Added concept of "transaction fingerprints"

The region allocator marks freed regions "Condemned" before actually freeing them.  Condemned regions are associated 
with a transaction; before they can be reused / coalesced the transaction that condemned them must complete.

The region allocator used "TisActiveTransaction(xid)" to determine whether a given transaction had completed.  The 
problem is that Stasis now reuses transaction id's, and condemned regions were never reclaimed in practice.  A transaction
fingerprint is a unique identifier (currently transaction id, rec_lsn) that can be used to distinguish transactions that
did not execute concurrently.

I considered an alternative design based upon end-of-transaction actions.  This would have worked as well, but would 
have been a more invasive change.

src/stasis/operations/regions.c

@@ -59,7 +59,8 @@ static int operate_dealloc_region_unlocked(int xid, const regionAllocArg *dat) {
   assert(ret);
 
   t.status = REGION_VACANT;
-  t.region_xid = xid;
+  int err = TgetTransactionFingerprint(xid, &t.region_xid_fp);
+  assert(!err);
 
   TsetBoundaryTag(xid, firstPage -1, &t);
 
@@ -132,7 +133,8 @@ static void TdeallocBoundaryTag(int xid, pageid_t page) {
   int ret = readBoundaryTag(xid, page, &t);
   assert(ret);
   t.status = REGION_CONDEMNED;
-  t.region_xid = xid;
+  int err = TgetTransactionFingerprint(xid, &t.region_xid_fp);
+  assert(!err);
   TsetBoundaryTag(xid, page, &t);
 
 }
@@ -146,7 +148,7 @@ void regionsInit(stasis_log_t *log) {
     t.size = PAGEID_T_MAX;
     t.prev_size = PAGEID_T_MAX;
     t.status = REGION_VACANT;
-    t.region_xid = INVALID_XID;
+    TgetTransactionFingerprint(INVALID_XID, &t.region_xid_fp);
     t.allocation_manager = 0;
 
     // This does what TallocBoundaryTag(-1, 0, &t); would do, but it
@@ -285,7 +287,7 @@ static void TregionAllocHelper(int xid, pageid_t page, pageid_t pageCount, int a
     //  - It could cause some fragmentation if interleaved transactions are allocating, and some abort.
     //  - Multiple transactions can allocate space at the end of the page file without blocking each other.
     new_tag.status = REGION_VACANT;
-    new_tag.region_xid = INVALID_XID;
+    TgetTransactionFingerprint(INVALID_XID, &new_tag.region_xid_fp);
     new_tag.allocation_manager = 0;
 
     TallocBoundaryTag(xid, newPageid, &new_tag);
@@ -293,7 +295,8 @@ static void TregionAllocHelper(int xid, pageid_t page, pageid_t pageCount, int a
   }
 
   t.status = REGION_ZONED;
-  t.region_xid = xid;
+  int err = TgetTransactionFingerprint(xid, &t.region_xid_fp);
+  assert(!err);
   t.allocation_manager = allocationManager;
   t.size = pageCount;
 
@@ -303,7 +306,7 @@ static void TregionAllocHelper(int xid, pageid_t page, pageid_t pageCount, int a
 
 static void consolidateRegions(int xid, pageid_t * firstPage, boundary_tag  *t) {
 
-  if(t->status != REGION_VACANT || TisActiveTransaction(t->region_xid)) { return; }
+  if(t->status != REGION_VACANT || TisActiveTransaction(&t->region_xid_fp)) { return; }
 
   //  (*firstPage)++;
 
@@ -324,7 +327,7 @@ static void consolidateRegions(int xid, pageid_t * firstPage, boundary_tag  *t)
       // TODO: Truncate page file.
       TdeallocBoundaryTag(xid, succ_page);
       mustWriteOriginalTag = 1;
-    } else if(succ_tag.status == REGION_VACANT && (!TisActiveTransaction(succ_tag.region_xid))) {
+    } else if(succ_tag.status == REGION_VACANT && (!TisActiveTransaction(&succ_tag.region_xid_fp))) {
 
       t->size = t->size + succ_tag.size + 1;
       pageid_t succ_succ_page = succ_page + succ_tag.size + 1;
@@ -357,7 +360,7 @@ static void consolidateRegions(int xid, pageid_t * firstPage, boundary_tag  *t)
     int ret = readBoundaryTag(xid, pred_page, &pred_tag);
     assert(ret);
 
-    if(pred_tag.status == REGION_VACANT && (!TisActiveTransaction(pred_tag.region_xid))) {
+    if(pred_tag.status == REGION_VACANT && (!TisActiveTransaction(&pred_tag.region_xid_fp))) {
 
       TdeallocBoundaryTag(xid, *firstPage);
 
@@ -452,7 +455,7 @@ pageid_t TregionAlloc(int xid, pageid_t pageCount, int allocationManager) {
 
   //  printf(" %d, {%d, %d, %d}\tpageCount=%d\n", pageid, t.size, t.prev_size, t.status, pageCount);
 
-  while(t.status != REGION_VACANT || t.size < pageCount || TisActiveTransaction(t.region_xid)) {
+  while(t.status != REGION_VACANT || t.size < pageCount || TisActiveTransaction(&t.region_xid_fp)) {
     // TODO: This while loop and the boundary tag manipulation below should be factored into two submodules.
 
