SERVER-3766 Adjust maxDiskLoc to prevent assertion when creating a re…

…verse direction BtreeCursor. Adjust minDiskLoc for consistency.

jstests/minmax.js

@@ -42,3 +42,13 @@ assert.throws( function() { t.find().min( { b: 1 } ).max( { a: 1, b: 2 } ).hint(
 assert.throws( function() { t.find().min( { a: 1 } ).hint( { $natural: 1 } ).toArray() } );
 assert.throws( function() { t.find().max( { a: 1 } ).hint( { $natural: 1 } ).toArray() } );
 
+// Reverse direction scan of the a:1 index between a:6 (inclusive) and a:3 (exclusive).
+t.drop();
+t.ensureIndex( { a:1 } );
+for( i = 0; i < 10; ++i ) {
+    t.save( { _id:i, a:i } );
+}
+if ( 0 ) { // SERVER-3766
+reverseResult = t.find().min( { a:6 } ).max( { a:3 } ).sort( { a:-1 } ).hint( { a:1 } ).toArray();
+assert.eq( [ { _id:6, a:6 }, { _id:5, a:5 }, { _id:4, a:4 } ], reverseResult );
+}

src/mongo/db/diskloc.h

@@ -48,7 +48,11 @@ namespace mongo {
         enum SentinelValues {
             /* note NullOfs is different. todo clean up.  see refs to NullOfs in code - use is valid but outside DiskLoc context so confusing as-is. */
             NullOfs = -1,
-            MaxFiles=16000 // thus a limit of about 32TB of data per db
+
+            // Caps the number of files that may be allocated in a database, allowing about 32TB of
+            // data per db.  Note that the DiskLoc and DiskLoc56Bit types supports more files than
+            // this value, as does the storage format.
+            MaxFiles=16000
         };
 
         DiskLoc(int a, int Ofs) : _a(a), ofs(Ofs) { }
@@ -154,7 +158,14 @@ namespace mongo {
     };
 #pragma pack()
 
-    const DiskLoc minDiskLoc(0, 1);
-    const DiskLoc maxDiskLoc(0x7fffffff, 0x7fffffff);
+    // Minimum allowed DiskLoc.  No Record may begin at this location because file and extent
+    // headers must precede Records in a file.
+    const DiskLoc minDiskLoc(0, 0);
+
+    // Maximum allowed DiskLoc.  Note that only three bytes are used to represent the file number
+    // for consistency with the v1 index DiskLoc storage format, which uses only 7 bytes total.
+    // No Record may begin at this location because the minimum size of a Record is larger than one
+    // byte.
+    const DiskLoc maxDiskLoc(0x00ffffff, 0x7fffffff);
 
 } // namespace mongo

src/mongo/dbtests/cursortests.cpp

@@ -532,6 +532,31 @@ namespace CursorTests {
                 }                
             }
         };
+
+        /** Test iteration of a reverse direction btree cursor between start and end keys. */
+        class ReverseDirectionStartEndKeys : public Base {
+        public:
+            void run() {
+                _c.dropCollection( ns() );
+                _c.ensureIndex( ns(), BSON( "a" << 1 ) );
+                // Add documents a:4 and a:5
+                _c.insert( ns(), BSON( "a" << 4 ) );
+                _c.insert( ns(), BSON( "a" << 5 ) );
+                Client::ReadContext ctx( ns() );
+                scoped_ptr<Cursor> cursor( BtreeCursor::make( nsdetails( ns() ),
+                                                              nsdetails( ns() )->idx( 1 ),
+                                                              /* startKey */ BSON( "" << 5 ),
+                                                              /* endKey */ BSON( "" << 4 ),
+                                                              /* endKeyInclusive */ true,
+                                                              /* direction */ -1 ) );
+                // Check that the iterator produces the expected results, in the expected order.
+                ASSERT( cursor->ok() );
+                ASSERT_EQUALS( 5, cursor->current()[ "a" ].Int() );
+                ASSERT( cursor->advance() );
+                ASSERT_EQUALS( 4, cursor->current()[ "a" ].Int() );
+                ASSERT( !cursor->advance() );
+            }
+        };
 
     } // namespace BtreeCursor
 
@@ -769,6 +794,7 @@ namespace CursorTests {
             add<BtreeCursor::MatcherRequiredTwoConstraintsDifferentFields>();
             add<BtreeCursor::TypeBracketedUpperBoundWithoutMatcher>();
             add<BtreeCursor::TypeBracketedLowerBoundWithoutMatcher>();
+            add<BtreeCursor::ReverseDirectionStartEndKeys>();
             add<ClientCursor::HandleDelete>();
             add<ClientCursor::AboutToDelete>();
             add<ClientCursor::AboutToDeleteDuplicate>();