lib: defer source map payload decoding until first use

benchmark/module/module-require-source-map.js

@@ -0,0 +1,77 @@
+'use strict';
+
+// Loading modules that carry source maps, with source map support enabled.
+// This is the cost paid at startup by applications bundled or transpiled with
+// source maps; the maps are only consulted if a stack trace is generated.
+
+const fs = require('fs');
+const path = require('path');
+const common = require('../common.js');
+const tmpdir = require('../../test/common/tmpdir');
+const benchmarkDirectory = tmpdir.resolve('nodejs-benchmark-module-source-map');
+
+const bench = common.createBenchmark(main, {
+  sourceMap: ['none', 'inline', 'external'],
+  n: [1000],
+}, {
+  setup(configs) {
+    tmpdir.refresh();
+    const maxN = configs.reduce((max, c) => Math.max(max, c.n), 0);
+    createModules(maxN);
+  },
+});
+
+function moduleSource(i) {
+  const methods = [];
+  for (let m = 0; m < 40; m++) {
+    methods.push(`  method${m}(input) { return [].concat(input).map((item) => ({ item, m: ${m}, service: ${i} })); }`);
+  }
+  return `'use strict';
+class Service${i} {
+  constructor(options = {}) { this.options = { retries: 3, ...options }; }
+${methods.join('\n')}
+}
+function helper${i}(list) { return list.filter(Boolean).slice(0, ${i % 7}); }
+module.exports = { Service${i}, helper${i} };
+`;
+}
+
+function sourceMapFor(i, source) {
+  return JSON.stringify({
+    version: 3,
+    file: `${i}.js`,
+    sources: [`../src/${i}.ts`],
+    sourcesContent: [source],
+    names: [],
+    mappings: 'AAAA;' + 'AACA,MAAM;'.repeat(44),
+  });
+}
+
+function createModules(n) {
+  for (const kind of ['none', 'inline', 'external']) {
+    const dir = path.join(benchmarkDirectory, kind);
+    fs.mkdirSync(dir, { recursive: true });
+    for (let i = 0; i < n; i++) {
+      const source = moduleSource(i);
+      let trailer = '';
+      if (kind === 'inline') {
+        const data = Buffer.from(sourceMapFor(i, source)).toString('base64');
+        trailer = `//# sourceMappingURL=data:application/json;base64,${data}\n`;
+      } else if (kind === 'external') {
+        fs.writeFileSync(path.join(dir, `${i}.js.map`), sourceMapFor(i, source));
+        trailer = `//# sourceMappingURL=${i}.js.map\n`;
+      }
+      fs.writeFileSync(path.join(dir, `${i}.js`), source + trailer);
+    }
+  }
+}
+
+function main({ sourceMap, n }) {
+  process.setSourceMapsEnabled(true);
+  const dir = path.join(benchmarkDirectory, sourceMap);
+  bench.start();
+  for (let i = 0; i < n; i++) {
+    require(path.join(dir, `${i}.js`));
+  }
+  bench.end(n);
+}

lib/internal/source_map/source_map_cache.js

@@ -9,6 +9,7 @@ const {
   RegExpPrototypeSymbolSplit,
   SafeMap,
   StringPrototypeCodePointAt,
+  StringPrototypeIndexOf,
   StringPrototypeSplit,
   StringPrototypeStartsWith,
 } = primordials;
@@ -176,16 +177,14 @@ function maybeCacheSourceMap(filename, content, moduleInstance, isGeneratedSourc
   // Normalize the sourceURL to a file URL if it is a path.
   sourceURL = normalizeReferrerURL(sourceURL);
 
-  const data = dataFromUrl(filename, sourceMapURL);
-  // `data` could be null if the source map is invalid.
-  // In this case, create a cache entry with null data with source url for test coverage.
-
+  // The payload is resolved on first use (see sourceMapData()), except under
+  // coverage, where it is serialized at exit when no more JS may run.
   const entry = {
     __proto__: null,
     lineLengths: lineLengths(content),
-    data,
-    // Save the source map url if it is not a data url.
-    sourceMapURL: data ? null : sourceMapURL,
+    data: process.env.NODE_V8_COVERAGE ? dataFromUrl(filename, sourceMapURL) : undefined,
+    filename,
+    sourceMapURL,
     sourceURL,
   };
 
@@ -254,20 +253,36 @@ function dataFromUrl(sourceURL, sourceMappingURL) {
   return sourceMapFromFile(mapURL);
 }
 
+const kUnicodeLineTerminators = /[\u2028\u2029]/;
+
 // Cache the length of each line in the file that a source map was extracted
 // from. This allows translation from byte offset V8 coverage reports,
 // to line/column offset Source Map V3.
 function lineLengths(content) {
+  if (RegExpPrototypeExec(kUnicodeLineTerminators, content) !== null) {
+    return lineLengthsWithUnicodeTerminators(content);
+  }
+  // We purposefully keep \r as part of the line-length calculation, in
+  // cases where there is a \r\n separator, so that this can be taken into
+  // account in coverage calculations.
+  const output = [];
+  let lineStart = 0;
+  let lineEnd;
+  while ((lineEnd = StringPrototypeIndexOf(content, '\n', lineStart)) !== -1) {
+    ArrayPrototypePush(output, lineEnd - lineStart);
+    lineStart = lineEnd + 1;
+  }
+  ArrayPrototypePush(output, content.length - lineStart);
+  return output;
+}
+
+function lineLengthsWithUnicodeTerminators(content) {
   const contentLength = content.length;
   const output = [];
   let lineLength = 0;
   for (let i = 0; i < contentLength; i++, lineLength++) {
     const codePoint = StringPrototypeCodePointAt(content, i);
-
-    // We purposefully keep \r as part of the line-length calculation, in
-    // cases where there is a \r\n separator, so that this can be taken into
-    // account in coverage calculations.
-    // codepoints for \n (new line), \u2028 (line separator) and \u2029 (paragraph separator)
+    // \n (new line), \u2028 (line separator) and \u2029 (paragraph separator)
     if (codePoint === 10 || codePoint === 0x2028 || codePoint === 0x2029) {
       ArrayPrototypePush(output, lineLength);
       lineLength = -1; // To not count the matched codePoint such as \n character
@@ -351,16 +366,31 @@ function sourceMapCacheToObject() {
 
   const obj = { __proto__: null };
   for (const { 0: k, 1: v } of moduleSourceMapCache) {
+    const data = v.data ?? null;
     obj[k] = {
       __proto__: null,
       lineLengths: v.lineLengths,
-      data: v.data,
-      url: v.sourceMapURL,
+      data,
+      // Save the source map url if it is not a data url.
+      url: data ? null : v.sourceMapURL,
     };
   }
   return obj;
 }
 
+/**
+ * Resolve and parse the payload of a cache entry the first time it is needed;
+ * `null` marks a source map that could not be loaded.
+ * @param {object} entry
+ * @returns {object|null}
+ */
+function sourceMapData(entry) {
+  if (entry.data === undefined) {
+    entry.data = dataFromUrl(entry.filename, entry.sourceMapURL);
+  }
+  return entry.data;
+}
+
 /**
  * Find a source map for a given actual source URL or path.
  *
@@ -390,7 +420,7 @@ function findSourceMap(sourceURL) {
       sourceURL = pathToFileURL(sourceURL).href;
     }
     const entry = getModuleSourceMapCache().get(sourceURL) ?? generatedSourceMapCache.get(sourceURL);
-    if (entry?.data == null) {
+    if (entry === undefined || sourceMapData(entry) === null) {
       return undefined;
     }