wasi: fstat{at,} support

Clean up the std.c.S flags mapped from wasi-libc, and share them with the
no-libc wasi POSIX code.  Fix the wasi-libc Stat.mode field.

Add a test for fstat{,at} with symlinks (for everyone, not just wasi)

Fixes #20890

Author: rootbeer

lib/std/c.zig

@@ -1704,17 +1704,44 @@ pub const S = switch (native_os) {
     .linux => linux.S,
     .emscripten => emscripten.S,
     .wasi => struct {
-        pub const IEXEC = @compileError("TODO audit this");
+        // Match wasi-libc's libc-bottom-half/headers/public/__mode_t.h
+        // NOTE: this S struct is also used by Wasi-without-libc
         pub const IFBLK = 0x6000;
         pub const IFCHR = 0x2000;
         pub const IFDIR = 0x4000;
-        pub const IFIFO = 0xc000;
         pub const IFLNK = 0xa000;
-        pub const IFMT = IFBLK | IFCHR | IFDIR | IFIFO | IFLNK | IFREG | IFSOCK;
         pub const IFREG = 0x8000;
-        /// There's no concept of UNIX domain socket but we define this value here
-        /// in order to line with other OSes.
-        pub const IFSOCK = 0x1;
+        pub const IFSOCK = 0xc000;
+        pub const IFIFO = 0x1000;
+        pub const IFMT = IFBLK | IFCHR | IFDIR | IFIFO | IFLNK | IFREG | IFSOCK;
+
+        pub fn ISBLK(m: u32) bool {
+            return m & IFMT == IFBLK;
+        }
+
+        pub fn ISCHR(m: u32) bool {
+            return m & IFMT == IFCHR;
+        }
+
+        pub fn ISDIR(m: u32) bool {
+            return m & IFMT == IFDIR;
+        }
+
+        pub fn ISFIFO(m: u32) bool {
+            return m & IFMT == IFIFO;
+        }
+
+        pub fn ISLNK(m: u32) bool {
+            return m & IFMT == IFLNK;
+        }
+
+        pub fn ISREG(m: u32) bool {
+            return m & IFMT == IFREG;
+        }
+
+        pub fn ISSOCK(m: u32) bool {
+            return m & IFMT == IFSOCK;
+        }
     },
     .macos, .ios, .tvos, .watchos, .visionos => struct {
         pub const IFMT = 0o170000;
@@ -6372,7 +6399,7 @@ pub const Stat = switch (native_os) {
         dev: dev_t,
         ino: ino_t,
         nlink: nlink_t,
-        mode: mode_t,
+        mode: c_uint, // wasi-libc only the file-type bits (S.IFMT) no permission bits
         uid: uid_t,
         gid: gid_t,
         __pad0: c_uint = 0,

lib/std/posix.zig

@@ -77,6 +77,7 @@ else switch (native_os) {
         };
 
         pub const Stat = std.c.Stat; // libc Stat has nice conversion code to/from Wasi stat structure
+        pub const S = std.c.S; // libc S has all nice file-type conversions
 
         pub const mode_t = void; // Wasi does not (yet) support file mode/permission bits
 

lib/std/posix/test.zig

@@ -25,6 +25,9 @@ const supports_chdir = (native_os != .wasi);
 // Filter to skip tests on platforms that don't support absolute paths
 const supports_absolute_paths = (native_os != .wasi);
 
+// Filter to skip tests on platforms that don't (yet) suppport fstat/fstatat
+const supports_fstat = (native_os != .windows);
+
 test "check WASI CWD" {
     if (native_os == .wasi) {
         if (std.options.wasiCwd() != 3) {
@@ -305,6 +308,8 @@ fn testReadlink(target_path: []const u8, symlink_path: []const u8) !void {
 }
 
 test "link with relative paths" {
+    if (!supports_fstat) return error.SkipZigTest;
+
     switch (native_os) {
         .wasi, .linux, .solaris, .illumos => {},
         else => return error.SkipZigTest,
@@ -347,6 +352,8 @@ test "link with relative paths" {
 }
 
 test "linkat with different directories" {
+    if (!supports_fstat) return error.SkipZigTest;
+
     switch (native_os) {
         .wasi, .linux, .solaris, .illumos => {},
         else => return error.SkipZigTest,
@@ -388,9 +395,8 @@ test "linkat with different directories" {
     }
 }
 
