Allow PID file to be disabled at compile-time

Stubby unconditionally attempts to write a PID file under the run state
directory when it starts, but this directory is not always writeable by
non-root or non-system account users (e.g. on Slackware). To aid with
running Stubby in environments where the PID file would otherwise be
unwriteable or unnecessary, allow it to be disabled at compile-time with
the `ENABLE_PID_FILE` option (which remains on by default).

CMakeLists.txt

@@ -47,6 +47,7 @@ find_package(Libsystemd)
 if (Libsystemd_FOUND)
   option(ENABLE_SYSTEMD "Enable systemd support." ON)
 endif()
+option(ENABLE_PID_FILE "Enable PID file support." ON)
 option(ENABLE_GETDNS_STATIC_LINK "Link GetDNS statically." ON)
 if (ENABLE_GETDNS_STATIC_LINK)
   set(GETDNS_STATIC ON)

cmake/include/cmakeconfig.h.in

@@ -24,6 +24,7 @@
 
 #cmakedefine SERVER_DEBUG	1
 
+#cmakedefine ENABLE_PID_FILE    1
 #cmakedefine ENABLE_SYSTEMD     1
 
 #cmakedefine STUBBYCONFDIR      "@STUBBYCONFDIR@"

src/stubby.c

@@ -50,7 +50,7 @@
 
 #if defined(ENABLE_WINDOWS_SERVICE)
 #include "windowsservice.h"
-#else
+#elif defined(ENABLE_PID_FILE)
 #define STUBBYPIDFILE RUNSTATEDIR"/stubby.pid"
 #endif
 
@@ -204,6 +204,7 @@ main(int argc, char **argv)
 #if !defined(STUBBY_ON_WINDOWS)
 	if (!run_in_foreground) {
 		pid_t pid;
+#if defined(ENABLE_PID_FILE)
 		char pid_str[1024], *endptr;
 		FILE *fh = fopen(STUBBYPIDFILE, "r");
 		do {
@@ -225,13 +226,15 @@ main(int argc, char **argv)
 		} while(0);
 		if (fh)
 			(void) fclose(fh);
+#endif
 
 		pid = fork();
 		if (pid == -1) {
 			perror("Could not fork of stubby daemon\n");
 			r = GETDNS_RETURN_GENERIC_ERROR;
 
 		} else if (pid) {
+#if defined(ENABLE_PID_FILE)
 			fh = fopen(STUBBYPIDFILE, "w");
 			if (fh) {
 				fprintf(fh, "%d", (int)pid);
@@ -242,6 +245,7 @@ main(int argc, char **argv)
 					     strerror(errno));
 				exit(EXIT_FAILURE);
 			}
+#endif
 		} else {
 #ifdef SIGPIPE
 			(void)signal(SIGPIPE, SIG_IGN);