wasi: fix wasm-wasi-musl constants

Zig's copy of the `SYMLINK_{NO,}FOLLOW` constants from wasi-musl was
wrong, as were the `IFIFO` and `IFSOCK` file type flags.  Fix these up,
and add comments pointing to exactly where they come from (as the
wasi-musl source has lots of unused, different definitions of these
constants).

Add tests for the Zig convention that WASM preopen 3 is the current
working directory.   This is true for WASM with or without libc.

Enable several fs and posix tests that are now passing (not necessarily
because of this change) on wasm targets.

Fixes #20890.

1 files changed, 41 insertions, 9 deletions

diff --git a/lib/std/c.zig b/lib/std/c.zig
index d3418c7d8a..a366c0ca63 100644
--- a/lib/std/c.zig
+++ b/lib/std/c.zig
@@ -692,6 +692,7 @@ pub const F = switch (native_os) {
     .linux => linux.F,
     .emscripten => emscripten.F,
     .wasi => struct {
+        // Match `F_*` constants from lib/libc/include/wasm-wasi-musl/__header_fcntl.h
         pub const GETFD = 1;
         pub const SETFD = 2;
         pub const GETFL = 3;
@@ -1723,17 +1724,43 @@ pub const S = switch (native_os) {
     .linux => linux.S,
     .emscripten => emscripten.S,
     .wasi => struct {
-        pub const IEXEC = @compileError("TODO audit this");
+        // Match `S_*` constants from lib/libc/include/wasm-wasi-musl/__mode_t.h
         pub const IFBLK = 0x6000;
         pub const IFCHR = 0x2000;
         pub const IFDIR = 0x4000;
-        pub const IFIFO = 0xc000;
+        pub const IFIFO = 0x1000;
         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 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;
@@ -6802,6 +6829,7 @@ pub const Stat = switch (native_os) {
     },
     .emscripten => emscripten.Stat,
     .wasi => extern struct {
+        // Match wasi-libc's `struct stat` in lib/libc/include/wasm-wasi-musl/__struct_stat.h
         dev: dev_t,
         ino: ino_t,
         nlink: nlink_t,
@@ -7502,9 +7530,11 @@ pub const AT = switch (native_os) {
         pub const RECURSIVE = 0x8000;
     },
     .wasi => struct {
-        pub const SYMLINK_NOFOLLOW = 0x100;
-        pub const SYMLINK_FOLLOW = 0x400;
-        pub const REMOVEDIR: u32 = 0x4;
+        // Match `AT_*` constants in lib/libc/include/wasm-wasi-musl/__header_fcntl.h
+        pub const EACCESS = 0x0;
+        pub const SYMLINK_NOFOLLOW = 0x1;
+        pub const SYMLINK_FOLLOW = 0x2;
+        pub const REMOVEDIR = 0x4;
         /// When linking libc, we follow their convention and use -2 for current working directory.
         /// However, without libc, Zig does a different convention: it assumes the
         /// current working directory is the first preopen. This behavior can be
@@ -7512,7 +7542,6 @@ pub const AT = switch (native_os) {
         /// file.
         pub const FDCWD: fd_t = if (builtin.link_libc) -2 else 3;
     },
-
     else => void,
 };
 
@@ -7541,6 +7570,7 @@ pub const O = switch (native_os) {
         _: u9 = 0,
     },
     .wasi => packed struct(u32) {
+        // Match `O_*` bits from lib/libc/include/wasm-wasi-musl/__header_fcntl.h
         APPEND: bool = false,
         DSYNC: bool = false,
         NONBLOCK: bool = false,
@@ -7557,6 +7587,8 @@ pub const O = switch (native_os) {
         read: bool = false,
         SEARCH: bool = false,
         write: bool = false,
+        // O_CLOEXEC, O_TTY_ININT, O_NOCTTY are 0 in wasi-musl, so they're silently
+        // ignored in C code.  Thus no mapping in Zig.
         _: u3 = 0,
     },
     .solaris, .illumos => packed struct(u32) {