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Make pg_mkdir_p() tolerant of a concurrent directory creation.

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pg_mkdir_p creates each missing path component with a stat() followed
by mkdir().  If the stat() reports the component as absent but another
process creates it in the window before this process's mkdir(), mkdir()
fails with EEXIST and pg_mkdir_p treated that as a hard error -- unlike
"mkdir -p", which is meant to be idempotent and race-tolerant.

This shows up when several processes concurrently create paths that
share an ancestor directory: for example, parallel initdb runs whose
data directories live under a common temporary directory.  One process
wins the race to create the shared ancestor and the others fail with
    could not create directory "...": File exists

Fix this race condition by first trying mkdir() and only attempting
stat() if it fails with EEXIST.

On Windows, there's an additional problem: stat() opens a file handle
and participates in share-mode locking, which means it can transiently
fail on a directory another process is concurrently creating.  Use
GetFileAttributes() instead: it requests only FILE_READ_ATTRIBUTES
and is exempt from share-mode denial, so it reliably sees a
concurrently-created directory.

I (tgl) also chose to back-patch 039f7ee0f's effects on this function,
so that pgmkdirp.c remains identical in all live branches.

Author: Andrew Dunstan <andrew@dunslane.net>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3ca004de-e49b-4471-b8aa-fd656e70f68c@dunslane.net
Backpatch-through: 14
This commit is contained in:
Tom Lane 2026-06-19 12:52:00 -04:00
parent dc51167808
commit f25a07b2d9
1 changed files with 34 additions and 13 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

47
src/port/pgmkdirp.c
View file

@ -56,7 +56,6 @@
int
pg_mkdir_p(char *path, int omode)
{
struct stat sb;
mode_t numask,
oumask;
int last,
@ -119,24 +118,46 @@ pg_mkdir_p(char *path, int omode)
if (last)
(void) umask(oumask);
/* check for pre-existing directory */
if (stat(path, &sb) == 0)
if (mkdir(path, last ? omode : S_IRWXU | S_IRWXG | S_IRWXO) < 0)
{
if (!S_ISDIR(sb.st_mode))
/*
* If we got EEXIST because there's already a directory there,
* don't complain.
*/
#ifndef WIN32
int save_errno = errno;
struct stat sb;
if (save_errno != EEXIST ||
stat(path, &sb) != 0 ||
!S_ISDIR(sb.st_mode))
{
if (last)
errno = EEXIST;
else
errno = ENOTDIR;
/* Don't let stat replace mkdir's errno */
errno = save_errno;
retval = -1;
break;
}
#else /* WIN32 */
/*
* On Windows, stat() opens a handle and can transiently fail on a
* directory another process is concurrently creating. Probe with
* a path-based attribute query instead: it requests only
* FILE_READ_ATTRIBUTES and is exempt from share-mode denial, so
* it reliably sees a concurrently-created directory. We assume
* GetFileAttributes() won't change errno.
*/
DWORD attr = GetFileAttributes(path);
if (errno != EEXIST ||
attr == INVALID_FILE_ATTRIBUTES ||
!(attr & FILE_ATTRIBUTE_DIRECTORY))
{
retval = -1;
break;
}
#endif /* WIN32 */
}
else if (mkdir(path, last ? omode : S_IRWXU | S_IRWXG | S_IRWXO) < 0)
{
retval = -1;
break;
}
if (!last)
*p = '/';
}