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src/libraries/Common/src/System/IO/PathInternal.Unix.cs

@@ -17,6 +17,7 @@ internal static partial class PathInternal
         internal const string DirectorySeparatorCharAsString = "/";
         internal const string ParentDirectoryPrefix = @"../";
         internal const string DirectorySeparators = DirectorySeparatorCharAsString;
+        internal static ReadOnlySpan<byte> Utf8DirectorySeparators => "/"u8;
 
         internal static int GetRootLength(ReadOnlySpan<char> path)
         {

src/libraries/Common/src/System/IO/PathInternal.Windows.cs

@@ -55,6 +55,7 @@ internal static partial class PathInternal
         internal const string DevicePathPrefix = @"\\.\";
         internal const string ParentDirectoryPrefix = @"..\";
         internal const string DirectorySeparators = @"\/";
+        internal static ReadOnlySpan<byte> Utf8DirectorySeparators => @"\/"u8;
 
         internal const int MaxShortPath = 260;
         internal const int MaxShortDirectoryPath = 248;

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

@@ -22,7 +22,6 @@ internal sealed partial class TarHeader
 
         private static ReadOnlySpan<byte> GnuMagicBytes => "ustar "u8;
         private static ReadOnlySpan<byte> GnuVersionBytes => " \0"u8;
-        private static ReadOnlySpan<byte> SeparatorsAsBytes => Encoding.UTF8.GetBytes(PathInternal.DirectorySeparators);
 
         // Predefined text for the Name field of a GNU long metadata entry. Applies for both LongPath ('L') and LongLink ('K').
         private const string GnuLongMetadataName = "././@LongLink";
@@ -398,9 +397,9 @@ private int WriteUstarName(Span<byte> buffer)
                 return WriteLeftAlignedBytesAndGetChecksum(nameAndPrefixBytes, buffer.Slice(FieldLocations.Name, FieldLengths.Name));
             }
 
-            int lastIdx = nameAndPrefixBytes.LastIndexOfAny(SeparatorsAsBytes);
-            ReadOnlySpan<byte> name = stackalloc byte[0];
-            ReadOnlySpan<byte> prefix = stackalloc byte[0];
+            int lastIdx = nameAndPrefixBytes.LastIndexOfAny(PathInternal.Utf8DirectorySeparators);
+            scoped ReadOnlySpan<byte> name;
+            scoped ReadOnlySpan<byte> prefix;
 
             if (lastIdx == -1)
             {
@@ -410,32 +409,21 @@ private int WriteUstarName(Span<byte> buffer)
             else
             {
                 name = nameAndPrefixBytes.Slice(lastIdx + 1);
-
-                if (lastIdx > 0)
-                {
-                    prefix = nameAndPrefixBytes.Slice(0, lastIdx);
-                }
-                else
-                {
-                    // We cannot neglect the separator if that's the only thing we can place in prefix.
-                    prefix = nameAndPrefixBytes.Slice(0, 1);
-                }
+                prefix = nameAndPrefixBytes.Slice(0, Math.Max(lastIdx, 1)); // need at least the separator
             }
 
+            // At this point nameAndPrefixBytes.Length > 100.
+            // Attempt to split it in a way it can use prefix.
             while (prefix.Length - name.Length > FieldLengths.Prefix)
             {
-                lastIdx = prefix.LastIndexOfAny(SeparatorsAsBytes);
-                if (lastIdx == -1) { break; }
-
-                name = nameAndPrefixBytes.Slice(lastIdx + 1);
-                if (lastIdx > 0)
-                {
-                    prefix = prefix.Slice(0, lastIdx);
-                }
-                else
+                lastIdx = prefix.LastIndexOfAny(PathInternal.Utf8DirectorySeparators);
+                if (lastIdx < 0)
                 {
-                    prefix = prefix.Slice(0, 1);
+                    break;
                 }
+
+                name = nameAndPrefixBytes.Slice(lastIdx + 1);
+                prefix = nameAndPrefixBytes.Slice(0, Math.Max(lastIdx, 1)); // need at least the separator
             }
 
             if (prefix.Length <= FieldLengths.Prefix && name.Length <= FieldLengths.Name)