Revise mythread.h.

This adds:

  - mythread_sync() macro to create synchronized blocks

  - mythread_cond structure and related functions
    and macros for condition variables with timed
    waiting using a relative timeout

  - mythread_create() to create a thread with all
    signals blocked

Some of these wouldn't need to be inline functions,
but I'll keep them this way for now for simplicity.

For timed waiting on a condition variable, librt is
now required on some systems to use clock_gettime().
configure.ac was updated to handle this.

configure.ac

@@ -435,6 +435,7 @@ if test "x$enable_threads" = xyes; then
 	LIBS="$LIBS $PTHREAD_LIBS"
 	AM_CFLAGS="$AM_CFLAGS $PTHREAD_CFLAGS"
 	CC="$PTHREAD_CC"
+	AC_SEARCH_LIBS([clock_gettime], [rt])
 fi
 
 echo

src/common/mythread.h

@@ -1,7 +1,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 //
 /// \file       mythread.h
-/// \brief      Wrappers for threads
+/// \brief      Some threading related helper macros and functions
 //
 //  Author:     Lasse Collin
 //
@@ -14,19 +14,182 @@
 
 
 #ifdef HAVE_PTHREAD
-#	include <pthread.h>
 
+////////////////////
+// Using pthreads //
+////////////////////
+
+#include <pthread.h>
+#include <signal.h>
+#include <time.h>
+#include <unistd.h>
+
+
+/// \brief      Set the process signal mask
+///
+/// If threads are disabled, sigprocmask() is used instead
+/// of pthread_sigmask().
+#define mythread_sigmask(how, set, oset) \
+	pthread_sigmask(how, set, oset)
+
+
+/// \brief      Call the given function once
+///
+/// If threads are disabled, a thread-unsafe version is used.
 #define mythread_once(func) \
 do { \
 	static pthread_once_t once_ = PTHREAD_ONCE_INIT; \
 	pthread_once(&once_, &func); \
 } while (0)
 
