SERVER-18277 Track elapsed time on cursors using microsecond resolution on OperationContext.

This completes the mechanics of moving max-time tracking to OperationContext and
switching the checkForInterrupt checks to use the service context's fast clock
source, while tracking the amount of execution time remaining on a cursor with
microsecond granularity to ensure that remaining execution time always declines,
even for very brief operations on cursors.

This patch does not complete the transition from wait_for waiting to wait_until
waiting in all places that do waiting based on operation deadlines.

1 files changed, 17 insertions, 4 deletions

diff --git a/src/mongo/db/clientcursor.h b/src/mongo/db/clientcursor.h
index b0244aeb8f7..673fd92bf06 100644
--- a/src/mongo/db/clientcursor.h
+++ b/src/mongo/db/clientcursor.h
@@ -135,10 +135,23 @@ public:
         return _idleAgeMillis;
     }
 
-    uint64_t getLeftoverMaxTimeMicros() const {
+    /**
+     * Returns the amount of time execution time available to this cursor. Only valid at the
+     * beginning of a getMore request, and only really for use by the maxTime tracking code.
+     *
+     * Microseconds::max() == infinity, values less than 1 mean no time left.
+     */
+    Microseconds getLeftoverMaxTimeMicros() const {
         return _leftoverMaxTimeMicros;
     }
-    void setLeftoverMaxTimeMicros(uint64_t leftoverMaxTimeMicros) {
+
+    /**
+     * Sets the amount of execution time available to this cursor. This is only called when an
+     * operation that uses a cursor is finishing, to update its remaining time.
+     *
+     * Microseconds::max() == infinity, values less than 1 mean no time left.
+     */
+    void setLeftoverMaxTimeMicros(Microseconds leftoverMaxTimeMicros) {
         _leftoverMaxTimeMicros = leftoverMaxTimeMicros;
     }
 
@@ -248,8 +261,8 @@ private:
     // How long has the cursor been idle?
     int _idleAgeMillis;
 
-    // TODO: Document.
-    uint64_t _leftoverMaxTimeMicros;
+    // Unused maxTime budget for this cursor.
+    Microseconds _leftoverMaxTimeMicros = Microseconds::max();
 
     //
     // The underlying execution machinery.