fileio: add 'dir_fd' parameter to read_full_file_full()

Let's introduce an "at" version of read_full_file().

src/basic/fileio.c

@@ -543,17 +543,19 @@ finalize:
         return r;
 }
 
-int read_full_file_full(const char *filename, ReadFullFileFlags flags, char **contents, size_t *size) {
+int read_full_file_full(int dir_fd, const char *filename, ReadFullFileFlags flags, char **contents, size_t *size) {
         _cleanup_fclose_ FILE *f = NULL;
         int r;
 
         assert(filename);
         assert(contents);
 
-        r = fopen_unlocked(filename, "re", &f);
+        r = xfopenat(dir_fd, filename, "re", 0, &f);
         if (r < 0)
                 return r;
 
+        (void) __fsetlocking(f, FSETLOCKING_BYCALLER);
+
         return read_full_stream_full(f, filename, flags, contents, size);
 }
 

src/basic/fileio.h

@@ -6,6 +6,7 @@
 #include <stddef.h>
 #include <stdio.h>
 #include <sys/stat.h>
+#include <sys/fcntl.h>
 #include <sys/types.h>
 
 #include "macro.h"
@@ -52,9 +53,9 @@ static inline int write_string_file(const char *fn, const char *line, WriteStrin
 int write_string_filef(const char *fn, WriteStringFileFlags flags, const char *format, ...) _printf_(3, 4);
 
 int read_one_line_file(const char *filename, char **line);
-int read_full_file_full(const char *filename, ReadFullFileFlags flags, char **contents, size_t *size);
+int read_full_file_full(int dir_fd, const char *filename, ReadFullFileFlags flags, char **contents, size_t *size);
 static inline int read_full_file(const char *filename, char **contents, size_t *size) {
-        return read_full_file_full(filename, 0, contents, size);
+        return read_full_file_full(AT_FDCWD, filename, 0, contents, size);
 }
 int read_full_virtual_file(const char *filename, char **ret_contents, size_t *ret_size);
 int read_full_stream_full(FILE *f, const char *filename, ReadFullFileFlags flags, char **contents, size_t *size);