Add retries around SetPathAsSharedOpaqueOutput (#1102)

Public/Src/Engine/Processes/SharedOpaqueOutputHelper.cs

@@ -180,11 +180,42 @@ namespace BuildXL.Processes
             }
         }
 
+        private const int MaxNumberOfAttemptsForMarkingSharedOpaqueOutputs = 3;
+        private static readonly TimeSpan SleepDurationBetweenMarkingAttempts = TimeSpan.FromMilliseconds(10);
+
+        private static TimeSpan Multiply(TimeSpan timeSpan, int multiplier)
+        {
+            return TimeSpan.FromMilliseconds(multiplier * timeSpan.TotalMilliseconds);
+        }
+
         /// <summary>
         /// Marks a given path as "shared opaque output"
         /// </summary>
+        /// <remarks>
+        /// Retries are needed because: (Win|Unix).SetPathAsSharedOpaqueOutput does something like
+        ///   - check if file has write access rights
+        ///   - if it doesn't, give it those rights
+        ///   - mark the file as shared opaque output
+        ///   - ...
+        /// There is a race between checking and setting access rights, so it is possible that
+        /// here we check its rights, we see that it has the correct rights, then someone else 
+        /// (e.g., the cache) revokes those rights, and so we fail to mark the file as shared opaque output.
+        /// </remarks>
         /// <exception cref="BuildXLException">When unsuccessful</exception>
         public static void SetPathAsSharedOpaqueOutput(string expandedPath)
+        {
+            int attempt = 0;
+            while (true)
+            {
+                attempt += 1;
+
+                // wait a bit between attempts
+                if (attempt > 1)
+                {
+                    System.Threading.Thread.Sleep(Multiply(SleepDurationBetweenMarkingAttempts, attempt - 1));
+                }
+
+                try
                 {
                     if (OperatingSystemHelper.IsUnixOS)
                     {
@@ -194,6 +225,17 @@ namespace BuildXL.Processes
                     {
                         Win.SetPathAsSharedOpaqueOutput(expandedPath);
                     }
+
+                    return;
+                } 
+                catch (BuildXLException e)
+                {
+                    if (attempt >= MaxNumberOfAttemptsForMarkingSharedOpaqueOutputs)
+                    {
+                        throw new BuildXLException($"Exceeded max number of attempts ({MaxNumberOfAttemptsForMarkingSharedOpaqueOutputs}) to mark '{expandedPath}' as shared opaque output", e);
+                    }
+                }
+            }
         }
 
         /// <summary>