-test "fstatat" {
-    // enable when `fstat` and `fstatat` are implemented on Windows
-    if (native_os == .windows) return error.SkipZigTest;
+test "fstat(at) file" {
+    if (!supports_fstat) return error.SkipZigTest;
 
     var tmp = tmpDir(.{});
     defer tmp.cleanup();
@@ -402,14 +408,66 @@ test "fstatat" {
     // fetch file's info on the opened fd directly
     const file = try tmp.dir.openFile("file.txt", .{});
     const stat = try posix.fstat(file.handle);
-    defer file.close();
+    file.close();
 
     // now repeat but using `fstatat` instead
-    const flags = posix.AT.SYMLINK_NOFOLLOW;
-    const statat = try posix.fstatat(tmp.dir.fd, "file.txt", flags);
+    const statat = try posix.fstatat(tmp.dir.fd, "file.txt", 0);
     try expectEqual(stat, statat);
 }
 
+test "fstat(at) symlink" {
+    if (!supports_fstat) return error.SkipZigTest;
+
+    var tmp = testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    try tmp.dir.writeFile(.{ .sub_path = "target.txt", .data = "irrelevant" });
+
+    const target = try tmp.dir.openFile("target.txt", .{});
+    const statTarget = try posix.fstat(target.handle);
+    target.close();
+
+    // Set up symlink
+    try tmp.dir.symLink("target.txt", "sym.lnk", .{});
+
+    // Openat (+follow) + fstat() the symlink
+    const linkFollowFd = try posix.openat(tmp.dir.fd, "sym.lnk", .{}, default_mode);
+    defer posix.close(linkFollowFd);
+    const statLinkFollow = try posix.fstat(linkFollowFd);
+
+    // fstatat (with and without follow) the symlink
+    const statatLinkFollow = try posix.fstatat(tmp.dir.fd, "sym.lnk", 0);
+    const statatLinkNoFollow = try posix.fstatat(tmp.dir.fd, "sym.lnk", posix.AT.SYMLINK_NOFOLLOW);
+
+    if (@hasField(posix.O, "PATH")) {
+        // Can only openat() a symlink with NOFOLLOW if O.PATH is
+        // supported.  Result should exactly match result from the
+        // no-follow fstatat() call.
+
+        const linkNoFollowFd = try posix.openat(tmp.dir.fd, "sym.lnk", .{ .NOFOLLOW = true, .PATH = true }, default_mode);
+        defer posix.close(linkNoFollowFd);
+
+        const statLinkNoFollow = try posix.fstat(linkNoFollowFd);
+        try testing.expectEqual(statLinkNoFollow, statatLinkNoFollow);
+    }
+
+    // Link following should have followed the link
+    try testing.expectEqual(statTarget, statLinkFollow);
+    try testing.expectEqual(statTarget, statatLinkFollow);
+
+    // symlink and target are different:
+    try testing.expect(statTarget.ino != statatLinkNoFollow.ino);
+    try testing.expect(statTarget.mode != statatLinkNoFollow.mode);
+
+    // target is a regular, non-link file:
+    try testing.expect(posix.S.ISREG(statTarget.mode));
+    try testing.expect(!posix.S.ISLNK(statTarget.mode));
+
+    // symlink is a non-regular, link file:
+    try testing.expect(!posix.S.ISREG(statatLinkNoFollow.mode));
+    try testing.expect(posix.S.ISLNK(statatLinkNoFollow.mode));
+}
+
 test "readlinkat" {
     var tmp = tmpDir(.{});
     defer tmp.cleanup();
@@ -1255,6 +1313,7 @@ fn expectMode(dir: posix.fd_t, file: []const u8, mode: posix.mode_t) !void {
 
 test "fchmodat smoke test" {
     if (!std.fs.has_executable_bit) return error.SkipZigTest;
+    if (!supports_fstat) return error.SkipZigTest; // for expectMode()
 
     var tmp = tmpDir(.{});
     defer tmp.cleanup();