GitHub auth: use token with the most remaining requests

Part of #529.

lib/github-auth.js

@@ -124,6 +124,7 @@ function sendTokenToAllServers(token) {
 
 // Track rate limit requests remaining.
 
+// Ideally, we would want priority queues here.
 var reqRemaining = new Map();  // From token to requests remaining.
 var reqReset = new Map();  // From token to timestamp.
 
@@ -146,22 +147,44 @@ function utcEpochSeconds() {
 
 var userTokenRateLimit = 12500;
 
+// Return false if the token cannot reasonably be expected to perform
+// a GitHub request.
+function isTokenUsable(token, now) {
+  var reqs = reqRemaining.get(token);
+  var reset = reqReset.get(token);
+  // We don't want to empty more than 3/4 of a user's rate limit.
+  var hasRemainingReqs = reqs > (userTokenRateLimit / 4);
+  var isBeyondRateLimitReset = reset < now;
+  return hasRemainingReqs || isBeyondRateLimitReset;
+}
+
+// Return a list of tokens (as strings) which can be used for a GitHub request,
+// with a reasonable chance that the request will succeed.
+function usableTokens() {
+  var now = utcEpochSeconds();
+  return githubUserTokens.data.filter(function(token) {
+    return isTokenUsable(token, now);
+  });
+}
+
 // Retrieve a user token if there is one for which we believe there are requests
 // remaining. Return undefined if we could not find one.
 function getReqRemainingToken() {
   // Go through the user tokens.
-  // Keep the first one which is usable or has reset.
-  var now = utcEpochSeconds();
-  for (var token of reqReset.keys()) {
+  // Among usable ones, use the one with the highest number of remaining
+  // requests.
+  var tokens = usableTokens();
+  var highestReq = -1;
+  var highestToken;
+  for (var i = 0; i < tokens.length; i++) {
+    var token = tokens[i];
     var reqs = reqRemaining.get(token);
-    var reset = reqReset.get(token);
-    // We don't want to empty more than 3/4 of a user's rate limit.
-    var hasRemainingReqs = reqs > (userTokenRateLimit / 4);
-    var isBeyondRateLimitReset = reset < now;
-    if (hasRemainingReqs || isBeyondRateLimitReset) {
-      return token;
+    if (reqs > highestReq) {
+      highestReq = reqs;
+      highestToken = token;
     }
   }
+  return highestToken;
 }
 
 function addGithubToken(token) {