From d0232028111ce4f5a066d9a302fec142ebe91037 Mon Sep 17 00:00:00 2001
From: Artemis Tosini <me@artem.ist>
Date: Sat, 26 Oct 2024 17:12:06 +0000
Subject: [PATCH] Add support for `utimensat` as an alternative to `lutimes`

OpenBSD doesn't support `lutimes`, but does support `utimensat` which
subsumes it. In fact, all the BSDs, Linux, and newer macOS all support
it. So lets make this our first choice for the implementation.

In addition, let's get rid of the `lutimes` `ENOSYS` special case. The
Linux manpage says

> ENOSYS
>
> The kernel does not support this call; Linux 2.6.22 or later is
> required.

which I think is the origin of this check, but that's a very old version
of Linux at this point. The code can be simplified a lot of we drop
support for it here (as we've done elsewhere, anyways).

Co-Authored-By: John Ericson <John.Ericson@Obsidian.Systems>
---
 configure.ac               |  7 ++--
 src/libutil/file-system.cc | 68 +++++++++++++++++++-------------------
 src/libutil/meson.build    |  4 +++
 3 files changed, 42 insertions(+), 37 deletions(-)

diff --git a/configure.ac b/configure.ac
index fc59904f358..746e953ddf9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -89,9 +89,10 @@ AC_LANG_POP(C++)
 AC_CHECK_FUNCS([statvfs pipe2 close_range])
 
 
-# Check for lutimes, optionally used for changing the mtime of
-# symlinks.
-AC_CHECK_FUNCS([lutimes])
+# Check for lutimes and utimensat, optionally used for changing the
+# mtime of symlinks.
+AC_CHECK_DECLS([AT_SYMLINK_NOFOLLOW], [], [], [[#include <fcntl.h>]])
+AC_CHECK_FUNCS([lutimes utimensat])
 
 
 # Check whether the store optimiser can optimise symlinks.
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index 224b78b23d5..fd51d7d3cbd 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -630,7 +630,28 @@ void setWriteTime(
     time_t modificationTime,
     std::optional<bool> optIsSymlink)
 {
-#ifndef _WIN32
+#ifdef _WIN32
+    // FIXME use `fs::last_write_time`.
+    //
+    // Would be nice to use std::filesystem unconditionally, but
+    // doesn't support access time just modification time.
+    //
+    // System clock vs File clock issues also make that annoying.
+    warn("Changing file times is not yet implemented on Windows, path is '%s'", path);
+#elif HAVE_UTIMENSAT && HAVE_DECL_AT_SYMLINK_NOFOLLOW
+    struct timespec times[2] = {
+        {
+            .tv_sec = accessedTime,
+            .tv_nsec = 0,
+        },
+        {
+            .tv_sec = modificationTime,
+            .tv_nsec = 0,
+        },
+    };
+    if (utimensat(AT_FDCWD, path.c_str(), times, AT_SYMLINK_NOFOLLOW) == -1)
+        throw SysError("changing modification time of '%s' (using `utimensat`)", path);
+#else
     struct timeval times[2] = {
         {
             .tv_sec = accessedTime,
@@ -641,42 +662,21 @@ void setWriteTime(
             .tv_usec = 0,
         },
     };
-#endif
-
-    auto nonSymlink = [&]{
-        bool isSymlink = optIsSymlink
-            ? *optIsSymlink
-            : fs::is_symlink(path);
-
-        if (!isSymlink) {
-#ifdef _WIN32
-            // FIXME use `fs::last_write_time`.
-            //
-            // Would be nice to use std::filesystem unconditionally, but
-            // doesn't support access time just modification time.
-            //
-            // System clock vs File clock issues also make that annoying.
-            warn("Changing file times is not yet implemented on Windows, path is '%s'", path);
+#if HAVE_LUTIMES
+    if (lutimes(path.c_str(), times) == -1)
+        throw SysError("changing modification time of '%s'", path);
 #else
-            if (utimes(path.c_str(), times) == -1) {
-
-                throw SysError("changing modification time of '%s' (not a symlink)", path);
-            }
-#endif
-        } else {
-            throw Error("Cannot modification time of symlink '%s'", path);
-        }
-    };
+    bool isSymlink = optIsSymlink
+        ? *optIsSymlink
+        : fs::is_symlink(path);
 
-#if HAVE_LUTIMES
-    if (lutimes(path.c_str(), times) == -1) {
-        if (errno == ENOSYS)
-            nonSymlink();
-        else
-            throw SysError("changing modification time of '%s'", path);
+    if (!isSymlink) {
+        if (utimes(path.c_str(), times) == -1)
+            throw SysError("changing modification time of '%s' (not a symlink)", path);
+    } else {
+        throw Error("Cannot modification time of symlink '%s'", path);
     }
-#else
-    nonSymlink();
+#endif
 #endif
 }
 
diff --git a/src/libutil/meson.build b/src/libutil/meson.build
index 57b741a50a4..08413783d04 100644
--- a/src/libutil/meson.build
+++ b/src/libutil/meson.build
@@ -42,6 +42,8 @@ check_funcs = [
   # Optionally used to try to close more file descriptors (e.g. before
   # forking) on Unix.
   'sysconf',
+  # Optionally used for changing the mtime of files and symlinks.
+  'utimensat',
 ]
 foreach funcspec : check_funcs
   define_name = 'HAVE_' + funcspec.underscorify().to_upper()
@@ -49,6 +51,8 @@ foreach funcspec : check_funcs
   configdata.set(define_name, define_value)
 endforeach
 
+configdata.set('HAVE_DECL_AT_SYMLINK_NOFOLLOW', cxx.has_header_symbol('fcntl.h', 'AT_SYMLINK_NOFOLLOW').to_int())
+
 subdir('build-utils-meson/threads')
 
 # Check if -latomic is needed