Fix prefix writing on TarHeaderWrite

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

@@ -374,12 +374,9 @@ private int WritePosixName(Span<byte> buffer)
 
             if (_name.Length > FieldLengths.Name)
             {
-                int prefixBytesLength = Math.Min(_name.Length - FieldLengths.Name, FieldLengths.Name);
-                Span<byte> remaining = prefixBytesLength <= 256 ?
-                    stackalloc byte[prefixBytesLength] :
-                    new byte[prefixBytesLength];
-
-                int encoded = Encoding.ASCII.GetBytes(_name.AsSpan(FieldLengths.Name), remaining);
+                int prefixBytesLength = Math.Min(_name.Length - FieldLengths.Name, FieldLengths.Prefix);
+                Span<byte> remaining = stackalloc byte[prefixBytesLength];
+                int encoded = Encoding.ASCII.GetBytes(_name.AsSpan(FieldLengths.Name, prefixBytesLength), remaining);
                 Debug.Assert(encoded == remaining.Length);
 
                 checksum += WriteLeftAlignedBytesAndGetChecksum(remaining, buffer.Slice(FieldLocations.Prefix, FieldLengths.Prefix));

src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntry.Tests.cs

@@ -299,5 +299,61 @@ public void WriteTimestampsBeyondOctalLimit(TarEntryFormat format)
                 }
             }
         }
+
+        [Theory]
+        [InlineData(TarEntryFormat.V7)]
+        // [InlineData(TarEntryFormat.Ustar)] https://github.com/dotnet/runtime/issues/75360
+        [InlineData(TarEntryFormat.Pax)]
+        [InlineData(TarEntryFormat.Gnu)]
+        public void WriteLongName(TarEntryFormat format)
+        {
+            string maxPathComponent = new string('a', 255);
+            WriteLongNameCore(format, maxPathComponent);
+
+            maxPathComponent = new string('a', 90) + new string('b', 165);
+            WriteLongNameCore(format, maxPathComponent);
+
+            maxPathComponent = new string('a', 165) + new string('b', 90);
+            WriteLongNameCore(format, maxPathComponent);
+        }
+
+        private void WriteLongNameCore(TarEntryFormat format, string maxPathComponent)
+        {
+            Assert.Equal(255, maxPathComponent.Length);
+
+            TarEntry entry;
+            MemoryStream ms = new();
+            using (TarWriter writer = new(ms, true))
+            {
+                TarEntryType entryType = format == TarEntryFormat.V7 ? TarEntryType.V7RegularFile : TarEntryType.RegularFile;
+                entry = InvokeTarEntryCreationConstructor(format, entryType, maxPathComponent);
+                writer.WriteEntry(entry);
+
+                entry = InvokeTarEntryCreationConstructor(format, entryType, Path.Join(maxPathComponent, maxPathComponent));
+                writer.WriteEntry(entry);
+            }
+
+            ms.Position = 0;
+            using TarReader reader = new(ms);
+
+            entry = reader.GetNextEntry();
+            string expectedName = GetExpectedNameForFormat(format, maxPathComponent);
+            Assert.Equal(expectedName, entry.Name);
+
+            entry = reader.GetNextEntry();
+            expectedName = GetExpectedNameForFormat(format, Path.Join(maxPathComponent, maxPathComponent));
+            Assert.Equal(expectedName, entry.Name);
+
+            Assert.Null(reader.GetNextEntry());
+
+            string GetExpectedNameForFormat(TarEntryFormat format, string expectedName)
+            {
+                if (format is TarEntryFormat.V7) // V7 truncates names at 100 characters.
+                {
+                    return expectedName.Substring(0, 100);
+                }
+                return expectedName;
+            }
+        }
     }
 }