chroot before systemd switches root

chroot to the directory where systemd switches root from the initramfs.
Otherwise the svg file cannot be saved.

Author: alfonsosanchezbeato

src/bootchart.c

@@ -333,6 +333,7 @@ int main(int argc, char *argv[]) {
                 .sa_handler = signal_handler,
         };
         bool has_procfs = false;
+        bool has_sysroot = false;
 
         parse_conf();
 
@@ -433,6 +434,25 @@ int main(int argc, char *argv[]) {
                         if (r < 0)
                                 return EXIT_FAILURE;
                 }
+                if (!has_sysroot) {
+                        // sysroot is where systemd will switch root when
+                        // switching from initramfs to the root fs. We chroot
+                        // there so we can write later the svg file.
+                        //
+                        // TODO instead of detecting this way, use signal by initramfs signal.
+                        has_sysroot = access("/sysroot", F_OK) == 0;
+                        if (has_sysroot) {
+                                // chroot does not change the cwd of the process, so we do that first
+                                if (chdir("/sysroot") != 0) {
+                                        log_info_errno(errno, "cannot chdir to /sysroot: %m\n");
+                                        return EXIT_FAILURE;
+                                }
+                                if (chroot(".") != 0)  {
+                                        log_info_errno(errno, "cannot chroot: %m\n");
+                                        return EXIT_FAILURE;
+                                }
+                        }
+                }
 
                 sample_stop = gettime_ns();