Fix GetTempFileName() on Windows

src/libraries/System.Private.CoreLib/src/Resources/Strings.resx

@@ -1244,7 +1244,7 @@
     <value>Invalid type for ParameterInfo member in Attribute class.</value>
   </data>
   <data name="Argument_InvalidPathChars" xml:space="preserve">
-    <value>Illegal characters in path.</value>
+    <value>Null character in path.</value>
   </data>
   <data name="Argument_InvalidResourceCultureName" xml:space="preserve">
     <value>The given culture name '{0}' cannot be used to locate a resource file. Resource filenames must consist of only letters, numbers, hyphens or underscores.</value>

src/libraries/System.Private.CoreLib/src/System/IO/Path.Windows.cs

@@ -208,25 +208,56 @@ static uint GetTempPathW(int bufferLen, ref char buffer)
         // name on disk.
         public static string GetTempFileName()
         {
-            var tempPathBuilder = new ValueStringBuilder(stackalloc char[PathInternal.MaxShortPath]);
-
-            GetTempPath(ref tempPathBuilder);
-
-            var builder = new ValueStringBuilder(stackalloc char[PathInternal.MaxShortPath]);
-
-            uint result = Interop.Kernel32.GetTempFileNameW(
-                ref tempPathBuilder.GetPinnableReference(), "tmp", 0, ref builder.GetPinnableReference());
-
-            tempPathBuilder.Dispose();
-
-            if (result == 0)
-                throw Win32Marshal.GetExceptionForLastWin32Error();
-
-            builder.Length = builder.RawChars.IndexOf('\0');
+            // Avoid GetTempFileNameW because it is limited to 0xFFFF possibilities, which both
+            // means that it may have to make many attempts to create the file before
+            // finding an unused name, and also that if an app "leaks" such temp files,
+            // it can prevent GetTempFileNameW succeeding at all.
+            //
+            // To make this a little more robust, generate our own name with more
+            // entropy. We could use GetRandomFileName() here, but for consistency
+            // with Unix and to retain the ".tmp" extension we will use the "tmpXXXXXX.tmp" pattern.
+            // Using 32 characters for convenience, that gives us 32^^6 ~= 10^^9 possibilities,
+            // but we'll still loop to handle the unlikely case the file already exists.
+
+            byte* bytes = stackalloc byte[KeyLength];
+            Span<char> span = stackalloc char[13]; // tmpXXXXXX.tmp
+
+            int i = 0;
+            while (true)
+            {
+                const int KeyLength = 4; // 4 bytes = more than 6 x 5 bits
+                Interop.GetRandomBytes(bytes, KeyLength);
+
+                byte b0 = bytes[0];
+                byte b1 = bytes[1];
+                byte b2 = bytes[2];
+                byte b3 = bytes[3];
+
+                span[0] = span[10] = 't';
+                span[1] = span[11] = 'm';
+                span[2] = span[12] = 'p';
+                span[3] = (char)Base32Char[b0 & 0b0001_1111];
+                span[4] = (char)Base32Char[b1 & 0b0001_1111];
+                span[5] = (char)Base32Char[b2 & 0b0001_1111];
+                span[6] = (char)Base32Char[b3 & 0b0001_1111];
+                span[7] = (char)Base32Char[((b0 & 0b1110_0000) >> 5) | ((b1 & 0b1100_0000) >> 3)];
+                span[8] = (char)Base32Char[((b2 & 0b1110_0000) >> 5) | ((b3 & 0b1100_0000) >> 3)];
+                span[9] = '.';
+
+                string path = string.Concat(Path.GetTempPath(), span);
+
+                try
+                {
+                    File.OpenHandle(path, FileMode.CreateNew, FileAccess.Write).Dispose();
+                }
+                catch (IOException) when (i < 100)
+                {
+                    i++; // If TMP is invalid, don't loop forever
+                    continue; // File already exists: very, very unlikely
+                }
 
-            string path = PathHelper.Normalize(ref builder);
-            builder.Dispose();
-            return path;
+                return path;
+            }
         }
 
         // Tests if the given path contains a root. A path is considered rooted

src/libraries/System.Runtime.Extensions/tests/System/IO/PathTests.cs

@@ -182,7 +182,17 @@ public void GetTempFileName()
             try
             {
                 Assert.True(File.Exists(tmpFile));
-                Assert.Equal(".tmp", Path.GetExtension(tmpFile), ignoreCase: true, ignoreLineEndingDifferences: false, ignoreWhiteSpaceDifferences: false);
+                string fileName = Path.GetFileName(tmpFile);
+                Assert.StartsWith("tmp", fileName);
+                Assert.Equal("tmpXXXXXX.tmp".Length, fileName.Length);
+                Assert.EndsWith(".tmp", fileName);
+
+                const string validChars = "abcdefghijklmnopqrstuvwxyz0123456789";
+                for (int i = 3; i < 9; i++)
+                {
+                    Assert.True(validChars.Contains(char.ToLowerInvariant(fileName[i])));
+                }
+
                 Assert.Equal(-1, tmpFile.IndexOfAny(Path.GetInvalidPathChars()));
                 using (FileStream fs = File.OpenRead(tmpFile))
                 {