Be more careful when checking for empty files

If we want to avoid reading a totally empty file, it seems better
to check after we have opened the file, not before.

6 files changed, 35 insertions, 19 deletions

diff --git a/src/network/networkd-netdev.c b/src/network/networkd-netdev.c
index 9974913f49..8b96d60dca 100644
--- a/src/network/networkd-netdev.c
+++ b/src/network/networkd-netdev.c
@@ -495,11 +495,6 @@ static int netdev_load_one(Manager *manager, const char *filename) {
         assert(manager);
         assert(filename);
 
-        if (null_or_empty_path(filename)) {
-                log_debug("skipping empty file: %s", filename);
-                return 0;
-        }
-
         file = fopen(filename, "re");
         if (!file) {
                 if (errno == ENOENT)
@@ -508,6 +503,11 @@ static int netdev_load_one(Manager *manager, const char *filename) {
                         return -errno;
         }
 
+        if (null_or_empty_fd(fileno(file))) {
+                log_debug("Skipping empty file: %s", filename);
+                return 0;
+        }
+
         netdev = new0(NetDev, 1);
         if (!netdev)
                 return log_oom();
diff --git a/src/network/networkd-network.c b/src/network/networkd-network.c
index 700577fccf..3e46a1a80e 100644
--- a/src/network/networkd-network.c
+++ b/src/network/networkd-network.c
@@ -48,8 +48,8 @@ static int network_load_one(Manager *manager, const char *filename) {
                         return -errno;
         }
 
-        if (null_or_empty_path(filename)) {
-                log_debug("skipping empty file: %s", filename);
+        if (null_or_empty_fd(fileno(file))) {
+                log_debug("Skipping empty file: %s", filename);
                 return 0;
         }
 
diff --git a/src/shared/util.c b/src/shared/util.c
index 75dc58b63d..4fda31c838 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -3604,6 +3604,17 @@ int null_or_empty_path(const char *fn) {
         return null_or_empty(&st);
 }
 
+int null_or_empty_fd(int fd) {
+        struct stat st;
+
+        assert(fd >= 0);
+
+        if (fstat(fd, &st) < 0)
+                return -errno;
+
+        return null_or_empty(&st);
+}
+
 DIR *xopendirat(int fd, const char *name, int flags) {
         int nfd;
         DIR *d;
diff --git a/src/shared/util.h b/src/shared/util.h
index b3187a9ea1..d9d525e8a5 100644
--- a/src/shared/util.h
+++ b/src/shared/util.h
@@ -499,6 +499,7 @@ noreturn void freeze(void);
 
 bool null_or_empty(struct stat *st) _pure_;
 int null_or_empty_path(const char *fn);
+int null_or_empty_fd(int fd);
 
 DIR *xopendirat(int dirfd, const char *name, int flags);
 
diff --git a/src/udev/net/link-config.c b/src/udev/net/link-config.c
index 73243fa107..512885f9c8 100644
--- a/src/udev/net/link-config.c
+++ b/src/udev/net/link-config.c
@@ -153,11 +153,6 @@ static int load_link(link_config_ctx *ctx, const char *filename) {
         assert(ctx);
         assert(filename);
 
-        if (null_or_empty_path(filename)) {
-                log_debug("skipping empty file: %s", filename);
-                return 0;
-        }
-
         file = fopen(filename, "re");
         if (!file) {
                 if (errno == ENOENT)
@@ -166,6 +161,11 @@ static int load_link(link_config_ctx *ctx, const char *filename) {
                         return -errno;
         }
 
+        if (null_or_empty_fd(fileno(file))) {
+                log_debug("Skipping empty file: %s", filename);
+                return 0;
+        }
+
         link = new0(link_config, 1);
         if (!link)
                 return log_oom();
diff --git a/src/udev/udev-rules.c b/src/udev/udev-rules.c
index 85f78bcdef..9864016d14 100644
--- a/src/udev/udev-rules.c
+++ b/src/udev/udev-rules.c
@@ -1533,15 +1533,19 @@ static int parse_file(struct udev_rules *rules, const char *filename)
         int line_nr = 0;
         unsigned int i;
 
-        if (null_or_empty_path(filename)) {
-                log_debug("skip empty file: %s", filename);
-                return 0;
+        f = fopen(filename, "re");
+        if (!f) {
+                if (errno == ENOENT)
+                        return 0;
+                else
+                        return -errno;
         }
-        log_debug("read rules file: %s", filename);
 
-        f = fopen(filename, "re");
-        if (f == NULL)
-                return -1;
+        if (null_or_empty_fd(fileno(f))) {
+                log_debug("Skipping empty file: %s", filename);
+                return 0;
+        } else
+                log_debug("Reading rules file: %s", filename);
 
         first_token = rules->token_cur;
         filename_off = rules_add_string(rules, filename);