Updating the prevFile pointer of upstream file(s)

With the current code it is possible that we can
miss updating the pointer of an upstream file if
an intermediate file is being removed. This change
corrects that logic.

Change-Id: Ic554dca3bb005bf12c31d83ec3f7f405d91866fb

Author: abhinavdangeti

src/filemgr.cc

@@ -1522,9 +1522,31 @@ uint64_t FileMgr::fetchPrevHeader(uint64_t bid, void *buf, size_t *len,
 }
 
 void FileMgr::updateFilePointers() {
+    /**
+     * Example:
+     *
+     *  null <-- | A.0 | <-- | A.1 | <-- | A.2 | <-- | A.3 | --> null
+     *              \           \           \----------->
+     *               \           \---------------------->
+     *                \--------------------------------->
+     *
+     * Scenarios to be considered:
+     * (1) If A.3 were being removed (as the latest file is being removed):
+     *      - A.2 should point to NULL
+     *      - A.1 can point to NULL
+     *      - A.0 can point to NULL
+     * (2) If A.2 were being removed:
+     *      - A.3 would need to point to A.1
+     * (3) If A.1 were being removed:
+     *      - A.2 would need to point to A.0
+     * (4) If A.0 were being removed:
+     *      - A.1 would need to point to NULL
+     *
+     */
+
     // Update new_file pointers of all previously redirected downstream files
     FileMgr *temp = getPrevFile();
-    while (temp != NULL) {
+    while (temp != nullptr) {
         temp->acquireSpinLock();
         if (temp->getNewFile() == this) {
             temp->setNewFile(getNewFile());
@@ -1533,13 +1555,16 @@ void FileMgr::updateFilePointers() {
         temp = temp->getPrevFile();
     }
     // Update prev_file pointer of the upstream file if any
-    FileMgr *new_file = getNewFile();
-    if (new_file != NULL) {
-        new_file->acquireSpinLock();
-        if (new_file->getPrevFile() == this) {
-            new_file->setPrevFile(getPrevFile());
+    temp = getNewFile();
+    while (temp != nullptr && temp != this) {
+        temp->acquireSpinLock();
+        if (temp->getPrevFile() == this) {
+            temp->setPrevFile(getPrevFile());
+            temp->releaseSpinLock();
+            break;
         }
-        new_file->releaseSpinLock();
+        temp->releaseSpinLock();
+        temp = temp->getPrevFile();
     }
 }