debug: Add support to log to file.

When VIRGL_LOG_FILE gets set, put log into it. This is similar
with MESA_LOG_FILE. An improvement over MESA_LOG_FILE is that
it supports "%PID%" in file name and will replace it with pid
of the current process.

Signed-off-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

src/vrend_debug.c

@@ -174,7 +174,34 @@ int  vrend_debug_can_override(void)
 static
 void vrend_default_debug_callback(const char *fmt, va_list va)
 {
-   vfprintf(stderr, fmt, va);
+   static FILE* fp = NULL;
+   if (NULL == fp) {
+      const char* log = getenv("VIRGL_LOG_FILE");
+      if (log) {
+         char *log_prefix = strdup(log);
+         char *log_suffix = strstr(log_prefix, "%PID%");
+         if (log_suffix) {
+            *log_suffix = 0;
+            log_suffix += 5;
+            int len = strlen(log) + 32;
+            char *name = malloc(len);
+            snprintf(name, len, "%s%d%s", log_prefix, getpid(), log_suffix);
+            fp = fopen(name, "a");
+            free(name);
+         } else {
+            fp = fopen(log, "a");
+         }
+         free(log_prefix);
+         if (NULL == fp) {
+            fprintf(stderr, "Can't open %s\n", log);
+            fp = stderr;
+         }
+      } else {
+            fp = stderr;
+      }
+   }
+   vfprintf(fp, fmt, va);
+   fflush(fp);
 }
 
 static virgl_debug_callback_type debug_callback = vrend_default_debug_callback;