hostinfo: reuse proc FD and memory when parsing

Re-opening the FD on every poll is pretty expensive. So we should avoid
doing that as well re-use the read buffer (page-aligned, <=1 page) for
the sample data.

lib/sources/sp-hostinfo-source.c

@@ -17,20 +17,27 @@
  */
 
 #include <ctype.h>
+#include <fcntl.h>
 #include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 #include "sources/sp-hostinfo-source.h"
 
+#define PROC_STAT_BUF_SIZE 4096
+
 struct _SpHostinfoSource
 {
   GObject          parent_instance;
 
   guint            handler;
   gint             n_cpu;
+  gint             stat_fd;
 
   SpCaptureWriter *writer;
   GArray          *cpu_info;
+  gchar           *stat_buf;
 };
 
 typedef struct
@@ -61,6 +68,30 @@ sp_hostinfo_source_new (void)
   return g_object_new (SP_TYPE_HOSTINFO_SOURCE, NULL);
 }
 
+static gboolean
+read_stat (SpHostinfoSource *self)
+{
+  gssize len;
+
+  g_assert (self != NULL);
+  g_assert (self->stat_fd != -1);
+  g_assert (self->stat_buf != NULL);
+
+  if (lseek (self->stat_fd, 0, SEEK_SET) != 0)
+    return FALSE;
+
+  len = read (self->stat_fd, self->stat_buf, PROC_STAT_BUF_SIZE);
+  if (len <= 0)
+    return FALSE;
+
+  if (len < PROC_STAT_BUF_SIZE)
+    self->stat_buf[len] = 0;
+  else
+    self->stat_buf[PROC_STAT_BUF_SIZE-1] = 0;
+
+  return TRUE;
+}
+
 static void
 poll_cpu (SpHostinfoSource *self)
 {
@@ -85,23 +116,24 @@ poll_cpu (SpHostinfoSource *self)
   glong steal_calc;
   glong guest_calc;
   glong guest_nice_calc;
-  gchar *buf = NULL;
   glong total;
   gchar *line;
   gint ret;
   gint id;
-  gint i;
 
-  if (g_file_get_contents("/proc/stat", &buf, NULL, NULL))
+  if (read_stat (self))
     {
-      line = buf;
-      for (i = 0; buf[i]; i++)
+      line = self->stat_buf;
+
+      for (gsize i = 0; self->stat_buf[i]; i++)
         {
-          if (buf[i] == '\n') {
-            buf[i] = '\0';
-            if (g_str_has_prefix(line, "cpu"))
+          if (self->stat_buf[i] == '\n')
             {
-                if (isdigit(line[3]))
+              self->stat_buf[i] = '\0';
+
+              if (strncmp (line, "cpu", 3) == 0)
+                {
+                  if (isdigit (line[3]))
                     {
                       CpuInfo *cpu_info;
 
@@ -144,18 +176,18 @@ poll_cpu (SpHostinfoSource *self)
                       cpu_info->last_guest = guest;
                       cpu_info->last_guest_nice = guest_nice;
                     }
-              } else {
+                }
+              else
+                {
                   /* CPU info comes first. Skip further lines. */
                   break;
                 }
 
             next:
-            line = &buf[i + 1];
+              line = &self->stat_buf[i + 1];
             }
         }
     }
-
-  g_free (buf);
 }
 
 static void
@@ -163,12 +195,11 @@ publish_cpu (SpHostinfoSource *self)
 {
   SpCaptureCounterValue *counter_values;
   guint *counter_ids;
-  gint i;
 
   counter_ids = alloca (sizeof *counter_ids * self->n_cpu * 2);
   counter_values = alloca (sizeof *counter_values * self->n_cpu * 2);
 
-  for (i = 0; i < self->n_cpu; i++)
+  for (guint i = 0; i < self->n_cpu; i++)
     {
       CpuInfo *info = &g_array_index (self->cpu_info, CpuInfo, i);
       SpCaptureCounterValue *value = &counter_values[i*2];
@@ -219,6 +250,7 @@ sp_hostinfo_source_finalize (GObject *object)
 
   g_clear_pointer (&self->writer, sp_capture_writer_unref);
   g_clear_pointer (&self->cpu_info, g_array_unref);
+  g_clear_pointer (&self->stat_buf, g_free);
 
   G_OBJECT_CLASS (sp_hostinfo_source_parent_class)->finalize (object);
 }
@@ -234,7 +266,9 @@ sp_hostinfo_source_class_init (SpHostinfoSourceClass *klass)
 static void
 sp_hostinfo_source_init (SpHostinfoSource *self)
 {
+  self->stat_fd = -1;
   self->cpu_info = g_array_new (FALSE, TRUE, sizeof (CpuInfo));
+  self->stat_buf = g_malloc (PROC_STAT_BUF_SIZE);
 }
 
 static void
@@ -270,6 +304,12 @@ sp_hostinfo_source_stop (SpSource *source)
   g_source_remove (self->handler);
   self->handler = 0;
 
+  if (self->stat_fd != -1)
+    {
+      close (self->stat_fd);
+      self->stat_fd = -1;
+    }
+
   sp_source_emit_finished (SP_SOURCE (self));
 }
 
@@ -278,17 +318,17 @@ sp_hostinfo_source_prepare (SpSource *source)
 {
   SpHostinfoSource *self = (SpHostinfoSource *)source;
   SpCaptureCounter *counters;
-  gint i;
 
   g_assert (SP_IS_HOSTINFO_SOURCE (self));
 
+  self->stat_fd = open ("/proc/stat", O_RDONLY);
   self->n_cpu = g_get_num_processors ();
 
   g_array_set_size (self->cpu_info, 0);
 
   counters = alloca (sizeof *counters * self->n_cpu * 2);
 
-  for (i = 0; i < self->n_cpu; i++)
+  for (guint i = 0; i < self->n_cpu; i++)
     {
       SpCaptureCounter *ctr = &counters[i*2];
       CpuInfo info = { 0 };