-#	define mythread_sigmask(how, set, oset) \
-		pthread_sigmask(how, set, oset)
+
+/// \brief      Lock a mutex for a duration of a block
+///
+/// Perform pthread_mutex_lock(&mutex) in the beginning of a block
+/// and pthread_mutex_unlock(&mutex) at the end of the block. "break"
+/// may be used to unlock the mutex and jump out of the block.
+/// mythread_sync blocks may be nested.
+///
+/// Example:
+///
+///     mythread_sync(mutex) {
+///         foo();
+///         if (some_error)
+///             break; // Skips bar()
+///         bar();
+///     }
+///
+/// At least GCC optimizes the loops completely away so it doesn't slow
+/// things down at all compared to plain pthread_mutex_lock(&mutex)
+/// and pthread_mutex_unlock(&mutex) calls.
+///
+#define mythread_sync(mutex) mythread_sync_helper(mutex, __LINE__)
+#define mythread_sync_helper(mutex, line) \
+	for (unsigned int mythread_i_ ## line = 0; \
+			mythread_i_ ## line \
+				? (pthread_mutex_unlock(&(mutex)), 0) \
+				: (pthread_mutex_lock(&(mutex)), 1); \
+			mythread_i_ ## line = 1) \
+		for (unsigned int mythread_j_ ## line = 0; \
+				!mythread_j_ ## line; \
+				mythread_j_ ## line = 1)
+
+
+typedef struct {
+	/// Condition variable
+	pthread_cond_t cond;
+
+	/// Clock ID (CLOCK_REALTIME or CLOCK_MONOTONIC) associated with
+	/// the condition variable
+	clockid_t clk_id;
+
+} mythread_cond;
+
+
+/// \brief      Initialize a condition variable to use CLOCK_MONOTONIC
+///
+/// Using CLOCK_MONOTONIC instead of the default CLOCK_REALTIME makes the
+/// timeout in pthread_cond_timedwait() work correctly also if system time
+/// is suddenly changed. Unfortunately CLOCK_MONOTONIC isn't available
+/// everywhere while the default CLOCK_REALTIME is, so the default is
+/// used if CLOCK_MONOTONIC isn't available.
+static inline int
+mythread_cond_init(mythread_cond *mycond)
+{
+#if defined(_POSIX_CLOCK_SELECTION) && defined(_POSIX_MONOTONIC_CLOCK)
+	struct timespec ts;
+	pthread_condattr_t condattr;
+
+	// POSIX doesn't seem to *require* that pthread_condattr_setclock()
+	// will fail if given an unsupported clock ID. Test that
+	// CLOCK_MONOTONIC really is supported using clock_gettime().
+	if (clock_gettime(CLOCK_MONOTONIC, &ts) == 0
+			&& pthread_condattr_init(&condattr) == 0) {
+		int ret = pthread_condattr_setclock(
+				&condattr, CLOCK_MONOTONIC);
+		if (ret == 0)
+			ret = pthread_cond_init(&mycond->cond, &condattr);
+
+		pthread_condattr_destroy(&condattr);
+
+		if (ret == 0) {
+			mycond->clk_id = CLOCK_MONOTONIC;
+			return 0;
+		}
+	}
+
+	// If anything above fails, fall back to the default CLOCK_REALTIME.
+#endif
+
+	mycond->clk_id = CLOCK_REALTIME;
+	return pthread_cond_init(&mycond->cond, NULL);
+}
+
+
+/// \brief      Convert relative time to absolute time for use with timed wait
+///
+/// The current time of the clock associated with the condition variable
+/// is added to the relative time in *ts.
+static inline void
+mythread_cond_abstime(const mythread_cond *mycond, struct timespec *ts)
+{
+	struct timespec now;
+	clock_gettime(mycond->clk_id, &now);
+
+	ts->tv_sec += now.tv_sec;
+	ts->tv_nsec += now.tv_nsec;
+
+	// tv_nsec must stay in the range [0, 999_999_999].
+	if (ts->tv_nsec >= 1000000000L) {
+		ts->tv_nsec -= 1000000000L;
+		++ts->tv_sec;
+	}
+
+	return;
+}
+
+
+#define mythread_cond_wait(mycondptr, mutexptr) \
+	pthread_cond_wait(&(mycondptr)->cond, mutexptr)
+
+#define mythread_cond_timedwait(mycondptr, mutexptr, abstimeptr) \
+	pthread_cond_timedwait(&(mycondptr)->cond, mutexptr, abstimeptr)
+
+#define mythread_cond_signal(mycondptr) \
+	pthread_cond_signal(&(mycondptr)->cond)
+
+#define mythread_cond_broadcast(mycondptr) \
+	pthread_cond_broadcast(&(mycondptr)->cond)
+
+#define mythread_cond_destroy(mycondptr) \
+	pthread_cond_destroy(&(mycondptr)->cond)
+
+
+/// \brief      Create a thread with all signals blocked
+static inline int
+mythread_create(pthread_t *thread, void *(*func)(void *arg), void *arg)
+{
+	sigset_t old;
+	sigset_t all;
+	sigfillset(&all);
+
+	pthread_sigmask(SIG_SETMASK, &all, &old);
+	const int ret = pthread_create(thread, NULL, func, arg);
+	pthread_sigmask(SIG_SETMASK, &old, NULL);
+
+	return ret;
+}
 
 #else
 
+//////////////////
+// No threading //
+//////////////////
+
+#define mythread_sigmask(how, set, oset) \
+	sigprocmask(how, set, oset)
+
+
 #define mythread_once(func) \
 do { \
 	static bool once_ = false; \
@@ -36,7 +199,4 @@
 	} \
 } while (0)
 
-#	define mythread_sigmask(how, set, oset) \
-		sigprocmask(how, set, oset)
-
 #endif