feat: readInitialCoverage - extracts empty coverage data from an inst…

…rumented file

In support of finally fixing istanbuljs/babel-plugin-istanbul#4.
Blocked on a babel-traverse release that includes babel/babel#4746

src/gen-var.js

@@ -0,0 +1,11 @@
+import {createHash} from 'crypto';
+
+// function to use for creating hashes
+const SHA = 'sha1';
+
+// generate a variable name from hashing the supplied file path
+export default function genVar(filename) {
+    var hash = createHash(SHA);
+    hash.update(filename);
+    return 'cov_' + parseInt(hash.digest('hex').substr(0, 12), 16).toString(36);
+}

src/index.js

@@ -1,5 +1,6 @@
 import Instrumenter from './instrumenter';
 import programVisitor from './visitor';
+import readInitialCoverage from './read-coverage';
 
 /**
  * createInstrumenter creates a new instrumenter with the
@@ -13,4 +14,4 @@ function createInstrumenter(opts) {
 
 export {createInstrumenter};
 export {programVisitor};
-
+export {readInitialCoverage};

src/read-coverage.js

@@ -0,0 +1,47 @@
+import genVar from './gen-var';
+import { parse } from 'babylon';
+import traverse from 'babel-traverse';
+import * as t from 'babel-types';
+
+export default function readInitialCoverage (code, filename) {
+    if (typeof code !== 'string') {
+        throw new Error('Code must be a string');
+    }
+
+    // Parse as leniently as possible
+    const ast = parse(code, {
+        allowImportExportEverywhere: true,
+        allowReturnOutsideFunction: true,
+        allowSuperOutsideMethod: true,
+        sourceType: "script", // I think ?
+        plugins: ["*"]
+    });
+
+    const varName = genVar(filename);
+    let covPath;
+    traverse(ast, {
+        VariableDeclarator: function (path) {
+            if (t.isIdentifier(path.node.id) && path.node.id.name === varName) {
+                covPath = path.get('init');
+                path.stop();
+            }
+        }
+    });
+
+    if (!covPath) {
+        return null;
+    }
+
+    const binding = covPath.get('callee.body').scope.getOwnBinding("coverageData");
+    if (!binding) {
+        return null;
+    }
+
+    const dataPath = binding.path.get('init');
+    const coverageData = dataPath.evaluate();
+    if (!coverageData.confident) {
+        return null;
+    }
+
+    return coverageData.value;
+}

src/visitor.js

@@ -1,4 +1,5 @@
 import {SourceCoverage} from './source-coverage';
+import genVar from './gen-var';
 import {createHash} from 'crypto';
 import template from 'babel-template';
 
@@ -9,12 +10,6 @@ const COMMENT_RE = /^\s*istanbul\s+ignore\s+(if|else|next)(?=\W|$)/;
 // source map URL pattern
 const SOURCE_MAP_RE = /[#@]\s*sourceMappingURL=(.*)\s*$/m;
 
-// generate a variable name from hashing the supplied file path
-function genVar(filename) {
-    var hash = createHash(SHA);
-    hash.update(filename);
-    return 'cov_' + parseInt(hash.digest('hex').substr(0, 12), 16).toString(36);
-}
 
 // VisitState holds the state of the visitor, provides helper functions
 // and is the `this` for the individual coverage visitors.

test/util/verifier.js

@@ -2,6 +2,7 @@ import Instrumenter from '../../src/instrumenter';
 import {classes} from 'istanbul-lib-coverage';
 import {assert} from 'chai';
 import clone from 'clone';
+import readInitialCoverage from '../../src/read-coverage';
 
 var FileCoverage = classes.FileCoverage;
 
@@ -47,6 +48,10 @@ class Verifier {
         assert.deepEqual(cov.f, expectedCoverage.functions || {}, 'Function coverage mismatch');
         assert.deepEqual(cov.b, expectedCoverage.branches || {}, 'Branch coverage mismatch');
         assert.deepEqual(cov.s, expectedCoverage.statements || {}, 'Statement coverage mismatch');
+
+        if (this.result.file) {
+            assert.deepEqual(readInitialCoverage(this.getGeneratedCode(), this.result.file), this.result.emptyCoverage);
+        }
     }
 
     getCoverage() {
@@ -134,7 +139,8 @@ function create(code, opts, instrumenterOpts) {
         code: code,
         generatedCode: instrumenterOutput,
         coverageVariable: coverageVariable,
-        baseline: clone(g[coverageVariable])
+        baseline: clone(g[coverageVariable]),
+        emptyCoverage: instrumenter.lastFileCoverage()
     });
 }