Keep an fd for the cwd in the parser

To support distinct parsers having different working directories, we need
to keep the working directory alive, and also retain a non-path reference
to it.

src/builtin_cd.cpp

@@ -2,6 +2,7 @@
 #include "config.h"  // IWYU pragma: keep
 
 #include <errno.h>
+#include <fcntl.h>
 #include <sys/stat.h>
 
 #include "builtin.h"
@@ -68,14 +69,17 @@ int builtin_cd(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
 
     wcstring norm_dir = normalize_path(dir);
 
-    if (wchdir(norm_dir) != 0) {
+    // We need to keep around the fd for this directory, in the parser.
+    autoclose_fd_t dir_fd(wopen_cloexec(norm_dir, O_RDONLY));
+    bool success = dir_fd.valid() && fchdir(dir_fd.fd()) == 0;
+
+    if (!success) {
         struct stat buffer;
         int status;
 
         status = wstat(dir, &buffer);
         if (!status && S_ISDIR(buffer.st_mode)) {
             streams.err.append_format(_(L"%ls: Permission denied: '%ls'\n"), cmd, dir_in.c_str());
-
         } else {
             streams.err.append_format(_(L"%ls: '%ls' is not a directory\n"), cmd, dir_in.c_str());
         }
@@ -87,6 +91,7 @@ int builtin_cd(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
         return STATUS_CMD_ERROR;
     }
 
+    parser.libdata().cwd_fd = std::make_shared<const autoclose_fd_t>(std::move(dir_fd));
     std::vector<event_t> evts;
     parser.vars().set_one(L"PWD", ENV_EXPORT | ENV_GLOBAL, std::move(norm_dir), &evts);
     for (const auto &evt : evts) {

src/parser.cpp

@@ -1,6 +1,7 @@
 // The fish parser. Contains functions for parsing and evaluating code.
 #include "config.h"  // IWYU pragma: keep
 
+#include <fcntl.h>
 #include <stdio.h>
 #include <cwchar>
 
@@ -102,6 +103,12 @@ wcstring parser_t::user_presentable_path(const wcstring &path) const {
 
 parser_t::parser_t(std::shared_ptr<env_stack_t> vars) : variables(std::move(vars)) {
     assert(variables.get() && "Null variables in parser initializer");
+    int cwd = open_cloexec(".", O_RDONLY);
+    if (cwd < 0) {
+        perror("Unable to open the current working directory");
+        abort();
+    }
+    libdata().cwd_fd = std::make_shared<const autoclose_fd_t>(cwd);
 }
 
 parser_t::parser_t() : parser_t(env_stack_t::principal_ref()) {}

src/parser.h

@@ -169,6 +169,10 @@ struct library_data_t {
     /// machinery when wrapping: e.g. if `tig` wraps `git` then git completions need to see git on
     /// the command line.
     wcstring_list_t transient_commandlines{};
+
+    /// A file descriptor holding the current working directory, for use in openat().
+    /// This is never null and never invalid.
+    std::shared_ptr<const autoclose_fd_t> cwd_fd{};
 };
 
 class parser_t : public std::enable_shared_from_this<parser_t> {