zig build docs fails on aarch64

[andrewrk]

./zig build docs

Outputs:

attempt to unwrap null
reached unreachable code
/deps/install/lib/zig/std/child_process.zig:182:49: 0x24512b in std.child_process.ChildProcess.exec (docgen)
        var stdout_file_in_stream = child.stdout.?.inStream();
                                                ^
/deps/install/lib/zig/std/os.zig:667:22: 0x24b343 in std.os.dup2 (docgen)
            EBADF => unreachable, // always a race condition
                     ^
/deps/zig/doc/docgen.zig:1485:41: 0x2462ff in exec (docgen)
    const result = try ChildProcess.exec(allocator, args, null, env_map, max_doc_file_size);
                                        ^
/deps/install/lib/zig/std/child_process.zig:609:38: 0x24a907 in std.child_process.ChildProcess.setUpChildIo (docgen)
            StdIo.Pipe => try os.dup2(pipe_fd, std_fileno),
                                     ^
/deps/zig/doc/docgen.zig:1510:28: 0x24278f in getBuiltinCode (docgen)
    const result = try exec(allocator, env_map, [_][]const u8{
                           ^
/deps/install/lib/zig/std/child_process.zig:340:25: 0x24a1d3 in std.child_process.ChildProcess.spawnPosix (docgen)
            setUpChildIo(self.stdout_behavior, stdout_pipe[1], os.STDOUT_FILENO, dev_null_fd) catch |err| forkChildErrReport(err_pipe[1], err);
                        ^
/deps/zig/doc/docgen.zig:981:44: 0x2336b3 in genHtml (docgen)
    const builtin_code = try getBuiltinCode(allocator, &env_map, zig_exe);
                                           ^
/deps/zig/doc/docgen.zig:56:16: 0x23006f in main (docgen)
    try genHtml(allocator, &tokenizer, &toc, &buffered_out_stream.stream, zig_exe);
      /deps/install/lib/zig/std/child_process.zig : 109 : 35 : 0x 249943  in  std.child_process.ChildProcess.spawn  (^
docgen)
            return self.spawnPosix();
                                  ^
/deps/install/lib/zig/std/special/start.zig:204:37: 0x22ee8b in std.special.posixCallMainAndExit (docgen)
            const result = root.main() catch |err| {
                                    ^
/deps/install/lib/zig/std/child_process.zig:175:24: 0x244f27 in std.child_process.ChildProcess.exec (docgen)
        try child.spawn();
                       ^
/deps/install/lib/zig/std/special/start.zig:102:5: 0x22ed23 in std.special._start (docgen)
    @noInlineCall(posixCallMainAndExit);
    ^
/deps/zig/doc/docgen.zig:1485:41: 0x2462ff in exec (docgen)
    const result = try ChildProcess.exec(allocator, args, null, env_map, max_doc_file_size);
                                        ^
/deps/zig/doc/docgen.zig:1510:28: 0x24278f in getBuiltinCode (docgen)
    const result = try exec(allocator, env_map, [_][]const u8{
                           ^
/deps/zig/doc/docgen.zig:981:44: 0x2336b3 in genHtml (docgen)
    const builtin_code = try getBuiltinCode(allocator, &env_map, zig_exe);
                                           ^
/deps/zig/doc/docgen.zig:56:16: 0x23006f in main (docgen)
    try genHtml(allocator, &tokenizer, &toc, &buffered_out_stream.stream, zig_exe);
               ^
/deps/install/lib/zig/std/special/start.zig:204:37: 0x22ee8b in std.special.posixCallMainAndExit (docgen)
            const result = root.main() catch |err| {
                                    ^
The following command terminated unexpectedly:
cd /deps/zig && /deps/zig/zig-cache/o/98n8r9vC4nGn5qF3Sd-2i3ST_zzI8KLzbeEedI2zkEMzolFqfiTyNgVs9aiY0WMX/docgen ../install/bin/zig doc/langref.html.in zig-cache/langref.html 

Build failed. The following command failed:
/deps/zig/zig-cache/o/fmWagUauxJYG5TDzHuY1dSsadLlVX8bK9e8mjM_uQMlxvk7UH62_aP9N2GSFQQaV/build /deps/install/bin/zig /deps/zig /deps/zig/zig-cache docs
/deps/install/lib/zig/std/special/start.zig:102:5: 0x22ed23 in std.special._start (docgen)
    @noInlineCall(posixCallMainAndExit);
    ^

I believe what we're seeing is 2 interleaved stack traces. One from the child and one from the parent (side note- we need locking in stack trace dumping code). The child hits an assertion error at dup2 and the parent hits one when it tries to null-unwrap stdout.

[LemonBoy]

Not our fault, it's LLVM that miscompiles the if (self.stdin_behavior == StdIo.Pipe) expressions in spawnPosix, leading to os.pipe never being called at all. The good news is that the problem is gone in LLVM 10 (I suspect it's just #3271 again)

[andrewrk]

aarch64 tarballs now ship with docs. There is one test in the docs disabled for aarch64 which is #3282.