Clean up TarHeader.GlobalHeadFormatPrefix (#77580)

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

@@ -3,7 +3,6 @@
 
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
-using System.Diagnostics;
 
 namespace System.Formats.Tar
 {
@@ -26,7 +25,7 @@ internal PaxGlobalExtendedAttributesTarEntry(TarHeader header, TarReader readerO
         /// <param name="globalExtendedAttributes">An enumeration of string key-value pairs that represents the metadata to include as Global Extended Attributes.</param>
         /// <exception cref="ArgumentNullException"><paramref name="globalExtendedAttributes"/> is <see langword="null"/>.</exception>
         public PaxGlobalExtendedAttributesTarEntry(IEnumerable<KeyValuePair<string, string>> globalExtendedAttributes)
-            : base(TarEntryType.GlobalExtendedAttributes, TarHeader.GlobalHeadFormatPrefix, TarEntryFormat.Pax, isGea: true)
+            : base(TarEntryType.GlobalExtendedAttributes, nameof(PaxGlobalExtendedAttributesTarEntry), TarEntryFormat.Pax, isGea: true) // Name == name of type for lack of a better temporary name until the entry is written
         {
             ArgumentNullException.ThrowIfNull(globalExtendedAttributes);
             _header.InitializeExtendedAttributesWithExisting(globalExtendedAttributes);

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

@@ -893,32 +893,22 @@ private string GenerateExtendedAttributeName()
         }
 
         // Gets the special name for the 'name' field in a global extended attribute entry.
-        // Format: "%d/GlobalHead.%p/%n"
+        // Format: "%d/GlobalHead.%p.%n"
         // - %d: The path of the $TMPDIR variable, if found. Otherwise, the value is '/tmp'.
         // - %p: The current process ID.
-        // - %n: The sequence number of the global extended header record of the archive, starting at 1. In our case, since we only generate one, the value is always 1.
+        // - %n: The sequence number of the global extended header record of the archive, starting at 1.
         // If the path of $TMPDIR makes the final string too long to fit in the 'name' field,
         // then the TMPDIR='/tmp' is used.
         private static string GenerateGlobalExtendedAttributeName(int globalExtendedAttributesEntryNumber)
         {
             Debug.Assert(globalExtendedAttributesEntryNumber >= 1);
 
-            string tmpDir = Path.GetTempPath();
-            if (Path.EndsInDirectorySeparator(tmpDir))
-            {
-                tmpDir = Path.TrimEndingDirectorySeparator(tmpDir);
-            }
-            int processId = Environment.ProcessId;
-
-            string result = string.Format(GlobalHeadFormatPrefix, tmpDir, processId);
-            string suffix = $".{globalExtendedAttributesEntryNumber}"; // GEA sequence number
-            if (result.Length + suffix.Length >= FieldLengths.Name)
-            {
-                result = string.Format(GlobalHeadFormatPrefix, "/tmp", processId);
-            }
-            result += suffix;
+            ReadOnlySpan<char> tmp = Path.TrimEndingDirectorySeparator(Path.GetTempPath());
 
-            return result;
+            string result = $"{tmp}/GlobalHead.{Environment.ProcessId}.{globalExtendedAttributesEntryNumber}";
+            return result.Length >= FieldLengths.Name ?
+                string.Concat("/tmp", result.AsSpan(tmp.Length)) :
+                result;
         }
 
         private static int GetUtf8TextLength(ReadOnlySpan<char> text)

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

@@ -39,11 +39,6 @@ internal sealed partial class TarHeader
         private const string PaxEaDevMajor = "devmajor";
         private const string PaxEaDevMinor = "devminor";
 
-        // Global Extended Attribute entries have a special format in the Name field:
-        // "{tmpFolder}/GlobalHead.{processId}.{GEAEntryNumber}"
-        // Excludes ".{GEAEntryNumber}" because the number gets added on write.
-        internal const string GlobalHeadFormatPrefix = "{0}/GlobalHead.{1}";
-
         internal Stream? _dataStream;
 
         // Position in the stream where the data ends in this header.

src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntryAsync.Entry.Pax.Tests.cs

@@ -378,7 +378,9 @@ public async Task Add_Empty_GlobalExtendedAttributes_Async()
             await using (TarWriter writer = new TarWriter(archive, leaveOpen: true))
             {
                 PaxGlobalExtendedAttributesTarEntry gea = new PaxGlobalExtendedAttributesTarEntry(new Dictionary<string, string>());
+                Assert.Equal("PaxGlobalExtendedAttributesTarEntry", gea.Name);
                 await writer.WriteEntryAsync(gea);
+                Assert.Matches(@".*/GlobalHead\.\d+\.\d+", gea.Name);
             }
 
             archive.Seek(0, SeekOrigin.Begin);