[input] try to use CLOCK_MONOTONIC for condvar

By default, Pthread condition variables use the wretched
CLOCK_REALTIME for pthread_cond_timedwait(). We reaaaaaaly
want it to use CLOCK_MONOTONIC instead. Where supported,
use pthread_condvar_setclock() to initialize the input
condvar with CLOCK_MONOTONIC, since every other operation
we perform is clocked against it. Closes #2291.

src/lib/in.c

@@ -1385,6 +1385,24 @@ getpipes(ipipe pipes[static 2]){
   return 0;
 }
 
+// attempt to set up a pthread_condattr_t such that pthread_cond_timedwait()
+// uses CLOCK_MONOTONIC rather than CLOCK_REALTIME. unfortunately, this isn't
+// available on all operating systems. return success so long as we can
+// initialize the condattr_t; it does not imply successful clock selection =\.
+static int
+prep_condattr(pthread_condattr_t* cattr){
+  int e;
+  if( (e = pthread_condattr_init(cattr)) ){
+    logerror("couldn't initialize condattr (%s)\n", strerror(e));
+    return -1;
+  }
+  // remember, return success even if this doesn't fly
+  if( (e = pthread_condattr_setclock(cattr, CLOCK_MONOTONIC)) ){
+    logwarn("warning: couldn't set CLOCK_MONOTONIC for condvar (%s)\n", strerror(e));
+  }
+  return 0;
+}
+
 static inline inputctx*
 create_inputctx(tinfo* ti, FILE* infp, int lmargin, int tmargin, int rmargin,
                 int bmargin, ncsharedstats* stats, unsigned drain,
@@ -1396,9 +1414,11 @@ create_inputctx(tinfo* ti, FILE* infp, int lmargin, int tmargin, int rmargin,
       i->isize = BUFSIZ;
       if( (i->inputs = malloc(sizeof(*i->inputs) * i->isize)) ){
         if(pthread_mutex_init(&i->ilock, NULL) == 0){
-          if(pthread_cond_init(&i->icond, NULL) == 0){
+          pthread_condattr_t condattr;
+          if(prep_condattr(&condattr) == 0){
+            if(pthread_cond_init(&i->icond, &condattr) == 0){
               if(pthread_mutex_init(&i->clock, NULL) == 0){
-              if(pthread_cond_init(&i->ccond, NULL) == 0){
+                if(pthread_cond_init(&i->ccond, &condattr) == 0){
                   if((i->stdinfd = fileno(infp)) >= 0){
                     if( (i->initdata = malloc(sizeof(*i->initdata))) ){
                       if(getpipes(i->readypipes) == 0){
@@ -1443,6 +1463,8 @@ create_inputctx(tinfo* ti, FILE* infp, int lmargin, int tmargin, int rmargin,
               }
               pthread_cond_destroy(&i->icond);
             }
+            pthread_condattr_destroy(&condattr);
+          }
           pthread_mutex_destroy(&i->ilock);
         }
         free(i->inputs);