darwin: lazily set process TTY name

Fetching the TTY name of a process is extremely expensive on darwin and
the call to devname accounts for 95% of htop's CPU usage when there is
high process turnover (this is mostly due to devname calling lstat,
which is incredibly slow). This can make htop unresponsive.

To mitigate this only set the process TTY name if the it is being
actively displayed (PROCESS_FLAG_TTY), which by default it is not
on darwin.

darwin/DarwinProcess.c

@@ -12,6 +12,7 @@ in the source distribution for its full text.
 #include <stdlib.h>
 #include <string.h>
 #include <mach/mach.h>
+#include <sys/dirent.h>
 
 #include "CRT.h"
 #include "Process.h"
@@ -26,7 +27,7 @@ const ProcessFieldData Process_fields[LAST_PROCESSFIELD] = {
    [PPID] = { .name = "PPID", .title = "PPID", .description = "Parent process ID", .flags = 0, .pidColumn = true, },
    [PGRP] = { .name = "PGRP", .title = "PGRP", .description = "Process group ID", .flags = 0, .pidColumn = true, },
    [SESSION] = { .name = "SESSION", .title = "SID", .description = "Process's session ID", .flags = 0, .pidColumn = true, },
-   [TTY] = { .name = "TTY", .title = "TTY      ", .description = "Controlling terminal", .flags = 0, },
+   [TTY] = { .name = "TTY", .title = "TTY      ", .description = "Controlling terminal", .flags = PROCESS_FLAG_TTY, },
    [TPGID] = { .name = "TPGID", .title = "TPGID", .description = "Process ID of the fg process group of the controlling terminal", .flags = 0, .pidColumn = true, },
    [MINFLT] = { .name = "MINFLT", .title = "     MINFLT ", .description = "Number of minor faults which have not required loading a memory page from disk", .flags = 0, .defaultSortDesc = true, },
    [MAJFLT] = { .name = "MAJFLT", .title = "     MAJFLT ", .description = "Number of major faults which have required loading a memory page from disk", .flags = 0, .defaultSortDesc = true, },
@@ -276,6 +277,18 @@ static long long int nanosecondsToCentiseconds(uint64_t nanoseconds) {
    return nanoseconds / nanoseconds_per_second * centiseconds_per_second;
 }
 
+static char* DarwinProcess_getDevname(dev_t dev) {
+   if (dev == NODEV) {
+      return NULL;
+   }
+   char buf[sizeof("/dev/") + MAXNAMLEN];
+   char *name = devname_r(dev, S_IFCHR, buf, MAXNAMLEN);
+   if (name) {
+      return xStrdup(name);
+   }
+   return NULL;
+}
+
 void DarwinProcess_setFromKInfoProc(Process* proc, const struct kinfo_proc* ps, bool exists) {
    DarwinProcess* dp = (DarwinProcess*)proc;
 
@@ -306,15 +319,8 @@ void DarwinProcess_setFromKInfoProc(Process* proc, const struct kinfo_proc* ps,
       proc->isKernelThread = false;
       proc->isUserlandThread = false;
       dp->translated = ps->kp_proc.p_flag & P_TRANSLATED;
-
       proc->tty_nr = ps->kp_eproc.e_tdev;
-      const char* name = (ps->kp_eproc.e_tdev != NODEV) ? devname(ps->kp_eproc.e_tdev, S_IFCHR) : NULL;
-      if (!name) {
-         free(proc->tty_name);
       proc->tty_name = NULL;
-      } else {
-         free_and_xStrdup(&proc->tty_name, name);
-      }
 
       proc->starttime_ctime = ep->p_starttime.tv_sec;
       Process_fillStarttimeBuffer(proc);
@@ -327,6 +333,23 @@ void DarwinProcess_setFromKInfoProc(Process* proc, const struct kinfo_proc* ps,
       }
    }
 
+   if (proc->tty_name == NULL && (dev_t)proc->tty_nr != NODEV) {
+      /* The call to devname() is extremely expensive (due to lstat)
+       * and represents ~95% of htop's CPU usage when there is high
+       * process turnover.
+       *
+       * To mitigate this we only fetch TTY information if the TTY
+       * field is enabled in the settings.
+       */
+      if (proc->settings->ss->flags & PROCESS_FLAG_TTY) {
+         proc->tty_name = DarwinProcess_getDevname(proc->tty_nr);
+         if (!proc->tty_name) {
+            /* devname failed: prevent us from calling it again */
+            proc->tty_nr = NODEV;
+         }
+      }
+   }
+
    /* Mutable information */
    proc->nice = ep->p_nice;
    proc->priority = ep->p_priority;

darwin/DarwinProcess.h

@@ -13,6 +13,8 @@ in the source distribution for its full text.
 #include "darwin/DarwinProcessList.h"
 
 
+#define PROCESS_FLAG_TTY 0x00000100
+
 typedef struct DarwinProcess_ {
    Process super;