Fix Stream.Seek implementations to reliably shift position as required

The bug is that `Stream.Read` was assumed to always fill the provided buffer, but in fact only a non-zero movement is contractually guaranteed.

Author: AArnott

src/NerdBank.GitVersioning/ManagedGit/GitPackDeltafiedStream.cs

@@ -135,11 +135,7 @@ public override long Seek(long offset, SeekOrigin origin)
             if (origin == SeekOrigin.Begin && offset > this.position)
             {
                 // We can optimise this by skipping over instructions rather than executing them
-                int length = (int)(offset - this.position);
-
-                byte[] buffer = ArrayPool<byte>.Shared.Rent(length);
-                this.Read(buffer, 0, length);
-                ArrayPool<byte>.Shared.Return(buffer);
+                this.ReadExactly(checked((int)(offset - this.position)));
                 return this.position;
             }
             else

src/NerdBank.GitVersioning/ManagedGit/GitPackMemoryCacheStream.cs

@@ -73,13 +73,9 @@ public override long Seek(long offset, SeekOrigin origin)
 
             if (offset > this.cacheStream.Length)
             {
-                var toRead = (int)(offset - this.cacheStream.Length);
-                byte[] buffer = ArrayPool<byte>.Shared.Rent(toRead);
-                int read = this.stream.Read(buffer, 0, toRead);
                 this.cacheStream.Seek(0, SeekOrigin.End);
-                this.cacheStream.Write(buffer, 0, read);
-                ArrayPool<byte>.Shared.Return(buffer);
-
+                int toRead = (int)(offset - this.cacheStream.Length);
+                this.stream.ReadExactly(toRead, this.cacheStream);
                 this.DisposeStreamIfRead();
                 return this.cacheStream.Position;
             }

src/NerdBank.GitVersioning/ManagedGit/StreamExtensions.cs

@@ -150,5 +150,32 @@ internal static bool TryAdd<TKey, TValue>(this System.Collections.Generic.IDicti
             return true;
         }
 #endif
+
+        /// <summary>
+        /// Reads the specified number of bytes from a stream, or until the end of the stream.
+        /// </summary>
+        /// <param name="readFrom">The stream to read from.</param>
+        /// <param name="length">The number of bytes to be read.</param>
+        /// <param name="copyTo">The stream to copy the read bytes to, if required.</param>
+        /// <returns>The number of bytes actually read. This will be less than <paramref name="length"/> only if the end of <paramref name="readFrom"/> is reached.</returns>
+        internal static int ReadExactly(this Stream readFrom, int length, Stream? copyTo = null)
+        {
+            int bytesRemaining = length;
+            byte[] buffer = ArrayPool<byte>.Shared.Rent(Math.Min(50 * 1024, bytesRemaining));
+            while (bytesRemaining > 0)
+            {
+                int read = readFrom.Read(buffer, 0, Math.Min(buffer.Length, bytesRemaining));
+                if (read == 0)
+                {
+                    break;
+                }
+
+                copyTo?.Write(buffer, 0, read);
+                bytesRemaining -= read;
+            }
+
+            ArrayPool<byte>.Shared.Return(buffer);
+            return length - bytesRemaining;
+        }
     }
 }

src/NerdBank.GitVersioning/ManagedGit/ZLibStream.cs

@@ -151,11 +151,7 @@ public override long Seek(long offset, SeekOrigin origin)
             if (origin == SeekOrigin.Begin && offset > this.position)
             {
                 // We may be able to optimize this by skipping over the compressed data
-                int length = (int)(offset - this.position);
-
-                byte[] buffer = ArrayPool<byte>.Shared.Rent(length);
-                this.Read(buffer, 0, length);
-                ArrayPool<byte>.Shared.Return(buffer);
+                this.ReadExactly(checked((int)(offset - this.position)));
                 return this.position;
             }
             else