lseek() on dir handle does not work as expected (#1224)

* Document lseek on dir will not work as expected

README.md

@@ -179,6 +179,7 @@ If your use-case involves updating/uploading file(s) through other means and you
 - chown  : Change of ownership is not supported by Azure Storage hence Blobfuse2 does not support this.
 - Creation of device files or pipes is not supported by Blobfuse2.
 - Blobfuse2 does not support extended-attributes (x-attrs) operations
+- Blobfuse2 does not support lseek() operation on directory handles. No error is thrown but it will not work as expected.
 
 ## Un-Supported Scenarios
 - Blobfuse2 does not support overlapping mount paths. While running multiple instances of Blobfuse2 make sure each instance has a unique and non-overlapping mount point.

test/test_utils/dir_list_seek.c

@@ -0,0 +1,66 @@
+#include <dirent.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+
+#define handle_error(msg) \
+	do { perror(msg); exit(EXIT_FAILURE); } while (0)
+
+// To compile this file use "gcc dir_list_seek.c -o dir_list_seek"
+// To run this util use "./dir_list_seek <path to mounted directory"
+
+struct linux_dirent {
+	long           d_ino;
+	off_t          d_off;
+	unsigned short d_reclen;
+	char           d_name[];
+};
+
+#define BUF_SIZE 1024
+
+int main(int argc, char **argv)
+{
+	int fd, nread;
+	char buf[BUF_SIZE];
+	struct linux_dirent *d;
+	int bpos;
+	char d_type;
+
+	fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
+	if(fd == -1)
+		handle_error("open");
+
+	for(int i = 0; i < 10; i++) {
+        lseek(fd, 0, SEEK_SET);
+		nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
+		if(nread == -1)
+			handle_error("getdents");
+
+		if(nread == 0)
+			break;
+
+		printf("--------------- nread=%d ---------------\n", nread);
+		printf("i-node#  file type  d_reclen  d_off   d_name\n");
+
+		for(bpos=0; bpos<nread; ) {
+			d = (struct linux_dirent *)(buf+bpos);
+			printf("%8ld  ", d->d_ino);
+			d_type = *(buf + bpos + d->d_reclen - 1);
+			printf("%-10s ", (d_type == DT_REG) ?  "regular" :
+                             (d_type == DT_DIR) ?  "directory" :
+							 (d_type == DT_FIFO) ? "FIFO" :
+							 (d_type == DT_SOCK) ? "socket" :
+							 (d_type == DT_LNK) ?  "symlink" :
+							 (d_type == DT_BLK) ?  "block dev" :
+							 (d_type == DT_CHR) ?  "char dev" : "???");
+            printf("%4d %10lld  %s\n", d->d_reclen,
+						(long long) d->d_off, d->d_name);
+            bpos += d->d_reclen;
+		}
+	}
+
+	return 0;
+}