Fix Icon(Stream) handling of partial reads (#82621)

Icon's ctor assumes Read on a stream will always produce as much data as was asked for, and that's not guaranteed to be the case.

src/libraries/System.Drawing.Common/src/System/Drawing/Icon.cs

@@ -127,7 +127,20 @@ public Icon(Stream stream, int width, int height) : this()
             ArgumentNullException.ThrowIfNull(stream);
 
             _iconData = new byte[(int)stream.Length];
-            stream.Read(_iconData, 0, _iconData.Length);
+#if NET7_0_OR_GREATER
+            stream.ReadExactly(_iconData);
+#else
+            int totalRead = 0;
+            while (totalRead < _iconData.Length)
+            {
+                int bytesRead = stream.Read(_iconData, totalRead, _iconData.Length - totalRead);
+                if (bytesRead <= 0)
+                {
+                    throw new EndOfStreamException();
+                }
+                totalRead += bytesRead;
+            }
+#endif
             Initialize(width, height);
         }
 

src/libraries/System.Drawing.Common/tests/IconTests.cs

@@ -113,6 +113,23 @@ public void Ctor_Stream()
             }
         }
 
+        [ConditionalFact(Helpers.IsDrawingSupported)]
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, "Bug fix in core")]
+        public void Ctor_Stream_Trickled()
+        {
+            var stream = new TrickleStream(File.ReadAllBytes(Helpers.GetTestBitmapPath("48x48_multiple_entries_4bit.ico")));
+            var icon = new Icon(stream);
+            Assert.Equal(32, icon.Width);
+            Assert.Equal(32, icon.Height);
+            Assert.Equal(new Size(32, 32), icon.Size);
+        }
+
+        private sealed class TrickleStream : MemoryStream
+        {
+            public TrickleStream(byte[] bytes) : base(bytes) { }
+            public override int Read(byte[] buffer, int offset, int count) => base.Read(buffer, offset, Math.Min(count, 1));
+        }
+
         [ConditionalTheory(Helpers.IsDrawingSupported)]
         [MemberData(nameof(Size_TestData))]
         public void Ctor_Stream_Width_Height(string fileName, Size size, Size expectedSize)