Zip async correctness fix

src/libraries/System.IO.Compression/src/System/IO/Compression/ZipBlocks.Async.cs

@@ -251,7 +251,7 @@ public static async Task<ZipEndOfCentralDirectoryBlock> ReadBlockAsync(Stream st
         }
         else if (readComment)
         {
-            stream.ReadExactly(eocdBlock._archiveComment);
+            await stream.ReadExactlyAsync(eocdBlock._archiveComment, cancellationToken).ConfigureAwait(false);
         }
         return eocdBlock;
     }

src/libraries/System.IO.Compression/tests/ZipArchive/zip_ReadTests.cs

@@ -919,5 +919,32 @@ public static async Task ReadAfterSeekingPastEnd_ReturnsZeroBytes(bool async)
 
             await DisposeStream(async, readStream);
         }
+
+        [Fact]
+        public static async Task ReadArchiveCommentAsync_DoesNotCallSyncRead()
+        {
+            const string ExpectedComment = "this is the archive-level comment";
+
+            byte[] zipBytes;
+            using (MemoryStream buildStream = new MemoryStream())
+            {
+                using (ZipArchive archive = new ZipArchive(buildStream, ZipArchiveMode.Create, leaveOpen: true))
+                {
+                    archive.CreateEntry("file.txt");
+                    archive.Comment = ExpectedComment;
+                }
+                zipBytes = buildStream.ToArray();
+            }
+
+            await using MemoryStream ms = new MemoryStream(zipBytes);
+            await using CallTrackingStream tracker = new CallTrackingStream(ms);
+
+            ZipArchive readArchive = await ZipArchive.CreateAsync(tracker, ZipArchiveMode.Read, leaveOpen: true, entryNameEncoding: null);
+            Assert.Equal(ExpectedComment, readArchive.Comment);
+            await readArchive.DisposeAsync();
+
+            Assert.Equal(0, tracker.TimesCalled(nameof(Stream.Read)));
+            Assert.Equal(0, tracker.TimesCalled(nameof(Stream.ReadByte)));
+        }
     }
 }