeFile);

var execCommand = JSON.stringify(common.config.execPath) + ' ' + JSON.stringify(scriptFile);
var script;
if (fs.existsSync(paramsFile)) common.unlinkSync(paramsFile);
if (fs.existsSync(stderrFile)) common.unlinkSync(stderrFile);
if (fs.existsSync(stdoutFile)) common.unlinkSync(stdoutFile);

opts.cwd = path.resolve(opts.cwd);
var optString = JSON.stringify(opts);

script = [
"var child = require('child_process')",
" , fs = require('fs');",
'var childProcess = child.exec(' + JSON.stringify(cmd) + ', ' + optString + ', function(err) {',
' var fname = ' + JSON.stringify(codeFile) + ';',
' if (!err) {',
' fs.writeFileSync(fname, "0");',
' } else if (err.code === undefined) {',
' fs.writeFileSync(fname, "1");',
' } else {',
' fs.writeFileSync(fname, err.code.toString());',
' }',
'});',
'var stdoutStream = fs.createWriteStream(' + JSON.stringify(stdoutFile) + ');',
'var stderrStream = fs.createWriteStream(' + JSON.stringify(stderrFile) + ');',
'childProcess.stdout.pipe(stdoutStream, {end: false});',
'childProcess.stderr.pipe(stderrStream, {end: false});',
'childProcess.stdout.pipe(process.stdout);',
'childProcess.stderr.pipe(process.stderr);',
].join('\n') +
(pipe ? '\nchildProcess.stdin.end(' + JSON.stringify(pipe) + ');\n' : '\n') +
[
'var stdoutEnded = false, stderrEnded = false;',
'function tryClosingStdout(){ if(stdoutEnded){ stdoutStream.end(); } }',
'function tryClosingStderr(){ if(stderrEnded){ stderrStream.end(); } }',
"childProcess.stdout.on('end', function(){ stdoutEnded = true; tryClosingStdout(); });",
"childProcess.stderr.on('end', function(){ stderrEnded = true; tryClosingStderr(); });",
].join('\n');

fs.writeFileSync(scriptFile, script);

var paramsToSerialize = {
command: cmd,
execOptions: opts,
pipe: pipe,
stdoutFile: stdoutFile,
stderrFile: stderrFile,
codeFile: codeFile,
};

fs.writeFileSync(paramsFile, JSON.stringify(paramsToSerialize), 'utf8');
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What is the advantage of storing the params in a separate file, as opposed to simply passing them in as a argument to the command?

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That was my original approach, but there's some issues with passing via CLI argument (at least in shells, via execSync). As an example, we could look at the multi-line string:

has"chars
.txt

JSON.stringify gives us "has\"chars\n.txt". So far, so good.

When this gets passed to the shell, it looks something like:

node exec-child.js "has\"chars\n.txt"

The shell interprets the string and supplies the interpreted value to the node process, so process.argv[2] looks like has"chars\n.txt (that's a one-line string with a literal \ followed by a literal n).

This is where we hit issues. Our string is partially deserialized, so we can't use JSON.parse, but it's still not equivalent to our original string. This is an unfortunate consequence of using both shell and JS--they have different opinions on how to escape strings.

We can consider passing these by CLI argument if we replace execSync() with execFileSync() within exec.js. This is one of my intended future optimizations, but I left it out to limit complexity. Such a change is backwards compatible (we just can't change exec() within exec-child.js).

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Ok, that makes sense. We eliminate the need to write the JS file every call, but in return we need to write the arguments to a file. Not the best, but it's an improvement overall.

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This is still strictly better. The JS file used to contain each argument written inside it (as literals), and sizeof(code) + sizeof(arguments) > sizeof(arguments).

I'll also send some PRs to trim down the number of arguments, which will be a bigger win.


var execCommand = [
JSON.stringify(common.config.execPath),
JSON.stringify(path.join(__dirname, 'exec-child.js')),
JSON.stringify(paramsFile),
].join(' ');

/* istanbul ignore else */
if (opts.silent) {
Expand All @@ -91,10 +73,10 @@ function execSync(cmd, opts, pipe) {
child.execSync(execCommand, opts);
} catch (e) {
// Clean up immediately if we have an exception
try { common.unlinkSync(scriptFile); } catch (e2) {}
try { common.unlinkSync(stdoutFile); } catch (e2) {}
try { common.unlinkSync(stderrFile); } catch (e2) {}
try { common.unlinkSync(codeFile); } catch (e2) {}
try { common.unlinkSync(paramsFile); } catch (e2) {}
try { common.unlinkSync(stderrFile); } catch (e2) {}
try { common.unlinkSync(stdoutFile); } catch (e2) {}
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We're not using e2 anyway, might as well call it e (it won't overwrite the e from above).

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I think there's an eslint warning against that...

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Oh, right.

throw e;
}

Expand All @@ -118,10 +100,10 @@ function execSync(cmd, opts, pipe) {
}

// No biggie if we can't erase the files now -- they're in a temp dir anyway
try { common.unlinkSync(scriptFile); } catch (e) {}
try { common.unlinkSync(stdoutFile); } catch (e) {}
try { common.unlinkSync(stderrFile); } catch (e) {}
try { common.unlinkSync(codeFile); } catch (e) {}
try { common.unlinkSync(paramsFile); } catch (e) {}
try { common.unlinkSync(stderrFile); } catch (e) {}
try { common.unlinkSync(stdoutFile); } catch (e) {}

if (code !== 0) {
common.error('', code, { continue: true });
Expand Down