[sysprof] capture: add sp_capture_writer_new_from_env()

[Christian Hergert <chergert src gnome org>]

commit 05314dd28c41819a167c97dbf764349a65de7cdd
Author: Christian Hergert <chergert redhat com>
Date:   Mon May 6 20:19:46 2019 -0700

    capture: add sp_capture_writer_new_from_env()
    
    This will return NULL if there is not sufficient data from the
    environment to setup tracing.

 lib/capture/sp-capture-writer.c | 23 +++++++++++++++++++++++
 lib/capture/sp-capture-writer.h |  1 +
 2 files changed, 24 insertions(+)
---
diff --git a/lib/capture/sp-capture-writer.c b/lib/capture/sp-capture-writer.c
index e45109c..24c0d43 100644
--- a/lib/capture/sp-capture-writer.c
+++ b/lib/capture/sp-capture-writer.c
@@ -23,6 +23,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <glib/gstdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ -415,6 +416,9 @@ sp_capture_writer_new_from_fd (int   fd,
   GTimeVal tv;
   gsize header_len = sizeof(*header);
 
+  if (fd < 0)
+    return NULL;
+
   if (buffer_size == 0)
     buffer_size = DEFAULT_BUFFER_SIZE;
 
@@ -1221,3 +1225,22 @@ do_end:
 
   return TRUE;
 }
+
+SpCaptureWriter *
+sp_capture_writer_new_from_env (gsize buffer_size)
+{
+  const gchar *fdstr;
+  int fd;
+
+  if (!(fdstr = g_getenv ("SYSPROF_TRACE_FD")))
+    return NULL;
+
+  if (!(fd = atoi (fdstr)))
+    return NULL;
+
+  /* ignore stdin/stdout/stderr */
+  if (fd < 2)
+    return NULL;
+
+  return sp_capture_writer_new_from_fd (dup (fd), buffer_size);
+}
diff --git a/lib/capture/sp-capture-writer.h b/lib/capture/sp-capture-writer.h
index 3b817e5..336794b 100644
--- a/lib/capture/sp-capture-writer.h
+++ b/lib/capture/sp-capture-writer.h
@@ -38,6 +38,7 @@ typedef struct
   gsize padding[48];
 } SpCaptureStat;
 
+SpCaptureWriter    *sp_capture_writer_new_from_env    (gsize                    buffer_size);
 SpCaptureWriter    *sp_capture_writer_new             (const gchar             *filename,
                                                        gsize                    buffer_size);
 SpCaptureWriter    *sp_capture_writer_new_from_fd     (int                      fd,