DevTools: hosted mode fallback if generated files aren't uploaded yet

third_party/WebKit/Source/devtools/scripts/hosted_mode/server.js

Index: third_party/WebKit/Source/devtools/scripts/hosted_mode/server.js
diff --git a/third_party/WebKit/Source/devtools/scripts/hosted_mode/server.js b/third_party/WebKit/Source/devtools/scripts/hosted_mode/server.js
index f821ba9a41a51dcbcc062d7fae9ea3e08680103f..c72e47ec6cb85335bb396731147111eb6c9f08df 100644
--- a/third_party/WebKit/Source/devtools/scripts/hosted_mode/server.js
+++ b/third_party/WebKit/Source/devtools/scripts/hosted_mode/server.js
@@ -116,7 +116,27 @@ function proxy(filePath)
         if (proxyFileCache.has(proxyFileURL))
             return Promise.resolve(proxyFileCache.get(proxyFileURL));
         return utils.fetch(proxyFileURL)
-            .then(cacheProxyFile.bind(null, proxyFileURL));
+            .then(cacheProxyFile.bind(null, proxyFileURL))
+            .catch(onMissingFile);
+    }
+
+    function onMissingFile() {
+        var isFullCheckout = utils.shellOutput("git config --get remote.origin.url") === "https://chromium.googlesource.com/chromium/src.git";
+        var earlierCommitHash;
+        var gitLogCommand = `git log --max-count=1 --grep="Commit-Position" --before="12 hours ago"`;

[chenwilliam, 2016/11/14 23:47:30]

fixed the git grep so it works on full & front-end checkout since the main repo
has "Cr-Commit-Position:" and the mirror repo has "Cr-Original-Commit-Position:"

+        if (isFullCheckout) {
+            earlierCommitHash = utils.shellOutput(`${gitLogCommand} --pretty=format:"%H"`);
+        } else {
+            var commitMessage = utils.shellOutput(`${gitLogCommand}`);
+            earlierCommitHash = commitMessage.match(/Cr-Mirrored-Commit: (.*)/)[1];
+        }
+        var fallbackURL = proxyFilePathToURL[filePath](earlierCommitHash);
+        console.log("WARNING: Unable to fetch generated file based on browser's revision");
+        console.log("Fetching earlier revision of same file as fallback");
+        console.log("There may be a mismatch between the front-end and back-end");
+        console.log(fallbackURL, "\n");
+        return utils.fetch(fallbackURL)
+            .then(cacheProxyFile.bind(null, fallbackURL));
     }
 
     function cacheProxyFile(proxyFileURL, data)