     //    printf("t.status = %d, REGION_VACANT = %d, t.size = %d, pageCount = %d\n", t.status, REGION_VACANT, t.size, pageCount);

src/stasis/transactional2.c

@@ -444,8 +444,27 @@ int TactiveThreadCount(void) {
 int* TlistActiveTransactions(int *count) {
   return stasis_transaction_table_list_active(stasis_transaction_table, count);
 }
-int TisActiveTransaction(int xid) {
+int TgetTransactionFingerprint(int xid, stasis_transaction_fingerprint_t * fp) {
-  return stasis_transaction_table_is_active(stasis_transaction_table, xid);
+  if(stasis_transaction_table_is_active(stasis_transaction_table, xid)) {
+    lsn_t rec_lsn = stasis_transaction_table_get(stasis_transaction_table, xid)->recLSN;
+    if(rec_lsn == INVALID_LSN) {  // Generate a dummy entry.
+      Tupdate(xid,0,0,0,OPERATION_NOOP);
+    }
+    fp->xid = xid;
+    rec_lsn = stasis_transaction_table_get(stasis_transaction_table, xid)->recLSN;
+    fp->rec_lsn = rec_lsn;
+    return 0;
+  } else if(xid == INVALID_XID) {
+    fp->xid = INVALID_XID;
+    fp->rec_lsn = INVALID_LSN;
+  } else {
+    return ENOENT;
+  }
+}
+int TisActiveTransaction(stasis_transaction_fingerprint_t * fp) {
+  // stasis_transaction_table_is_active returns false for INVALID_XID, which is what we want.
+  return stasis_transaction_table_is_active(stasis_transaction_table, fp->xid)
+         && stasis_transaction_table_get(stasis_transaction_table, fp->xid)->recLSN == fp->rec_lsn;
 }
 
 

stasis/operations/regions.h

@@ -16,7 +16,7 @@ typedef struct boundary_tag {
   pageid_t size;
   pageid_t prev_size;
   int status;
-  int region_xid;
+  stasis_transaction_fingerprint_t region_xid_fp;
   int allocation_manager;
 } boundary_tag;
 

stasis/transactional.h

@@ -727,13 +727,25 @@ lsn_t TendNestedTopAction(int xid, void * handle);
  */
 int* TlistActiveTransactions(int *count);
 
+/**
+ * A unique (for all time) transaction descriptor.
+ *
+ * Unlike xids, fingerprints are not reused.
+ *
+ */
+typedef struct stasis_transaction_fingerprint_t {
+  lsn_t rec_lsn;  // The lsn of the first entry in the transaction
+  int xid; // The transaction id.  Lets us look up the rec_lsn in the transaction table.
+} stasis_transaction_fingerprint_t;
+
+int TgetTransactionFingerprint(int xid, stasis_transaction_fingerprint_t * fp);
 /**
  * Checks to see if a transaction is still active.
  *
  * @param xid The transaction id to be tested.
  * @return true if the transaction is still running, false otherwise.
  */
-int TisActiveTransaction(int xid);
+int TisActiveTransaction(stasis_transaction_fingerprint_t* fp);
 /*
  * @return the number of currently active transactions.
  */

test/stasis/check_regions.c

@@ -186,12 +186,6 @@ START_TEST(regions_randomizedTest) {
   Tcommit(xid);
 
   Tdeinit();
-  if((double)max_size/(double)max_ideal_size > 5) {
-    // max_blowup isn't what we want here; it measures the peak
-    // percentage of the file that is unused.  Instead, we want to
-    // measure the actual and ideal page file sizes for this run.
-    printf("WARNING: Excessive blowup ");
-  }
   printf("Max # of regions = %lld, page file size = %5.2fM, ideal page file size = %5.2fM, (blowup = %5.2f)\n",
 	 //peak bytes wasted = %5.2fM, blowup = %3.2f\n",
 	 max_region_count,
@@ -200,6 +194,13 @@ START_TEST(regions_randomizedTest) {
 	 (double)max_size/(double)max_ideal_size);
   //	 ((double)max_waste * PAGE_SIZE)/(1024.0*1024.0),
   //	 max_blowup);
+  if((double)max_size/(double)max_ideal_size > 5) {
+    // max_blowup isn't what we want here; it measures the peak
+    // percentage of the file that is unused.  Instead, we want to
+    // measure the actual and ideal page file sizes for this run.
+    printf("ERROR: Excessive blowup (max allowable is 5)\n");
+    abort();
+  }
 
 } END_TEST