Use fstatat(2) for gufi_dir2index and gufi_dir2trace

Pathname resolution has a significant overhead during
GUFI indexing. Using the _at variant of stat(2) when processing
files means less work is required to resolve the paths.

Profiling GUFI runs with flamegraphs suggests that this is an
effective change to reduce the overhead of pathname resolution.
I observed the time spent resolving names go from ~ 14% using
lstat(2) down to ~ 6% using fstatat(2), in one example run on an
NFS filestystem with approx. 450,000 files.

include/bf.h

@@ -301,6 +301,7 @@ struct work {
 
 /* extra data used by entries that does not depend on data from other directories */
 struct entry_data {
+   int           parent_fd;    /* holds an FD that can be used for fstatat(2), etc. */
    char          type;
    char          linkname[MAXPATH];
    uint8_t       lstat_called;

src/gufi_dir2index.c

@@ -64,6 +64,7 @@ OF SUCH DAMAGE.
 
 #include <errno.h>
 #include <dirent.h>
+#include <fcntl.h>
 #include <inttypes.h>
 #include <stdlib.h>
 #include <string.h>
@@ -129,7 +130,9 @@ static int process_nondir(struct work *entry, struct entry_data *ed, void *args)
     struct input *in = nda->in;
 
     if (!ed->lstat_called) {
-        if (lstat(entry->name, &ed->statuso) != 0) {
+        char *basename = entry->name + entry->name_len - entry->basename_len;
+
+        if (fstatat(ed->parent_fd, basename, &ed->statuso, AT_SYMLINK_NOFOLLOW) != 0) {
             return 1;
         }
     }

src/gufi_dir2trace.c

@@ -106,8 +106,10 @@ static int process_external(struct input *in, void *args,
 static int process_nondir(struct work *entry, struct entry_data *ed, void *args) {
     struct NondirArgs *nda = (struct NondirArgs *) args;
     if (!ed->lstat_called) {
-        if (lstat(entry->name, &ed->statuso) != 0) {
-            return 0;
+        char *basename = entry->name + entry->name_len - entry->basename_len;
+
+        if (fstatat(ed->parent_fd, basename, &ed->statuso, AT_SYMLINK_NOFOLLOW) != 0) {
+            return 1;
         }
 
         if (ed->type == 'l') {

src/utils.c

@@ -412,6 +412,17 @@ int descend(QPTPool_t *ctx, const size_t id, void *args,
         const size_t next_level = work->level + 1;
         const size_t recursion_level = work->recursion_level + 1;
 
+        int d_fd = dirfd(dir);
+        if (d_fd < 0) {
+            /*
+             * We should never get here. glibc's dirfd(3) never return errors, and
+             * Apple's libc only returns an error if the DIR * is invalid, which would
+             * indicate a bug in GUFI.
+             */
+            fprintf(stderr, "BUG: dirfd(3) failed: errno = %d\n", errno);
+            return 1;
+        }
+
         struct dirent *dir_child = NULL;
         while ((dir_child = readdir(dir))) {
             const size_t len = strlen(dir_child->d_name);
@@ -428,6 +439,7 @@ int descend(QPTPool_t *ctx, const size_t id, void *args,
 
             struct entry_data child_ed;
             memset(&child_ed, 0, sizeof(child_ed));
+            child_ed.parent_fd = -1;
 
             /* get child path */
             child.name_len = SNFORMAT_S(child.name, MAXPATH, 3,
@@ -520,6 +532,7 @@ int descend(QPTPool_t *ctx, const size_t id, void *args,
                     xattrs_get(child.name, &child_ed.xattrs);
                 }
 
+                child_ed.parent_fd = d_fd;
                 processnondir(&child, &child_ed, nondir_args);
                 ctrs.nondirs_processed++;