test_runner: adds built in lcov reporter

Fixes nodejs#49626

doc/api/test.md

@@ -402,6 +402,18 @@ 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:
 
@@ -635,6 +647,10 @@ 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.
 

lib/internal/test_runner/reporter/lcov.js

@@ -0,0 +1,104 @@
+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 });
+  }
+
+  _transform(event, _encoding, callback) {
+    if (event.type === 'test:coverage') {
+      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:${file.path.replace(workingDirectory + '/', '')}\n`;
+
+          // Following is a list of line numbers for each function name found in
+          // the source file:
+          // ## FN:\<line number of function start\>,\<function name\>
+          //
+          // After, there is a list of execution counts for each instrumented
+          // function:
+          // ## FNDA:\<execution count\>,\<function name\>
+          //
+          // This loop adds the FN lines to the lcov variable as it goes and
+          // gathers the FNDA lines to be added later. This way we only loop
+          // through the list of functions once.
+          let fnda = '';
+          for (let j = 0; j < file.functions.length; j++) {
+            const func = file.functions[j];
+            const name = func.name || `anonymous_${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:${file.coveredBranchCount}\n`;
+
+          // Then there is a list of execution counts for each instrumented line
+          // (i.e. a line which resulted in executable code):
+          // ## DA:\<line number\>,\<execution count\>[,\<checksum\>]
+          const sortedLines = [...file.lines].sort((a, b) => a.line - b.line);
+          for (let j = 0; j < sortedLines.length; j++) {
+            lcov += `DA:${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) {
+        callback(error);
+      }
+      return callback(null, lcov);
+    }
+    callback(null);
+  }
+}
+
+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 lcov;
+    }
+  }
 });

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

@@ -0,0 +1,11 @@
+'use strict';
+require('../../../common');
+const fixtures = require('../../../common/fixtures');
+const spawn = require('node:child_process').spawn;
+
+const child = spawn(process.execPath,
+                    ['--no-warnings', '--experimental-test-coverage', '--test-reporter', 'lcov', fixtures.path('test-runner/output/output.js')],
+                    { stdio: 'pipe' });
+// eslint-disable-next-line no-control-regex
+child.stdout.on('data', (d) => process.stdout.write(d.toString().replace(/[^\x00-\x7F]/g, '').replace(/\u001b\[\d+m/g, '')));
+child.stderr.pipe(process.stderr);