Backed out changeset 481e9149f22f (bug 1647470) for bc failures on browser_resource_uri.js.

dom/base/ProcessSelector.jsm

@@ -39,16 +39,19 @@ MinTabSelector.prototype = {
   QueryInterface: ChromeUtils.generateQI([Ci.nsIContentProcessProvider]),
 
   provideProcess(aType, aOpener, aProcesses, aMaxCount) {
+    if (aProcesses.length < aMaxCount) {
+      return Ci.nsIContentProcessProvider.NEW_PROCESS;
+    }
+
     let min = Number.MAX_VALUE;
     let candidate = Ci.nsIContentProcessProvider.NEW_PROCESS;
 
-    // The reason for not directly using aProcesses.length here is because if
-    // we keep processes alive for testing but want a test to use only single
+    // Note, that at this point aMaxCount is in the valid range and
+    // the reason for not using aProcesses.length here is because if we keep
+    // processes alive for testing but want a test to use only single
     // content process we can just keep relying on dom.ipc.processCount = 1
     // this way.
-    let numIters = Math.min(aProcesses.length, aMaxCount);
-
-    for (let i = 0; i < numIters; i++) {
+    for (let i = 0; i < aMaxCount; i++) {
       let process = aProcesses[i];
       let tabCount = process.tabCount;
       if (process.opener === aOpener && tabCount < min) {
@@ -57,13 +60,6 @@ MinTabSelector.prototype = {
       }
     }
 
-    // If all current processes have at least one tab and we have not yet
-    // reached the maximum, spawn a new process.
-    if (min > 0 && aProcesses.length < aMaxCount) {
-      return Ci.nsIContentProcessProvider.NEW_PROCESS;
-    }
-
-    // Otherwise we use candidate.
     return candidate;
   },
 };

dom/ipc/ContentParent.cpp

@@ -860,14 +860,6 @@ already_AddRefed<ContentParent> ContentParent::MinTabSelect(
     }
   }
 
-  // If all current processes have at least one tab and we have not yet reached
-  // the maximum, use a new process.
-  if (min > 0 &&
-      aContentParents.Length() < static_cast<uint32_t>(aMaxContentParents)) {
-    return nullptr;
-  }
-
-  // Otherwise we return candidate.
   return candidate.forget();
 }
 
@@ -924,7 +916,8 @@ already_AddRefed<ContentParent> ContentParent::GetUsedBrowserProcess(
     // selection.
     NS_WARNING("nsIContentProcessProvider failed to return a process");
     RefPtr<ContentParent> random;
-    if ((random = MinTabSelect(aContentParents, aOpener, aMaxContentParents))) {
+    if (aContentParents.Length() >= aMaxContentParents &&
+        (random = MinTabSelect(aContentParents, aOpener, aMaxContentParents))) {
       MOZ_LOG(ContentParent::GetLog(), LogLevel::Debug,
               ("GetUsedProcess: Reused random process %p (%d) for %s",
                random.get(), (unsigned int)random->ChildID(),