[libglnx/replace-increasing-mtime] Add GLNX_FILE_REPLACE_INCREASING_MTIME

[Alexander Larsson <alexl src gnome org>]

commit f54ac24c2d3b10d97835ace20091a5d3668e52d1
Author: Alexander Larsson <alexl redhat com>
Date:   Mon Nov 2 14:37:26 2020 +0100

    Add GLNX_FILE_REPLACE_INCREASING_MTIME
    
    This make replaced files have a strictly increasing st_mtime. The main
    usecase I have for this is to ensure the summary file mtime increases
    because the flatpak tests are failing due to the python httpd used
    in the tests rely on st_mtime for the http If-Modified-Since header.
    
    For the tests this breaks all the time since we're just doing a lot of
    summary updates. However, I can see this accidentally happening in the
    wild too, so i think its proper to always ensure the new summary is
    "newer", even though it means it will be timestamped slightly in the
    future. In practice this will not happen regularly, and the times it
    *does* happen we really do need it.

 glnx-fdio.c | 37 +++++++++++++++++++++++++++++++------
 glnx-fdio.h |  2 ++
 2 files changed, 33 insertions(+), 6 deletions(-)
---
diff --git a/glnx-fdio.c b/glnx-fdio.c
index 422bc2d..02013c6 100644
--- a/glnx-fdio.c
+++ b/glnx-fdio.c
@@ -1105,6 +1105,11 @@ glnx_file_replace_contents_with_perms_at (int                   dfd,
 {
   char *dnbuf = strdupa (subpath);
   const char *dn = dirname (dnbuf);
+  gboolean increasing_mtime = (flags & GLNX_FILE_REPLACE_INCREASING_MTIME) != 0;
+  gboolean nodatasync = (flags & GLNX_FILE_REPLACE_NODATASYNC) != 0;
+  gboolean datasync_new = (flags & GLNX_FILE_REPLACE_DATASYNC_NEW) != 0;
+  struct stat stbuf;
+  gboolean has_stbuf = FALSE;
 
   dfd = glnx_dirfd_canonicalize (dfd);
 
@@ -1128,15 +1133,18 @@ glnx_file_replace_contents_with_perms_at (int                   dfd,
   if (glnx_loop_write (tmpf.fd, buf, len) < 0)
     return glnx_throw_errno_prefix (error, "write");
 
-  if (!(flags & GLNX_FILE_REPLACE_NODATASYNC))
+  if (!nodatasync || increasing_mtime)
     {
-      struct stat stbuf;
-      gboolean do_sync;
-
       if (!glnx_fstatat_allow_noent (dfd, subpath, &stbuf, AT_SYMLINK_NOFOLLOW, error))
         return FALSE;
-      if (errno == ENOENT)
-        do_sync = (flags & GLNX_FILE_REPLACE_DATASYNC_NEW) > 0;
+      has_stbuf = errno != ENOENT;
+    }
+
+  if (!nodatasync)
+    {
+      gboolean do_sync;
+      if (!has_stbuf)
+        do_sync = datasync_new;
       else
         do_sync = TRUE;
 
@@ -1156,6 +1164,23 @@ glnx_file_replace_contents_with_perms_at (int                   dfd,
   if (fchmod (tmpf.fd, mode) != 0)
     return glnx_throw_errno_prefix (error, "fchmod");
 
+  if (increasing_mtime && has_stbuf)
+    {
+      struct stat fd_stbuf;
+
+      if (fstat (tmpf.fd, &fd_stbuf) != 0)
+        return glnx_throw_errno_prefix (error, "fstat");
+
+      /* We want to ensure that the new file has a st_mtime (i.e. the second precision)
+       * is incrementing to avoid mtime check issues when files change often.
+       */
+      if (fd_stbuf.st_mtime <= stbuf.st_mtime)
+        {
+          struct timespec ts[2] = { {0, UTIME_OMIT}, {stbuf.st_mtime + 1, 0} };
+          (void) futimens (tmpf.fd, ts);
+        }
+    }
+
   if (!glnx_link_tmpfile_at (&tmpf, GLNX_LINK_TMPFILE_REPLACE,
                              dfd, subpath, error))
     return FALSE;
diff --git a/glnx-fdio.h b/glnx-fdio.h
index f95e473..cc1ed4b 100644
--- a/glnx-fdio.h
+++ b/glnx-fdio.h
@@ -143,12 +143,14 @@ glnx_file_get_contents_utf8_at (int                   dfd,
  * GLnxFileReplaceFlags:
  * @GLNX_FILE_REPLACE_DATASYNC_NEW: Call fdatasync() even if the file did not exist
  * @GLNX_FILE_REPLACE_NODATASYNC: Never call fdatasync()
+ * @GLNX_FILE_REPLACE_INCREASING_MTIME: Ensure that st_mtime increases (in second precision)
  *
  * Flags controlling file replacement.
  */
 typedef enum {
   GLNX_FILE_REPLACE_DATASYNC_NEW = (1 << 0),
   GLNX_FILE_REPLACE_NODATASYNC = (1 << 1),
+  GLNX_FILE_REPLACE_INCREASING_MTIME = (1 << 2),
 } GLnxFileReplaceFlags;
 
 gboolean