Check at startup that standard streams are open

Various code that manipulates file descriptors assumes that they _are_
open.  When changing this code, I don't want to consider the
consequences of them not being open.  Just check at startup that they
are open and exit if they are not.

Author: DemiMarie

daemon/qrexec-daemon.c

@@ -1541,12 +1541,10 @@ int main(int argc, char **argv)
         int null_fd = open("/dev/null", O_RDONLY|O_NOCTTY);
         if (null_fd < 0)
             err(1, "open(%s)", "/dev/null");
-        if (null_fd > 0) {
-            if (dup2(null_fd, 0) != 0)
-                err(1, "dup2(%d, 0)", null_fd);
-            if (null_fd > 2 && close(null_fd) != 0)
-                err(1, "close(%d)", null_fd);
-        }
+        if (dup2(null_fd, 0) != 0)
+            err(1, "dup2(%d, 0)", null_fd);
+        if (close(null_fd) != 0)
+            err(1, "close(%d)", null_fd);
     }
 
     setup_logging("qrexec-daemon");

libqrexec/log.c

@@ -26,6 +26,8 @@
 #include <unistd.h>
 #include <string.h>
 #include <errno.h>
+#include <fcntl.h>
+#include <err.h>
 
 #include "libqrexec-utils.h"
 
@@ -79,5 +81,12 @@ void qrexec_log(int level, int errnoval, const char *file, int line,
 }
 
 void setup_logging(const char *program_name) {
+    /* Make sure FD 0, 1 and 2 are open.  Various code that manipulates
+     * FDs breaks if they are not. */
+    for (int i = 0; i < 3; ++i) {
+        if (fcntl(i, F_GETFD) == -1 && errno == EBADF)
+            errx(125, "File descriptor %d is closed, cannot continue", i);
+    }
+
     qrexec_program_name = program_name;
 }