[v12.x backport] v8: implement v8.takeCoverage() and v8.stopCoverage()

doc/api/v8.md

@@ -214,6 +214,32 @@ v8.setFlagsFromString('--trace_gc');
 setTimeout(() => { v8.setFlagsFromString('--notrace_gc'); }, 60e3);
 ```
 
+## `v8.takeCoverage()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+The `v8.takeCoverage()` method allows the user to write the coverage started by
+[`NODE_V8_COVERAGE`][] to disk on demand. This method can be invoked multiple
+times during the lifetime of the process, each time the execution counter will
+be reset and a new coverage report will be written to the directory specified
+by [`NODE_V8_COVERAGE`][].
+
+When the process is about to exit, one last coverage will still be written to
+disk, unless [`v8.stopCoverage()`][] is invoked before the process exits.
+
+## `v8.stopCoverage()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+The `v8.stopCoverage()` method allows the user to stop the coverage collection
+started by [`NODE_V8_COVERAGE`][], so that V8 can release the execution count
+records and optimize code. This can be used in conjunction with
+`v8.takeCoverage()` if the user wants to collect the coverage on demand.
+
 ## `v8.writeHeapSnapshot([filename])`
 <!-- YAML
 added: v11.13.0
@@ -509,6 +535,7 @@ A subclass of [`Deserializer`][] corresponding to the format written by
 [`Deserializer`]: #v8_class_v8_deserializer
 [`Error`]: errors.html#errors_class_error
 [`GetHeapSpaceStatistics`]: https://v8docs.nodesource.com/node-13.2/d5/dda/classv8_1_1_isolate.html#ac673576f24fdc7a33378f8f57e1d13a4
+[`NODE_V8_COVERAGE`]: cli.md#cli_node_v8_coverage_dir
 [`Serializer`]: #v8_class_v8_serializer
 [`deserializer._readHostObject()`]: #v8_deserializer_readhostobject
 [`deserializer.transferArrayBuffer()`]: #v8_deserializer_transferarraybuffer_id_arraybuffer
@@ -518,6 +545,7 @@ A subclass of [`Deserializer`][] corresponding to the format written by
 [`serializer.releaseBuffer()`]: #v8_serializer_releasebuffer
 [`serializer.transferArrayBuffer()`]: #v8_serializer_transferarraybuffer_id_arraybuffer
 [`serializer.writeRawBytes()`]: #v8_serializer_writerawbytes_buffer
+[`v8.stopCoverage()`]: #v8_v8_stopcoverage
 [`vm.Script`]: vm.html#vm_new_vm_script_code_options
 [HTML structured clone algorithm]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm
 [V8]: https://developers.google.com/v8/

lib/v8.js

@@ -37,6 +37,12 @@ const {
   Serializer: _Serializer,
   Deserializer: _Deserializer
 } = internalBinding('serdes');
+
+let profiler = {};
+if (internalBinding('config').hasInspector) {
+  profiler = internalBinding('profiler');
+}
+
 const assert = require('internal/assert');
 const { copy } = internalBinding('buffer');
 const { inspect } = require('internal/util/inspect');
@@ -286,6 +292,8 @@ module.exports = {
   DefaultSerializer,
   DefaultDeserializer,
   deserialize,
+  takeCoverage: profiler.takeCoverage,
+  stopCoverage: profiler.stopCoverage,
   serialize,
   writeHeapSnapshot,
 };