test_runner: adds built in lcov test reporter

doc/api/test.md

@@ -403,6 +403,18 @@ if (anAlwaysFalseCondition) {
 }
 ```
 
+### Coverage reporters
+
+The tap and spec reporters will print a summary of the coverage statistics.
+There is also an lcov reporter that will generate an lcov file which can be
+used as an in depth coverage report.
+
+```bash
+node --test --experimental-test-coverage --test-reporter=lcov --test-reporter-destination=lcov.info
+```
+
+### Limitations
+
 The test runner's code coverage functionality has the following limitations,
 which will be addressed in a future Node.js release:
 
@@ -850,6 +862,10 @@ The following built-reporters are supported:
 * `junit`
   The junit reporter outputs test results in a jUnit XML format
 
+* `lcov`
+  The `lcov` reporter outputs test coverage when used with the
+  [`--experimental-test-coverage`][] flag.
+
 When `stdout` is a [TTY][], the `spec` reporter is used by default.
 Otherwise, the `tap` reporter is used by default.
 
@@ -861,11 +877,11 @@ to the test runner's output is required, use the events emitted by the
 The reporters are available via the `node:test/reporters` module:
 
 ```mjs
-import { tap, spec, dot, junit } from 'node:test/reporters';
+import { tap, spec, dot, junit, lcov } from 'node:test/reporters';
 ```
 
 ```cjs
-const { tap, spec, dot, junit } = require('node:test/reporters');
+const { tap, spec, dot, junit, lcov } = require('node:test/reporters');
 ```
 
 ### Custom reporters

lib/internal/test_runner/reporter/lcov.js

@@ -0,0 +1,107 @@
+'use strict';
+
+const { relative } = require('path');
+const Transform = require('internal/streams/transform');
+
+// This reporter is based on the LCOV format, as described here:
+// https://ltp.sourceforge.net/coverage/lcov/geninfo.1.php
+// Excerpts from this documentation are included in the comments that make up
+// the _transform function below.
+class LcovReporter extends Transform {
+  constructor(options) {
+    super({ ...options, writableObjectMode: true, __proto__: null });
+  }
+
+  _transform(event, _encoding, callback) {
+    if (event.type !== 'test:coverage') {
+      return callback(null);
+    }
+    let lcov = '';
+    // A tracefile is made up of several human-readable lines of text, divided
+    // into sections. If available, a tracefile begins with the testname which
+    // is stored in the following format:
+    // ## TN:\<test name\>
+    lcov += 'TN:\n';
+    const {
+      data: {
+        summary: { workingDirectory },
+      },
+    } = event;
+    try {
+      for (let i = 0; i < event.data.summary.files.length; i++) {
+        const file = event.data.summary.files[i];
+        // For each source file referenced in the .da file, there is a section
+        // containing filename and coverage data:
+        // ## SF:\<path to the source file\>
+        lcov += `SF:${relative(workingDirectory, file.path)}\n`;
+
+        // Following is a list of line numbers for each function name found in
+        // the source file:
+        // ## FN:\<line number of function start\>,\<function name\>
+        //
+        // After, there is a list of execution counts for each instrumented
+        // function:
+        // ## FNDA:\<execution count\>,\<function name\>
+        //
+        // This loop adds the FN lines to the lcov variable as it goes and
+        // gathers the FNDA lines to be added later. This way we only loop
+        // through the list of functions once.
+        let fnda = '';
+        for (let j = 0; j < file.functions.length; j++) {
+          const func = file.functions[j];
+          const name = func.name || `anonymous_${j}`;
+          lcov += `FN:${func.line},${name}\n`;
+          fnda += `FNDA:${func.count},${name}\n`;
+        }
+        lcov += fnda;
+
+        // This list is followed by two lines containing the number of
+        // functions found and hit:
+        // ## FNF:\<number of functions found\>
+        // ## FNH:\<number of function hit\>
+        lcov += `FNF:${file.totalFunctionCount}\n`;
+        lcov += `FNH:${file.coveredFunctionCount}\n`;
+
+        // Branch coverage information is stored which one line per branch:
+        // ## BRDA:\<line number\>,\<block number\>,\<branch number\>,\<taken\>
+        // Block number and branch number are gcc internal IDs for the branch.
+        // Taken is either '-' if the basic block containing the branch was
+        // never executed or a number indicating how often that branch was
+        // taken.
+        for (let j = 0; j < file.branches.length; j++) {
+          lcov += `BRDA:${file.branches[j].line},${j},0,${file.branches[j].count}\n`;
+        }
+
+        // Branch coverage summaries are stored in two lines:
+        // ## BRF:\<number of branches found\>
+        // ## BRH:\<number of branches hit\>
+        lcov += `BRF:${file.totalBranchCount}\n`;
+        lcov += `BRH:${file.coveredBranchCount}\n`;
+
+        // Then there is a list of execution counts for each instrumented line
+        // (i.e. a line which resulted in executable code):
+        // ## DA:\<line number\>,\<execution count\>[,\<checksum\>]
+        const sortedLines = file.lines.toSorted((a, b) => a.line - b.line);
+        for (let j = 0; j < sortedLines.length; j++) {
+          lcov += `DA:${sortedLines[j].line},${sortedLines[j].count}\n`;
+        }
+
+        // At the end of a section, there is a summary about how many lines
+        // were found and how many were actually instrumented:
+        // ## LH:\<number of lines with a non-zero execution count\>
+        // ## LF:\<number of instrumented lines\>
+        lcov += `LH:${file.coveredLineCount}\n`;
+        lcov += `LF:${file.totalLineCount}\n`;
+
+        // Each sections ends with:
+        // end_of_record
+        lcov += 'end_of_record\n';
+      }
+    } catch (error) {
+      return callback(error);
+    }
+    return callback(null, lcov);
+  }
+}
+
+module.exports = LcovReporter;

lib/internal/test_runner/utils.js

@@ -112,6 +112,7 @@ const kBuiltinReporters = new SafeMap([
   ['dot', 'internal/test_runner/reporter/dot'],
   ['tap', 'internal/test_runner/reporter/tap'],
   ['junit', 'internal/test_runner/reporter/junit'],
+  ['lcov', 'internal/test_runner/reporter/lcov'],
 ]);
 
 const kDefaultReporter = process.stdout.isTTY ? 'spec' : 'tap';

lib/test/reporters.js

@@ -6,6 +6,7 @@ let dot;
 let junit;
 let spec;
 let tap;
+let lcov;
 
 ObjectDefineProperties(module.exports, {
   __proto__: null,
@@ -45,4 +46,13 @@ ObjectDefineProperties(module.exports, {
       return tap;
     },
   },
+  lcov: {
+    __proto__: null,
+    configurable: true,
+    enumerable: true,
+    get() {
+      lcov ??= require('internal/test_runner/reporter/lcov');
+      return ReflectConstruct(lcov, arguments);
+    },
+  },
 });

test/fixtures/test-runner/output/lcov_reporter.js

@@ -0,0 +1,7 @@
+'use strict';
+require('../../../common');
+const fixtures = require('../../../common/fixtures');
+const spawn = require('node:child_process').spawn;
+
+spawn(process.execPath,
+      ['--no-warnings', '--experimental-test-coverage', '--test-reporter', 'lcov', fixtures.path('test-runner/output/output.js')], { stdio: 'inherit' });