Add verification mode to screenshotter

Now Travis can run the screenshotter in verification mode.  The files in the
repository will be seen as the expected outcome, and if the actual result
differs from that, it might be attempted four more times before the test
case is actually deemed failed.  A timeout between page load and screenshot
should allow any possible font issues to settle down.

.travis.yml

@@ -9,5 +9,4 @@ before_script:
   - docker pull selenium/standalone-chrome:2.46.0
 script:
   - npm test
-  - dockers/Screenshotter/screenshotter.sh
-  - git diff --name-only --exit-code
+  - dockers/Screenshotter/screenshotter.sh --verify

dockers/Screenshotter/screenshotter.js

@@ -63,6 +63,10 @@ var opts = require("nomnom")
         abbr: "x",
         help: "Comma-separated list of test cases to exclude"
     })
+    .option("verify", {
+        flag: true,
+        help: "Check whether screenshot matches current file content"
+    })
     .parse();
 
 var listOfCases;
@@ -204,7 +208,9 @@ function buildDriver() {
         builder.usingServer(seleniumURL);
     }
     driver = builder.build();
-    setSize(targetW, targetH);
+    driver.manage().timeouts().setScriptTimeout(3000).then(function() {
+        setSize(targetW, targetH);
+    });
 }
 
 //////////////////////////////////////////////////////////////////////
@@ -243,6 +249,9 @@ function imageDimensions(img) {
 
 var countdown = listOfCases.length;
 
+var exitStatus = 0;
+var listOfFailed = [];
+
 function takeScreenshots() {
     listOfCases.forEach(takeScreenshot);
 }
@@ -253,9 +262,27 @@ function takeScreenshot(key) {
         console.error("Test case " + key + " not known!");
         return;
     }
+
+    var file = path.join(dstDir, key + "-" + opts.browser + ".png");
+    var retry = 0;
+    var loadExpected = null;
+    if (opts.verify) {
+        loadExpected = promisify(fs.readFile, file);
+    }
+
     var url = katexURL + "test/screenshotter/test.html?" + itm.query;
     driver.get(url);
-    driver.takeScreenshot().then(function haveScreenshot(img) {
+    driver.takeScreenshot().then(haveScreenshot).then(function() {
+        if (--countdown === 0) {
+            if (listOfFailed.length) {
+                console.error("Failed: " + listOfFailed.join(" "));
+            }
+            // devServer.close(cb) will take too long.
+            process.exit(exitStatus);
+        }
+    }, check);
+
+    function haveScreenshot(img) {
         img = imageDimensions(img);
         if (img.width !== targetW || img.height !== targetH) {
             throw new Error("Excpected " + targetW + " x " + targetH +
@@ -270,27 +297,63 @@ function takeScreenshot(key) {
              * output file name for one of these cases, we accept both.
              */
             key += "_alt";
+            file = path.join(dstDir, key + "-" + opts.browser + ".png");
+            if (loadExpected) {
+                loadExpected = promisify(fs.readFile, file);
+            }
         }
-        var file = path.join(dstDir, key + "-" + opts.browser + ".png");
-        var deferred = new selenium.promise.Deferred();
         var opt = new jspngopt.Optimizer({
             pako: pako
         });
         var buf = opt.bufferSync(img.buf);
-        fs.writeFile(file, buf, function(err) {
-            if (err) {
-                deferred.reject(err);
-            }
-            else {
-                deferred.fulfill();
-            }
-        });
-        return deferred.promise;
-    }).then(function() {
-        console.log(key);
-        if (--countdown === 0) {
-            // devServer.close(cb) will take too long.
-            process.exit(0);
+        if (loadExpected) {
+            return loadExpected.then(function(expected) {
+                if (!buf.equals(expected)) {
+                    if (++retry === 5) {
+                        console.error("FAIL! " + key);
+                        listOfFailed.push(key);
+                        exitStatus = 3;
+                    } else {
+                        console.log("error " + key);
+                        driver.get(url);
+                        browserSideWait(500 * retry);
+                        return driver.takeScreenshot().then(haveScreenshot);
+                    }
+                } else {
+                    console.log("* ok  " + key);
+                }
+            });
+        } else {
+            return promisify(fs.writeFile, file, buf).then(function() {
+                console.log(key);
+            });
         }
-    }, check);
+    }
+}
+
+// Wait using a timeout call in the browser, to ensure that the wait
+// time doesn't start before the page has reportedly been loaded.
+function browserSideWait(milliseconds) {
+    // The last argument (arguments[1] here) is the callback to selenium
+    return driver.executeAsyncScript(
+        "window.setTimeout(arguments[1], arguments[0]);",
+        milliseconds);
+}
+
+// Turn node callback style into a call returning a promise,
+// like Q.nfcall but using Selenium promises instead of Q ones.
+// Second and later arguments are passed to the function named in the
+// first argument, and a callback is added as last argument.
+function promisify(f) {
+    var args = Array.prototype.slice.call(arguments, 1);
+    var deferred = new selenium.promise.Deferred();
+    args.push(function(err, val) {
+        if (err) {
+            deferred.reject(err);
+        } else {
+            deferred.fulfill(val);
+        }
+    });
+    f.apply(null, args);
+    return deferred.promise;
 }