perl syntax checking in parallel, but limit the number of processes.

The syntax checking is slow (5-6 mins on github), but checking over 100 scripts simultaneously is also unworkable:
openfoodfacts#3368

The limit is configurable, in case the environment running the checks ever has more than the 2 cores it currently does. Increasing the limit on 2 cores works (at least with 3 & 4), but is no faster.

scripts/check_perl.js

@@ -2,37 +2,109 @@
 /*eslint no-await-in-loop: "off"*/
 
 const process = require("process");
-const util = require("util");
+const util = require('util');
 const glob = util.promisify(require("glob").glob);
-const { spawn } = require("child_process");
-
-function checkFile(path, doneCallback) {
-  console.log(`Checking ${path}`);
-  try {
-    const child = spawn(`perl`, ["-c", "-CS", "-Ilib", path]);
-    child.stdout.on("data", (data) => console.log("[" + path + "] " + data));
-    child.stderr.on("data", (data) => console.error("[" + path + "] " + data));
-    child.on("close", (code) => {
-      console.log("[" + path + "] result: " + code);
-      process.exitCode = code;
-      doneCallback();
-    });
-  } catch (e) {
-    console.error("[" + path + "] " + e);
-    process.exitCode = e.code;
-    throw e;
-  }
-}
+const { spawn } = require('child_process');
 
-const check = util.promisify(checkFile);
+// Github actions have 2 CPUs: 
+// https://docs.github.com/en/free-pro-team@latest/actions/reference/specifications-for-github-hosted-runners#supported-runners-and-hardware-resources
+// Travis also seems to have 2 CPUs:
+// https://docs.travis-ci.com/user/reference/overview/#virtualisation-environment-vs-operating-system
+
+const maxRunning = 2;
+
+function sleep(ms) {
+  return new Promise((resolve) => setTimeout(resolve, ms));
+}
 
 async function main() {
+
+  const filesToCheck = [];
+
+  // build array of paths from given arguments
   // 0 = node; 1 = check_perl.js
   for (const arg of process.argv.slice(2)) {
     const files = await glob(arg);
     for (var i = 0; i < files.length; ++i) {
-      const path = files[i];
-      await check(path);
+      filesToCheck.push(files[i]);
+    }
+  }
+  const numFilesToCheck = filesToCheck.length;
+
+  const spawns = [];
+
+  console.log(`Checking ${numFilesToCheck} files, max ${maxRunning} processes...`);
+
+  let running = Number.MAX_SAFE_INTEGER;
+
+  // loop until we've popped all the files off the array
+  while (filesToCheck.length > 0) {
+
+    // count how many spawned processes don't have an exit code yet
+    running = spawns.reduce((pv, cv) => {
+      if (cv.exitCode === null) {
+        return pv + 1;
+      }
+      else {
+        return pv;
+      }
+    }, 0);
+
+    // if we're not at the maximum number of processes, fire off another
+    if (running < maxRunning) {
+      try {
+
+        const file = filesToCheck.shift();
+        console.log(`Queueing ${file}`);
+        const perl = spawn(
+          'perl', 
+          ["-c", "-CS", "-Ilib", file ],
+          { stdio: ['pipe', 'inherit', 'inherit'] }
+        );
+        spawns.push(perl);
+
+        perl.on('close', (code, signal) => {
+          if (code !== null) {
+            console.log(`[${file}] child process exited with code ${code}`);
+            if (code != 0) {
+              process.exitCode = code;
+            }
+          }
+          else if ( signal !== null ) {
+            console.log(`[${file}] child process exited with signal ${signal}`);
+          }
+        });
+
+      } catch (e) {
+        console.error(e);
+        process.exitCode = e.code;
+        throw e;
+      }
+    }
+
+    // wait before looping
+    if (running > 0) {
+      await sleep(100);
+    }
+
+  }
+
+  console.log('Finished queueing files.');
+
+  // don't return until all child processes have finished
+  running = Number.MAX_SAFE_INTEGER;
+  while (running > 0) {
+    running = spawns.reduce((pv, cv) => {
+      if (cv.exitCode === null) {
+        return pv + 1;
+      }
+      else {
+        return pv;
+      }
+    }, 0);
+
+    if (running > 0) {
+      await sleep(100);
     }
   }