perf: terminate tesseract worker after a delay (#1449)

fixes #1447

src/routes/_utils/runTesseract.js

@@ -1,5 +1,7 @@
 import { importTesseractWorker } from '../_utils/asyncModules'
 
+const DESTROY_WORKER_DELAY = 300000 // 5 minutes
+
 // TODO: it's flaky to try to estimate tesseract's total progress this way
 const steps = [
   { status: 'loading tesseract core', proportion: 0.05 },
@@ -9,6 +11,36 @@ const steps = [
   { status: 'recognizing text', proportion: 0.6 }
 ]
 
+let worker
+let destroyWorkerHandle
+
+async function initWorker () {
+  if (!worker) {
+    worker = (await importTesseractWorker())()
+  }
+}
+
+function destroyWorker () {
+  console.log('destroying tesseract worker')
+  if (worker) {
+    worker.terminate()
+    worker = null
+  }
+}
+
+// destroy the worker after a delay to reduce memory usage
+function scheduleDestroyWorker () {
+  cancelDestroyWorker()
+  destroyWorkerHandle = setTimeout(destroyWorker, DESTROY_WORKER_DELAY)
+}
+
+function cancelDestroyWorker () {
+  if (destroyWorkerHandle) {
+    clearTimeout(destroyWorkerHandle)
+    destroyWorkerHandle = null
+  }
+}
+
 function getTotalProgress (progressInfo) {
   const idx = steps.findIndex(({ status }) => progressInfo.status === status)
   let total = 0
@@ -19,9 +51,7 @@ function getTotalProgress (progressInfo) {
   return total
 }
 
-export async function runTesseract (url, onProgress) {
-  const worker = await importTesseractWorker()
-
+function recognize (url, onProgress) {
   // TODO: have to trick tesseract into not creating a blob URL because that would break our CSP
   // see https://github.com/naptha/tesseract.js/pull/322
   let promise
@@ -38,6 +68,16 @@ export async function runTesseract (url, onProgress) {
       onProgress(getTotalProgress(progressInfo))
     }
   })
-  const res = await promise
-  return res.text
+  return promise
+}
+
+export async function runTesseract (url, onProgress) {
+  cancelDestroyWorker()
+  await initWorker()
+  try {
+    const { text } = await recognize(url, onProgress)
+    return text
+  } finally {
+    scheduleDestroyWorker()
+  }
 }

src/routes/_utils/tesseractWorker.js

@@ -12,10 +12,9 @@ import { TesseractWorker } from 'tesseract.js'
 // which seems excessive. So we just live with the bug for now.
 // https://github.com/naptha/tesseract.js/issues/325
 const { origin } = location
-const tesseractWorker = new TesseractWorker({
+
+export default () => new TesseractWorker({
   workerPath: `${origin}/${workerPath}`,
   langPath: `${origin}/`,
   corePath: `${origin}/${corePath}`
 })
-
-export default tesseractWorker