xz: Add SIGTSTP handler for progress indicator time keeping.

This way, if xz is stopped the elapsed time and estimated time
remaining won't get confused by the amount of time spent in
the stopped state.

This raises SIGSTOP. It's not clear to me if this is the correct way.
POSIX and glibc docs say that SIGTSTP shouldn't stop the process if
it is orphaned but this commit doesn't attempt to handle that.

Search for SIGTSTP in section 2.4.3:

https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html

src/xz/mytime.c

@@ -20,7 +20,12 @@
 
 uint64_t opt_flush_timeout = 0;
 
+#ifdef USE_SIGTSTP_HANDLER
+static volatile uint64_t start_time;
+#else
 static uint64_t start_time;
+#endif
+
 static uint64_t next_flush;
 
 
@@ -48,10 +53,49 @@ mytime_now(void)
 }
 
 
+#ifdef USE_SIGTSTP_HANDLER
+extern void
+mytime_sigtstp_handler(int sig lzma_attribute((__unused__)))
+{
+	// Measure how long the process stays in the stopped state and add
+	// that amount to start_time. This way the the progress indicator
+	// won't count the stopped time as elapsed time and the estimated
+	// remaining time won't be confused by the time spent in the
+	// stopped state.
+	//
+	// FIXME? Is raising SIGSTOP the correct thing to do? POSIX.1-2017
+	// says that orphan processes shouldn't stop on SIGTSTP. So perhaps
+	// the most correct thing to do could be to revert to the default
+	// handler for SIGTSTP, unblock SIGTSTP, and then raise(SIGTSTP).
+	// It's quite a bit more complicated than just raising SIGSTOP though.
+	//
+	// The difference between raising SIGTSTP vs. SIGSTOP can be seen on
+	// the shell command line too by running "echo $?" after stopping
+	// a process but perhaps that doesn't matter.
+	const uint64_t t = mytime_now();
+	raise(SIGSTOP);
+	start_time += mytime_now() - t;
+	return;
+}
+#endif
+
+
 extern void
 mytime_set_start_time(void)
 {
+#ifdef USE_SIGTSTP_HANDLER
+	// Block the signals when accessing start_time so that we cannot
+	// end up with a garbage value. start_time is volatile but access
+	// to it isn't atomic at least on 32-bit systems.
+	signals_block();
+#endif
+
 	start_time = mytime_now();
+
+#ifdef USE_SIGTSTP_HANDLER
+	signals_unblock();
+#endif
+
 	return;
 }
 
@@ -59,7 +103,17 @@ mytime_set_start_time(void)
 extern uint64_t
 mytime_get_elapsed(void)
 {
-	return mytime_now() - start_time;
+#ifdef USE_SIGTSTP_HANDLER
+	signals_block();
+#endif
+
+	const uint64_t t = mytime_now() - start_time;
+
+#ifdef USE_SIGTSTP_HANDLER
+	signals_unblock();
+#endif
+
+	return t;
 }
 
 

src/xz/mytime.h

@@ -21,6 +21,12 @@
 extern uint64_t opt_flush_timeout;
 
 
+#ifdef USE_SIGTSTP_HANDLER
+/// \brief      Signal handler for SIGTSTP
+extern void mytime_sigtstp_handler(int sig);
+#endif
+
+
 /// \brief      Store the time when (de)compression was started
 ///
 /// The start time is also stored as the time of the first flush.

src/xz/private.h

@@ -49,6 +49,18 @@
 #	define ENABLE_SANDBOX 1
 #endif
 
+// Handling SIGTSTP keeps time-keeping for progress indicator correct
+// if xz is stopped. It requires use of clock_gettime() as that is
+// async-signal safe in POSIX. Require also SIGALRM support since
+// on systems where SIGALRM isn't available, progress indicator code
+// polls the time and the SIGTSTP handling adds slight overhead to
+// that code. Most (all?) systems that have SIGTSTP also have SIGALRM
+// so this requirement won't exclude many systems.
+#if defined(HAVE_CLOCK_GETTIME) && defined(HAVE_CLOCK_MONOTONIC) \
+		&& defined(SIGTSTP) && defined(SIGALRM)
+#	define USE_SIGTSTP_HANDLER 1
+#endif
+
 #include "main.h"
 #include "mytime.h"
 #include "coder.h"

src/xz/signals.c

@@ -82,6 +82,11 @@ signals_init(void)
 		sigaddset(&hooked_signals, message_progress_sigs[i]);
 #endif
 
+#ifdef USE_SIGTSTP_HANDLER
+	// Add the SIGTSTP handler from mytime.c to hooked_signals.
+	sigaddset(&hooked_signals, SIGTSTP);
+#endif
+
 	// Using "my_sa" because "sa" may conflict with a sockaddr variable
 	// from system headers on Solaris.
 	struct sigaction my_sa;
@@ -96,10 +101,11 @@ signals_init(void)
 	my_sa.sa_flags = 0;
 	my_sa.sa_handler = &signal_handler;
 
+	struct sigaction old;
+
 	for (size_t i = 0; i < ARRAY_SIZE(sigs); ++i) {
 		// If the parent process has left some signals ignored,
 		// we don't unignore them.
-		struct sigaction old;
 		if (sigaction(sigs[i], NULL, &old) == 0
 				&& old.sa_handler == SIG_IGN)
 			continue;
@@ -109,6 +115,15 @@ signals_init(void)
 			message_signal_handler();
 	}
 
+#ifdef USE_SIGTSTP_HANDLER
+	if (!(sigaction(SIGTSTP, NULL, &old) == 0
+				&& old.sa_handler == SIG_IGN)) {
+		my_sa.sa_handler = &mytime_sigtstp_handler;
+		if (sigaction(SIGTSTP, &my_sa, NULL))
+			message_signal_handler();
+	}
+#endif
+
 	signals_are_initialized = true;
 
 	return;