Enable long path names on Windows without the registry value

Windows applies the `longPathAware` manifest setting only when `LongPathsEnabled` is set in the registry, so paths longer than 260 characters still fail on a stock machine. Set the undocumented `IsLongPathAwareProcess` bit in the PEB at startup instead, which is what `RtlAreLongPathsEnabled` reads and what Go's runtime does since 1.23.

[Bug #18947] [Bug #18923]

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

NEWS.md

@@ -244,6 +244,11 @@ Ruby 4.0 bundled RubyGems and Bundler version 4. see the following links for det
     * SunOS 4 (Solaris, i.e. SunOS 5, is unaffected)
     * BSD/OS (BSDi)
 
+* Windows 10 1703 or later no longer needs the `LongPathsEnabled` registry
+  value to use paths longer than 260 characters.  Each path component is still
+  limited to 255 characters, and a child process still starts with the
+  `MAX_PATH` limited current directory. [[Bug #18947]]
+
 ## Compatibility issues
 
 * `Kernel#at_exit` and `END {}` now raise `Ractor::IsolationError` when called
@@ -327,6 +332,7 @@ A lot of work has gone into making Ractors more stable, performant, and usable.
 
 ## JIT
 
+[Bug #18947]: https://bugs.ruby-lang.org/issues/18947
 [Feature #8948]: https://bugs.ruby-lang.org/issues/8948
 [Feature #9779]: https://bugs.ruby-lang.org/issues/9779
 [Feature #15330]: https://bugs.ruby-lang.org/issues/15330

test/ruby/test_dir.rb

@@ -590,6 +590,22 @@ class TestDir < Test::Unit::TestCase
       assert_empty(entries - Dir.glob("#{wild}/Common*", File::FNM_SHORTNAME), bug10819)
     end
 
+    def test_glob_long_path
+      bug18923 = '[Bug #18923]'
+      # the whole path is longer than MAX_PATH, while each component is
+      # within the 255 chars limit of NTFS
+      deep = File.join(@root, "a" * 200, "b" * 200)
+      begin
+        FileUtils.mkdir_p(deep)
+      rescue SystemCallError
+        omit "long path names are not available"
+      end
+      file = File.join(deep, "c.txt")
+      File.write(file, "")
+      assert_equal([file], Dir.glob(File.join(@root, "**", "*.txt")), bug18923)
+      assert_equal(["c.txt"], Dir.children(deep), bug18923)
+    end
+
     def test_home_windows
       setup_envs(%w[HOME USERPROFILE HOMEDRIVE HOMEPATH])
 

win32/win32.c

@@ -843,6 +843,44 @@ static int w32_cmdvector(const WCHAR *, char ***, UINT, rb_encoding *);
 //
 // Initialization stuff
 //
+
+/* License: Ruby's */
+/* Enable long path names for this process, whatever the registry value
+ * HKLM\SYSTEM\CurrentControlSet\Control\FileSystem\LongPathsEnabled is.  The
+ * loader sets the same PEB bit for longPathAware in the manifest, but only
+ * when that registry value is set, so the manifest alone is not enough on a
+ * stock machine.  Undocumented, and taken from initLongPathSupport in Go's
+ * runtime <https://go.dev/src/runtime/os_windows.go>, whose maintainers have
+ * proposed to drop it <https://github.com/golang/go/issues/66560>. */
+static void
+init_long_path_support(void)
+{
+    /* PEB.BitField, and the IsLongPathAwareProcess bit in it */
+    enum {peb_bit_field_offset = 3, is_long_path_aware_process = 0x80};
+    typedef long (WINAPI version_func)(OSVERSIONINFOW *);
+    typedef void *(WINAPI peb_func)(void);
+    version_func *pRtlGetVersion;
+    peb_func *pRtlGetCurrentPeb;
+    OSVERSIONINFOW osvi;
+    BYTE *bit_field;
+
+    pRtlGetVersion = (version_func *)get_proc_address("ntdll.dll", "RtlGetVersion", NULL);
+    if (!pRtlGetVersion) return;
+    memset(&osvi, 0, sizeof(osvi));
+    osvi.dwOSVersionInfoSize = sizeof(osvi);
+    if (pRtlGetVersion(&osvi)) return;
+
+    /* the PEB bit is honored since Windows 10 1703 (10.0.15063) */
+    if (osvi.dwMajorVersion < 10) return;
+    if (osvi.dwMajorVersion == 10 && osvi.dwMinorVersion == 0 &&
+        osvi.dwBuildNumber < 15063) return;
+
+    pRtlGetCurrentPeb = (peb_func *)get_proc_address("ntdll.dll", "RtlGetCurrentPeb", NULL);
+    if (!pRtlGetCurrentPeb) return;
+    bit_field = (BYTE *)pRtlGetCurrentPeb() + peb_bit_field_offset;
+    *bit_field |= is_long_path_aware_process;
+}
+
 /* License: Ruby's */
 void
 rb_w32_sysinit(int *argc, char ***argv)
@@ -854,6 +892,7 @@ rb_w32_sysinit(int *argc, char ***argv)
     SetErrorMode(SEM_FAILCRITICALERRORS|SEM_NOGPFAULTERRORBOX);
 
     get_version();
+    init_long_path_support();
 
     //
     // subvert cmd.exe's feeble attempt at command line parsing