Expose the TarEntry start of the data stream

src/libraries/System.Formats.Tar/ref/System.Formats.Tar.cs

@@ -46,6 +46,7 @@ internal TarEntry() { }
         public System.IO.UnixFileMode Mode { get { throw null; } set { } }
         public System.DateTimeOffset ModificationTime { get { throw null; } set { } }
         public string Name { get { throw null; } set { } }
+        public long DataOffset { get { throw null; } }
         public int Uid { get { throw null; } set { } }
         public void ExtractToFile(string destinationFileName, bool overwrite) { }
         public System.Threading.Tasks.Task ExtractToFileAsync(string destinationFileName, bool overwrite, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarEntry.cs

@@ -281,6 +281,12 @@ public Stream? DataStream
             }
         }
 
+        /// <summary>
+        /// The starting position of the data stream respective to the archive stream.
+        /// If the entry does not come from an archive stream, or the archive stream is unseekable, returns -1.
+        /// </summary>
+        public long DataOffset => _header._dataOffset;
+
         /// <summary>
         /// A string that represents the current entry.
         /// </summary>

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHeader.Read.cs

@@ -25,7 +25,7 @@ internal sealed partial class TarHeader
 
             archiveStream.ReadExactly(buffer);
 
-            TarHeader? header = TryReadAttributes(initialFormat, buffer);
+            TarHeader? header = TryReadAttributes(initialFormat, buffer, archiveStream);
             if (header != null && processDataBlock)
             {
                 header.ProcessDataBlock(archiveStream, copyData);
@@ -47,7 +47,7 @@ internal sealed partial class TarHeader
 
             await archiveStream.ReadExactlyAsync(buffer, cancellationToken).ConfigureAwait(false);
 
-            TarHeader? header = TryReadAttributes(initialFormat, buffer.Span);
+            TarHeader? header = TryReadAttributes(initialFormat, buffer.Span, archiveStream);
             if (header != null && processDataBlock)
             {
                 await header.ProcessDataBlockAsync(archiveStream, copyData, cancellationToken).ConfigureAwait(false);
@@ -58,7 +58,7 @@ internal sealed partial class TarHeader
             return header;
         }
 
-        private static TarHeader? TryReadAttributes(TarEntryFormat initialFormat, ReadOnlySpan<byte> buffer)
+        private static TarHeader? TryReadAttributes(TarEntryFormat initialFormat, ReadOnlySpan<byte> buffer, Stream archiveStream)
         {
             // Confirms if v7 or pax, or tentatively selects ustar
             TarHeader? header = TryReadCommonAttributes(buffer, initialFormat);
@@ -86,6 +86,8 @@ internal sealed partial class TarHeader
                     }
                     // In PAX, there is nothing to read in this section (empty space)
                 }
+                // Finished reading the header metadata, next byte belongs to the data section, save the position
+                SetDataOffset(header, archiveStream);
             }
             return header;
         }

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHeader.Write.cs

@@ -83,6 +83,9 @@ private void WriteWithUnseekableDataStream(TarEntryFormat format, Stream destina
             // We know the exact location where the data starts depending on the format
             long dataStartPosition = headerStartPosition + dataLocation;
 
+            // Before writing, update the offset field now that the entry belongs to an archive
+            _dataOffset = dataStartPosition + 1;
+
             // Move to the data start location and write the data
             destinationStream.Seek(dataLocation, SeekOrigin.Current);
             _dataStream.CopyTo(destinationStream); // The data gets copied from the current position
@@ -132,6 +135,9 @@ private async Task WriteWithUnseekableDataStreamAsync(TarEntryFormat format, Str
             // We know the exact location where the data starts depending on the format
             long dataStartPosition = headerStartPosition + dataLocation;
 
+            // Before writing, update the offset field now that the entry belongs to an archive
+            _dataOffset = dataStartPosition + 1;
+
             // Move to the data start location and write the data
             destinationStream.Seek(dataLocation, SeekOrigin.Current);
             await _dataStream.CopyToAsync(destinationStream, cancellationToken).ConfigureAwait(false); // The data gets copied from the current position
@@ -758,6 +764,9 @@ private int WriteGnuFields(Span<byte> buffer)
         // Writes the current header's data stream into the archive stream.
         private void WriteData(Stream archiveStream, Stream dataStream)
         {
+            // Before writing, update the offset field now that the entry belongs to an archive
+            SetDataOffset(this, archiveStream);
+
             dataStream.CopyTo(archiveStream); // The data gets copied from the current position
             WriteEmptyPadding(archiveStream);
         }
@@ -798,6 +807,9 @@ private async Task WriteDataAsync(Stream archiveStream, Stream dataStream, Cance
         {
             cancellationToken.ThrowIfCancellationRequested();
 
+            // Before writing, update the offset field now that the entry belongs to an archive
+            SetDataOffset(this, archiveStream);
+
             await dataStream.CopyToAsync(archiveStream, cancellationToken).ConfigureAwait(false); // The data gets copied from the current position
 
             int paddingAfterData = TarHelpers.CalculatePadding(_size);

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHeader.cs

@@ -40,6 +40,7 @@ internal sealed partial class TarHeader
         private const string PaxEaDevMinor = "devminor";
 
         internal Stream? _dataStream;
+        internal long _dataOffset;
 
         // Position in the stream where the data ends in this header.
         internal long _endOfHeaderAndDataAndBlockAlignment;
@@ -95,6 +96,7 @@ internal TarHeader(TarEntryFormat format, string name = "", int mode = 0, DateTi
             _typeFlag = typeFlag;
             _magic = GetMagicForFormat(format);
             _version = GetVersionForFormat(format);
+            _dataOffset = -1;
         }
 
         // Constructor called when creating an entry using the common fields from another entry.
@@ -149,5 +151,13 @@ internal void InitializeExtendedAttributesWithExisting(IEnumerable<KeyValuePair<
             TarEntryFormat.Gnu => GnuVersion,
             _ => string.Empty,
         };
+
+        // Stores the archive stream's position where we know the current entry's data section begins,
+        // if the archive stream is seekable. Otherwise, -1.
+        private static void SetDataOffset(TarHeader header, Stream archiveStream) => header._dataOffset =
+            archiveStream.CanSeek
+            // Add one because the last byte read is still part of the header
+            ? archiveStream.Position + 1
+            : -1;
     }
 }