trace: Specify trace file name

Allow users to specify a file for trace-outputs at configuration.
Also, allow trace files to be annotated by <pid> so each qemu instance has
unique traces.

The trace file name can be passed as a config option:
--trace-file=/path/to/file
(Default: trace )
At runtime, the pid of the qemu process is appended to the filename so
that mutiple qemu instances do not have overlapping logs.

Eg : trace-1234 for qemu launched with pid 1234.

I have yet to test this on windows. getpid() is used at many places
in code(including vnc.c), so I'm hoping this would be okay too.
Edited-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -318,6 +318,7 @@ check_utests="no"
 user_pie="no"
 zero_malloc=""
 trace_backend="nop"
+trace_file="trace"
 
 # OS specific
 if check_define __linux__ ; then
@@ -522,6 +523,8 @@ for opt do
   ;;
   --trace-backend=*) trace_backend="$optarg"
   ;;
+  --trace-file=*) trace_file="$optarg"
+  ;;
   --enable-gprof) gprof="yes"
   ;;
   --static)
@@ -901,6 +904,8 @@ echo "  --disable-docs           disable documentation build"
 echo "  --disable-vhost-net      disable vhost-net acceleration support"
 echo "  --enable-vhost-net       enable vhost-net acceleration support"
 echo "  --trace-backend=B        Trace backend nop simple"
+echo "  --trace-file=NAME        Full PATH,NAME of file to store traces"
+echo "                           Default:trace-<pid>"
 echo ""
 echo "NOTE: The object files are built at the place where configure is launched"
 exit 1
@@ -2206,6 +2211,7 @@ echo "fdatasync         $fdatasync"
 echo "uuid support      $uuid"
 echo "vhost-net support $vhost_net"
 echo "Trace backend     $trace_backend"
+echo "Trace output file $trace_file-<pid>"
 
 if test $sdl_too_old = "yes"; then
 echo "-> Your SDL version is too old - please upgrade to have SDL support"
@@ -2471,6 +2477,12 @@ echo "TRACE_BACKEND=$trace_backend" >> $config_host_mak
 if test "$trace_backend" = "simple"; then
   echo "CONFIG_SIMPLE_TRACE=y" >> $config_host_mak
 fi
+# Set the appropriate trace file.
+if test "$trace_backend" = "simple"; then
+  trace_file="\"$trace_file-%u\""
+fi
+echo "CONFIG_TRACE_FILE=$trace_file" >> $config_host_mak
+
 echo "TOOLS=$tools" >> $config_host_mak
 echo "ROMS=$roms" >> $config_host_mak
 echo "MAKE=$make" >> $config_host_mak

simpletrace.c

@@ -54,13 +54,26 @@ static bool write_header(FILE *fp)
     return fwrite(&header, sizeof header, 1, fp) == 1;
 }
 
-static void flush_trace_buffer(void)
+static bool open_trace_file(void)
 {
+    char *filename;
+
+    if (asprintf(&filename, CONFIG_TRACE_FILE, getpid()) < 0) {
+        return false;
+    }
+
+    trace_fp = fopen(filename, "w");
+    free(filename);
     if (!trace_fp) {
-        trace_fp = fopen("trace.log", "w");
-        if (trace_fp) {
-            write_header(trace_fp);
+        return false;
     }
+    return write_header(trace_fp);
+}
+
+static void flush_trace_buffer(void)
+{
+    if (!trace_fp) {
+        open_trace_file();
     }
     if (trace_fp) {
         size_t unused; /* for when fwrite(3) is declared warn_unused_result */