Bug 1647470: Fix MinTabSelector to take into account empty but reusable content processes; r=nika

When the current number of existing content processes is less than the maximum,
`MinTabSelector` previously always created a new process. This is inefficient in
the case where we have `dom.ipc.keepProcessesAlive.web` set and there are
content processes that are idle with zero tabs -- we should allow those to be
reused.

Differential Revision: https://phabricator.services.mozilla.com/D80560

dom/base/ProcessSelector.jsm

@@ -28,19 +28,16 @@ MinTabSelector.prototype = {
   QueryInterface: ChromeUtils.generateQI([Ci.nsIContentProcessProvider]),
 
   provideProcess(aType, aProcesses, aMaxCount) {
-    if (aProcesses.length < aMaxCount) {
-      return Ci.nsIContentProcessProvider.NEW_PROCESS;
-    }
-
     let min = Number.MAX_VALUE;
     let candidate = Ci.nsIContentProcessProvider.NEW_PROCESS;
 
-    // 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
+    // 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
     // content process we can just keep relying on dom.ipc.processCount = 1
     // this way.
-    for (let i = 0; i < aMaxCount; i++) {
+    let numIters = Math.min(aProcesses.length, aMaxCount);
+
+    for (let i = 0; i < numIters; i++) {
       let process = aProcesses[i];
       let tabCount = process.tabCount;
       if (tabCount < min) {
@@ -49,6 +46,13 @@ 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

@@ -848,6 +848,14 @@ 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();
 }
 
@@ -901,8 +909,7 @@ already_AddRefed<ContentParent> ContentParent::GetUsedBrowserProcess(
     // selection.
     NS_WARNING("nsIContentProcessProvider failed to return a process");
     RefPtr<ContentParent> random;
-    if (aContentParents.Length() >= aMaxContentParents &&
-        (random = MinTabSelect(aContentParents, aMaxContentParents))) {
+    if ((random = MinTabSelect(aContentParents, aMaxContentParents))) {
       MOZ_LOG(ContentParent::GetLog(), LogLevel::Debug,
               ("GetUsedProcess: Reused random process %p (%d) for %s",
                random.get(), (unsigned int)random->ChildID(),