Windows: Fix Iterator name buffer size not handling all possible file name components

lib/std/c/darwin.zig

@@ -1014,6 +1014,7 @@ pub const vm_machine_attribute_val_t = isize;
 pub const CALENDAR_CLOCK = 1;
 
 pub const PATH_MAX = 1024;
+pub const NAME_MAX = 255;
 pub const IOV_MAX = 16;
 
 pub const STDIN_FILENO = 0;

lib/std/c/dragonfly.zig

@@ -234,6 +234,7 @@ pub const SA = struct {
 };
 
 pub const PATH_MAX = 1024;
+pub const NAME_MAX = 255;
 pub const IOV_MAX = KERN.IOV_MAX;
 
 pub const ino_t = c_ulong;

lib/std/c/haiku.zig

@@ -266,6 +266,7 @@ pub const area_info = extern struct {
 };
 
 pub const MAXPATHLEN = PATH_MAX;
+pub const MAXNAMLEN = NAME_MAX;
 
 pub const image_info = extern struct {
     id: u32,
@@ -371,6 +372,9 @@ pub const KERN = struct {};
 pub const IOV_MAX = 1024;
 
 pub const PATH_MAX = 1024;
+/// NOTE: Contains room for the terminating null character (despite the POSIX
+/// definition saying that NAME_MAX does not include the terminating null).
+pub const NAME_MAX = 256; // limits.h
 
 pub const STDIN_FILENO = 0;
 pub const STDOUT_FILENO = 1;

lib/std/fs.zig

@@ -48,6 +48,30 @@ pub const MAX_PATH_BYTES = switch (builtin.os.tag) {
         @compileError("PATH_MAX not implemented for " ++ @tagName(builtin.os.tag)),
 };
 
+/// This represents the maximum size of a UTF-8 encoded file name component that
+/// the platform's common file systems support. File name components returned by file system
+/// operations are likely to fit into a UTF-8 encoded array of this length, but
+/// (depending on the platform) this assumption may not hold for every configuration.
+/// The byte count does not include a null sentinel byte.
+pub const MAX_NAME_BYTES = switch (builtin.os.tag) {
+    .linux, .macos, .ios, .freebsd, .dragonfly => os.NAME_MAX,
+    // Haiku's NAME_MAX includes the null terminator, so subtract one.
+    .haiku => os.NAME_MAX - 1,
+    .netbsd, .openbsd, .solaris => os.MAXNAMLEN,
+    // Each UTF-16LE character may be expanded to 3 UTF-8 bytes.
+    // If it would require 4 UTF-8 bytes, then there would be a surrogate
+    // pair in the UTF-16LE, and we (over)account 3 bytes for it that way.
+    .windows => os.windows.NAME_MAX * 3,
+    // For WASI, the MAX_NAME will depend on the host OS, so it needs to be
+    // as large as the largest MAX_NAME_BYTES (Windows) in order to work on any host OS.
+    // TODO determine if this is a reasonable approach
+    .wasi => os.windows.NAME_MAX * 3,
+    else => if (@hasDecl(root, "os") and @hasDecl(root.os, "NAME_MAX"))
+        root.os.NAME_MAX
+    else
+        @compileError("NAME_MAX not implemented for " ++ @tagName(builtin.os.tag)),
+};
+
 pub const base64_alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_".*;
 
 /// Base64 encoder, replacing the standard `+/` with `-_` so that it can be used in a file name on any filesystem.
@@ -680,7 +704,7 @@ pub const IterableDir = struct {
             index: usize,
             end_index: usize,
             first_iter: bool,
-            name_data: [256]u8,
+            name_data: [MAX_NAME_BYTES]u8,
 
             const Self = @This();
 

lib/std/fs/test.zig

@@ -703,6 +703,50 @@ test "makePath in a directory that no longer exists" {
     try testing.expectError(error.FileNotFound, tmp.dir.makePath("sub-path"));
 }
 
+fn testFilenameLimits(iterable_dir: IterableDir, maxed_filename: []const u8) !void {
+    // setup, create a dir and a nested file both with maxed filenames, and walk the dir
+    {
+        var maxed_dir = try iterable_dir.dir.makeOpenPath(maxed_filename, .{});
+        defer maxed_dir.close();
+
+        try maxed_dir.writeFile(maxed_filename, "");
+
+        var walker = try iterable_dir.walk(testing.allocator);
+        defer walker.deinit();
+
+        var count: usize = 0;
+        while (try walker.next()) |entry| {
+            try testing.expectEqualStrings(maxed_filename, entry.basename);
+            count += 1;
+        }
+        try testing.expectEqual(@as(usize, 2), count);
+    }
+
+    // ensure that we can delete the tree
+    try iterable_dir.dir.deleteTree(maxed_filename);
+}
+
+test "max file name component lengths" {
+    var tmp = tmpIterableDir(.{});
+    defer tmp.cleanup();
+
+    if (builtin.os.tag == .windows) {
+        // € is the character with the largest codepoint that is encoded as a single u16 in UTF-16,
+        // so Windows allows for NAME_MAX of them
+        const maxed_windows_filename = ("€".*) ** std.os.windows.NAME_MAX;
+        try testFilenameLimits(tmp.iterable_dir, &maxed_windows_filename);
+    } else if (builtin.os.tag == .wasi) {
+        // On WASI, the maxed filename depends on the host OS, so in order for this test to
+        // work on any host, we need to use a length that will work for all platforms
+        // (i.e. the minimum MAX_NAME_BYTES of all supported platforms).
+        const maxed_wasi_filename = [_]u8{'1'} ** 255;
+        try testFilenameLimits(tmp.iterable_dir, &maxed_wasi_filename);
+    } else {
+        const maxed_ascii_filename = [_]u8{'1'} ** std.fs.MAX_NAME_BYTES;
+        try testFilenameLimits(tmp.iterable_dir, &maxed_ascii_filename);
+    }
+}
+
 test "writev, readv" {
     var tmp = tmpDir(.{});
     defer tmp.cleanup();

lib/std/os/windows.zig

@@ -2977,6 +2977,24 @@ pub const PMEMORY_BASIC_INFORMATION = *MEMORY_BASIC_INFORMATION;
 /// from https://docs.microsoft.com/en-us/windows/desktop/FileIO/naming-a-file#maximum-path-length-limitation
 pub const PATH_MAX_WIDE = 32767;
 
+/// > [Each file name component can be] up to the value returned in the
+/// > lpMaximumComponentLength parameter of the GetVolumeInformation function
+/// > (this value is commonly 255 characters)
+/// from https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation
+///
+/// > The value that is stored in the variable that *lpMaximumComponentLength points to is
+/// > used to indicate that a specified file system supports long names. For example, for
+/// > a FAT file system that supports long names, the function stores the value 255, rather
+/// > than the previous 8.3 indicator. Long names can also be supported on systems that use
+/// > the NTFS file system.
+/// from https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getvolumeinformationw
+///
+/// The assumption being made here is that while lpMaximumComponentLength may vary, it will never
+/// be larger than 255.
+///
+/// TODO: More verification of this assumption.
+pub const NAME_MAX = 255;
+
 pub const FORMAT_MESSAGE_ALLOCATE_BUFFER = 0x00000100;
 pub const FORMAT_MESSAGE_ARGUMENT_ARRAY = 0x00002000;
 pub const FORMAT_MESSAGE_FROM_HMODULE = 0x00000800;