Rename UnknownFormat resource string to TarInvalidFormat, include for…

…mat value. Add tests to verify malformed archive.

src/libraries/System.Formats.Tar/src/Resources/Strings.resx

@@ -219,6 +219,9 @@
   <data name="TarGnuFormatExpected" xml:space="preserve">
     <value>A GNU format was expected, but could not be reliably determined for entry '{0}'.</value>
   </data>
+  <data name="TarInvalidFormat" xml:space="preserve">
+    <value>The archive format is invalid: '{0}'</value>
+  </data>
   <data name="TarMalformedTwoExtendedAttributesEntries" xml:space="preserve">
     <value>The archive is malformed. It contains two extended attributes entries in a row.</value>
   </data>
@@ -246,7 +249,4 @@
   <data name="UnauthorizedAccess_IODenied_Path" xml:space="preserve">
     <value>Access to the path '{0}' is denied.</value>
   </data>
-  <data name="UnknownFormat" xml:space="preserve">
-    <value>The archive format is unknown.</value>
-  </data>
 </root>

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

@@ -283,7 +283,7 @@ TarEntryType.RegularFile or
 
                 case TarFormat.Unknown:
                 default:
-                    throw new NotSupportedException(SR.UnknownFormat);
+                    throw new FormatException(string.Format(SR.TarInvalidFormat, archiveFormat));
             }
 
             throw new NotSupportedException(string.Format(SR.TarEntryTypeNotSupported, entryType, archiveFormat));

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarWriter.Unix.cs

@@ -37,7 +37,7 @@ partial void ReadFileFromDiskAndWriteToArchiveStreamAsEntry(string fullPath, str
                 TarFormat.Ustar => new UstarTarEntry(entryType, entryName),
                 TarFormat.Pax => new PaxTarEntry(entryType, entryName),
                 TarFormat.Gnu => new GnuTarEntry(entryType, entryName),
-                _ => throw new NotSupportedException(SR.UnknownFormat),
+                _ => throw new FormatException(string.Format(SR.TarInvalidFormat, Format)),
             };
 
             if ((entryType is TarEntryType.BlockDevice or TarEntryType.CharacterDevice) && status.Dev > 0)

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarWriter.Windows.cs

@@ -40,7 +40,7 @@ partial void ReadFileFromDiskAndWriteToArchiveStreamAsEntry(string fullPath, str
                 TarFormat.Ustar => new UstarTarEntry(entryType, entryName),
                 TarFormat.Pax => new PaxTarEntry(entryType, entryName),
                 TarFormat.Gnu => new GnuTarEntry(entryType, entryName),
-                _ => throw new NotSupportedException(SR.UnknownFormat),
+                _ => throw new FormatException(string.Format(SR.TarInvalidFormat, Format)),
             };
 
             FileSystemInfo info = attributes.HasFlag(FileAttributes.Directory) ? new DirectoryInfo(fullPath) : new FileInfo(fullPath);

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

@@ -186,7 +186,7 @@ public void WriteEntry(TarEntry entry)
                     break;
                 case TarFormat.Unknown:
                 default:
-                    throw new FormatException(SR.UnknownFormat);
+                    throw new FormatException(string.Format(SR.TarInvalidFormat, Format));
             }
 
             _wroteEntries = true;

src/libraries/System.Formats.Tar/tests/TarReader/TarReader.GetNextEntry.Tests.cs

@@ -8,6 +8,40 @@ namespace System.Formats.Tar.Tests
 {
     public class TarReader_GetNextEntry_Tests : TarTestsBase
     {
+        [Fact]
+        public void MalformedArchive_TooSmall()
+        {
+            using MemoryStream malformed = new MemoryStream();
+            byte[] buffer = new byte[] { 0x1 };
+            malformed.Write(buffer);
+            malformed.Seek(0, SeekOrigin.Begin);
+
+            using TarReader reader = new TarReader(malformed);
+            Assert.Throws<EndOfStreamException>(() => reader.GetNextEntry());
+        }
+
+        [Fact]
+        public void MalformedArchive_HeaderSize()
+        {
+            using MemoryStream malformed = new MemoryStream();
+            byte[] buffer = new byte[512]; // Minimum length of any header
+            Array.Fill<byte>(buffer, 0x1);
+            malformed.Write(buffer);
+            malformed.Seek(0, SeekOrigin.Begin);
+
+            using TarReader reader = new TarReader(malformed);
+            Assert.Throws<FormatException>(() => reader.GetNextEntry());
+        }
+
+        [Fact]
+        public void EmptyArchive()
+        {
+            using MemoryStream empty = new MemoryStream();
+
+            using TarReader reader = new TarReader(empty);
+            Assert.Null(reader.GetNextEntry());
+        }
+
         [Fact]
         public void GetNextEntry_CopyDataTrue_SeekableArchive()
         {