From ca8503f168d0632c606110da909aba3057777395 Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Mon, 21 Mar 2022 18:11:26 +0100
Subject: [PATCH] fs-util: add openat_report_new() wrapper around openat()

This is a wrapper around openat(). It works mostly the same, except for
one thing: it race-freely reports whether we just created the indicated
file in case O_CREAT is passed without O_EXCL.
---
 src/basic/fs-util.c     | 39 ++++++++++++++++++++++++++++++++
 src/basic/fs-util.h     |  2 ++
 src/test/test-fs-util.c | 50 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 91 insertions(+)

diff --git a/src/basic/fs-util.c b/src/basic/fs-util.c
index 11039fd75d..14aea54e26 100644
--- a/src/basic/fs-util.c
+++ b/src/basic/fs-util.c
@@ -1083,3 +1083,42 @@ int open_mkdir_at(int dirfd, const char *path, int flags, mode_t mode) {
 
         return TAKE_FD(fd);
 }
+
+int openat_report_new(int dirfd, const char *pathname, int flags, mode_t mode, bool *ret_newly_created) {
+        unsigned attempts = 7;
+
+        /* Just like openat(), but adds one thing: optionally returns whether we created the file anew or if
+         * it already existed before. This is only relevant of O_CREAT is set without O_EXCL, and thus will
+         * shortcut to openat() otherwise */
+
+        if (!FLAGS_SET(flags, O_CREAT) || FLAGS_SET(flags, O_EXCL) || !ret_newly_created)
+                return RET_NERRNO(openat(dirfd, pathname, flags, mode));
+
+        for (;;) {
+                int fd;
+
+                /* First, attempt to open without O_CREAT/O_EXCL, i.e. open existing file */
+                fd = openat(dirfd, pathname, flags & ~(O_CREAT | O_EXCL), mode);
+                if (fd >= 0) {
+                        *ret_newly_created = false;
+                        return fd;
+                }
+                if (errno != ENOENT)
+                        return -errno;
+
+                /* So the file didn't exist yet, hence create it with O_CREAT/O_EXCL. */
+                fd = openat(dirfd, pathname, flags | O_CREAT | O_EXCL, mode);
+                if (fd >= 0) {
+                        *ret_newly_created = true;
+                        return fd;
+                }
+                if (errno != EEXIST)
+                        return -errno;
+
+                /* Hmm, so now we got EEXIST? So it apparently exists now? If so, let's try to open again
+                 * without the two flags. But let's not spin forever, hnce put a limit on things */
+
+                if (--attempts == 0) /* Give up eventually, somebody is playing with us */
+                        return -EEXIST;
+        }
+}
diff --git a/src/basic/fs-util.h b/src/basic/fs-util.h
index 0bbb3f6298..e48cf6800f 100644
--- a/src/basic/fs-util.h
+++ b/src/basic/fs-util.h
@@ -110,3 +110,5 @@ int posix_fallocate_loop(int fd, uint64_t offset, uint64_t size);
 int parse_cifs_service(const char *s, char **ret_host, char **ret_service, char **ret_path);
 
 int open_mkdir_at(int dirfd, const char *path, int flags, mode_t mode);
+
+int openat_report_new(int dirfd, const char *pathname, int flags, mode_t mode, bool *ret_newly_created);
diff --git a/src/test/test-fs-util.c b/src/test/test-fs-util.c
index 67b6996907..c93723ad29 100644
--- a/src/test/test-fs-util.c
+++ b/src/test/test-fs-util.c
@@ -970,6 +970,56 @@ TEST(open_mkdir_at) {
         assert_se(subsubdir_fd >= 0);
 }
 
+TEST(openat_report_new) {
+        _cleanup_free_ char *j = NULL;
+        _cleanup_(rm_rf_physical_and_freep) char *d = NULL;
+        _cleanup_close_ int fd = -1;
+        bool b;
+
+        assert_se(mkdtemp_malloc(NULL, &d) >= 0);
+
+        j = path_join(d, "test");
+        assert_se(j);
+
+        fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
+        assert_se(fd >= 0);
+        fd = safe_close(fd);
+        assert_se(b);
+
+        fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
+        assert_se(fd >= 0);
+        fd = safe_close(fd);
+        assert_se(!b);
+
+        fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
+        assert_se(fd >= 0);
+        fd = safe_close(fd);
+        assert_se(!b);
+
+        assert_se(unlink(j) >= 0);
+
+        fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
+        assert_se(fd >= 0);
+        fd = safe_close(fd);
+        assert_se(b);
+
+        fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
+        assert_se(fd >= 0);
+        fd = safe_close(fd);
+        assert_se(!b);
+
+        assert_se(unlink(j) >= 0);
+
+        fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, NULL);
+        assert_se(fd >= 0);
+        fd = safe_close(fd);
+
+        fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
+        assert_se(fd >= 0);
+        fd = safe_close(fd);
+        assert_se(!b);
+}
+
 static int intro(void) {
         arg_test_dir = saved_argv[1];
         return EXIT_SUCCESS;