time-util: rework clock conversion logic

Let's split this out into its own helper function we can reuse at
various places.

Also, let's avoid signed values where we can so that we can cover more
of the available time range.

src/basic/time-util.c

@@ -82,43 +82,82 @@ triple_timestamp* triple_timestamp_get(triple_timestamp *ts) {
         return ts;
 }
 
+static usec_t map_clock_usec_internal(usec_t from, usec_t from_base, usec_t to_base) {
+
+        /* Maps the time 'from' between two clocks, based on a common reference point where the first clock
+         * is at 'from_base' and the second clock at 'to_base'. Basically calculates:
+         *
+         *         from - from_base + to_base
+         *
+         * But takes care of overflows/underflows and avoids signed operations. */
+
+        if (from >= from_base) { /* In the future */
+                usec_t delta = from - from_base;
+
+                if (to_base >= USEC_INFINITY - delta) /* overflow? */
+                        return USEC_INFINITY;
+
+                return to_base + delta;
+
+        } else { /* In the past */
+                usec_t delta = from_base - from;
+
+                if (to_base <= delta) /* underflow? */
+                        return 0;
+
+                return to_base - delta;
+        }
+}
+
+usec_t map_clock_usec(usec_t from, clockid_t from_clock, clockid_t to_clock) {
+
+        /* Try to avoid any inaccuracy needlessly added in case we convert from effectively the same clock
+         * onto itself */
+        if (map_clock_id(from_clock) == map_clock_id(to_clock))
+                return from;
+
+        /* Keep infinity as is */
+        if (from == USEC_INFINITY)
+                return from;
+
+        return map_clock_usec_internal(from, now(from_clock), now(to_clock));
+}
+
 dual_timestamp* dual_timestamp_from_realtime(dual_timestamp *ts, usec_t u) {
-        int64_t delta;
         assert(ts);
 
-        if (u == USEC_INFINITY || u <= 0) {
+        if (u == USEC_INFINITY || u == 0) {
                 ts->realtime = ts->monotonic = u;
                 return ts;
         }
 
         ts->realtime = u;
-
-        delta = (int64_t) now(CLOCK_REALTIME) - (int64_t) u;
-        ts->monotonic = usec_sub_signed(now(CLOCK_MONOTONIC), delta);
-
+        ts->monotonic = map_clock_usec(u, CLOCK_REALTIME, CLOCK_MONOTONIC);
         return ts;
 }
 
 triple_timestamp* triple_timestamp_from_realtime(triple_timestamp *ts, usec_t u) {
-        int64_t delta;
+        usec_t nowr;
 
         assert(ts);
 
-        if (u == USEC_INFINITY || u <= 0) {
+        if (u == USEC_INFINITY || u == 0) {
                 ts->realtime = ts->monotonic = ts->boottime = u;
                 return ts;
         }
 
+        nowr = now(CLOCK_REALTIME);
+
         ts->realtime = u;
-        delta = (int64_t) now(CLOCK_REALTIME) - (int64_t) u;
-        ts->monotonic = usec_sub_signed(now(CLOCK_MONOTONIC), delta);
-        ts->boottime = clock_boottime_supported() ? usec_sub_signed(now(CLOCK_BOOTTIME), delta) : USEC_INFINITY;
+        ts->monotonic = map_clock_usec_internal(u, nowr, now(CLOCK_MONOTONIC));
+        ts->boottime = clock_boottime_supported() ?
+                map_clock_usec_internal(u, nowr, now(CLOCK_BOOTTIME)) :
+                USEC_INFINITY;
 
         return ts;
 }
 
 dual_timestamp* dual_timestamp_from_monotonic(dual_timestamp *ts, usec_t u) {
-        int64_t delta;
         assert(ts);
 
         if (u == USEC_INFINITY) {
@@ -127,25 +166,28 @@ dual_timestamp* dual_timestamp_from_monotonic(dual_timestamp *ts, usec_t u) {
         }
 
         ts->monotonic = u;
-        delta = (int64_t) now(CLOCK_MONOTONIC) - (int64_t) u;
-        ts->realtime = usec_sub_signed(now(CLOCK_REALTIME), delta);
-
+        ts->realtime = map_clock_usec(u, CLOCK_MONOTONIC, CLOCK_REALTIME);
         return ts;
 }
 
 dual_timestamp* dual_timestamp_from_boottime_or_monotonic(dual_timestamp *ts, usec_t u) {
-        int64_t delta;
+        clockid_t cid;
+        usec_t nowm;
 
         if (u == USEC_INFINITY) {
                 ts->realtime = ts->monotonic = USEC_INFINITY;
                 return ts;
         }
 
-        dual_timestamp_get(ts);
-        delta = (int64_t) now(clock_boottime_or_monotonic()) - (int64_t) u;
-        ts->realtime = usec_sub_signed(ts->realtime, delta);
-        ts->monotonic = usec_sub_signed(ts->monotonic, delta);
+        cid = clock_boottime_or_monotonic();
+        nowm = now(cid);
 
+        if (cid == CLOCK_MONOTONIC)
+                ts->monotonic = u;
+        else
+                ts->monotonic = map_clock_usec_internal(u, nowm, now(CLOCK_MONOTONIC));
+
+        ts->realtime = map_clock_usec_internal(u, nowm, now(CLOCK_REALTIME));
         return ts;
 }
 

src/basic/time-util.h

@@ -29,8 +29,8 @@ typedef struct triple_timestamp {
         usec_t boottime;
 } triple_timestamp;
 
-#define USEC_INFINITY ((usec_t) -1)
-#define NSEC_INFINITY ((nsec_t) -1)
+#define USEC_INFINITY ((usec_t) UINT64_MAX)
+#define NSEC_INFINITY ((nsec_t) UINT64_MAX)
 
 #define MSEC_PER_SEC  1000ULL
 #define USEC_PER_SEC  ((usec_t) 1000000ULL)
@@ -67,6 +67,8 @@ typedef struct triple_timestamp {
 usec_t now(clockid_t clock);
 nsec_t now_nsec(clockid_t clock);
 
+usec_t map_clock_usec(usec_t from, clockid_t from_clock, clockid_t to_clock);
+
 dual_timestamp* dual_timestamp_get(dual_timestamp *ts);
 dual_timestamp* dual_timestamp_from_realtime(dual_timestamp *ts, usec_t u);
 dual_timestamp* dual_timestamp_from_monotonic(dual_timestamp *ts, usec_t u);