[PATCH 5/5] xfsprogs: libxcmd: use fs_device_number() consistently

[Alex Elder <aelder@xxxxxxx>]

The libxcmd code builds up a table that records information about
all filesystems that might be subject to quotas, as well as a set
of directories that are the roots of project quota trees.

When building the table, the device number for each affected
filesystem is determined (in fs_device_number()) using a call to
stat64().  It turns out that in all cases when doing this, a
directory path (and *not* a device special file path) is specified,
in which case the appropriate filesystem device id is found in the
st_dev field produce by the call to stat64() (i.e., the device id
for the mounted filesystem containing the path).  Accordingly,
fs_device_number() always returns the st_dev field.

Another routine, fs_table_lookup(), looks up an entry in this table
based on the path name provided.  However this function allows a
path to a device special file be provided.  In that case the right
device id to use is found in the st_rdev field returned by stat64().

I found this to be confusing, and it took a while to convince
myself that this wasn't actually bug.  (It wasn't initially clear
that device special files were never passed to fs_device_number().)

In order to prevent myself and others from ever wasting time like
this again, use fs_device_number() every time a device number is
needed, and in doing so determine it consistently in all cases (that
is--use st_rdev for device special files and st_dev otherwise).

In the process, change fs_device_number() to return an zero on
success (or an errno) rather than its first argument (or NULL).

Signed-off-by: Alex Elder <aelder@xxxxxxx>
---
 libxcmd/paths.c |   46 +++++++++++++++++++++-------------------------
 1 files changed, 21 insertions(+), 25 deletions(-)

diff --git a/libxcmd/paths.c b/libxcmd/paths.c
index aa0aeb6..f4110d4 100644
--- a/libxcmd/paths.c
+++ b/libxcmd/paths.c
@@ -37,18 +37,26 @@ struct fs_path *fs_path;
 char *mtab_file;
 #define PROC_MOUNTS    "/proc/self/mounts"
 
-static char *
+static int
 fs_device_number(
-       char            *name,
+       const char      *name,
        dev_t           *devnum)
 {
        struct stat64   sbuf;
 
        if (stat64(name, &sbuf) < 0)
-               return NULL;
-       *devnum = sbuf.st_dev;
+               return errno;
+       /*
+        * We want to match st_rdev if the path provided is a device
+        * special file.  Otherwise we are looking for the the
+        * device id for the containing filesystem, in st_dev.
+        */
+       if (S_ISBLK(sbuf.st_mode) || S_ISCHR(sbuf.st_mode))
+               *devnum = sbuf.st_rdev;
+       else
+               *devnum = sbuf.st_dev;
 
-       return name;
+       return 0;
 }
 
 /*
@@ -61,23 +69,12 @@ fs_table_lookup(
        const char      *dir,
        uint            flags)
 {
-       struct stat64   sbuf;
        uint            i;
-       dev_t           dev;
+       dev_t           dev = 0;
 
-       if (stat64(dir, &sbuf) < 0)
+       if (fs_device_number(dir, &dev))
                return NULL;
 
-       /*
-        * We want to match st_rdev if the directory provided is a
-        * device special file.  Otherwise we are looking for the
-        * the device id for the containing filesystem, in st_dev.
-        */
-       if (S_ISBLK(sbuf.st_mode) || S_ISCHR(sbuf.st_mode))
-               dev = sbuf.st_rdev;
-       else
-               dev = sbuf.st_dev;
-
        for (i = 0; i < fs_count; i++) {
                if ((flags & fs_table[i].fs_flags) == 0)
                        continue;
@@ -103,11 +100,11 @@ fs_table_insert(
                return EINVAL;
 
        datadev = logdev = rtdev = 0;
-       if (!fs_device_number(dir, &datadev))
+       if (fs_device_number(dir, &datadev))
                return errno;
-       if (fslog && !fs_device_number(fslog, &logdev))
+       if (fslog && fs_device_number(fslog, &logdev))
                return errno;
-       if (fsrt && !fs_device_number(fsrt, &rtdev))
+       if (fsrt && fs_device_number(fsrt, &rtdev))
                return errno;
 
        tmp_fs_table = realloc(fs_table, sizeof(fs_path_t) * (fs_count + 1));
@@ -293,15 +290,14 @@ fs_mount_point_from_path(
 {
        fs_cursor_t     cursor;
        fs_path_t       *fs;
-       struct stat64   s;
+       dev_t           dev = 0;
 
-       if (stat64(dir, &s) < 0) {
+       if (fs_device_number(dir, &dev))
                return NULL;
-       }
 
        fs_cursor_initialise(NULL, FS_MOUNT_POINT, &cursor);
        while ((fs = fs_cursor_next_entry(&cursor))) {
-               if (fs->fs_datadev == s.st_dev)
+               if (fs->fs_datadev == dev)
                        break;
        }
        return fs;
-- 
1.7.6.2