libsysprof-profile: add trace_fd helper for spawnable

I'd like to stop using tracefd, but since we need to maintain compatability
with older tooling, lets at least make it more ergonomic than adding an
instrument(s) for it.

src/libsysprof-profile/sysprof-spawnable.c

@@ -22,6 +22,8 @@
 
 #include <unistd.h>
 
+#include <glib/gstdio.h>
+
 #include "sysprof-spawnable.h"
 
 typedef struct
@@ -328,3 +330,51 @@ sysprof_spawnable_set_cwd (SysprofSpawnable *self,
       self->cwd = g_strdup (cwd);
     }
 }
+
+/**
+ * sysprof_spawnable_add_trace_fd:
+ * @self: a #SysprofSpawnable
+ * @envvar: (nullable): the environment variable
+ *
+ * Adds an environment variable to the spawnable that will contain a
+ * "tracing file-descriptor". The spawned process can use
+ * `sysprof_capture_writer_new_from_env()` if @envvar is %NULL
+ * or with `getenv()` and `sysprof_capture_writer_new_from_fd()`.
+ *
+ * If @envvar is %NULL, "SYSPROF_TRACE_FD" will be used.
+ *
+ * The caller is responsible for closin the resulting FD.
+ *
+ * Returns: A file-descriptor which can be used to read the trace or
+ *   -1 upon failure and `errno` is set. Caller must `close()` the
+ *   FD if >= 0.
+ */
+int
+sysprof_spawnable_add_trace_fd (SysprofSpawnable *self,
+                                const char       *envvar)
+{
+  g_autofd int fd = -1;
+  g_autofd int dest = -1;
+  g_autofree char *name = NULL;
+  g_autofree char *fdstr = NULL;
+
+  g_return_val_if_fail (SYSPROF_IS_SPAWNABLE (self), -1);
+
+  if (envvar == NULL)
+    envvar = "SYSPROF_TRACE_FD";
+
+  name = g_strdup_printf ("[sysprof-tracefd:%s]", envvar);
+
+  if (-1 == (fd = sysprof_memfd_create (name)))
+    return -1;
+
+  if (-1 == (dest = dup (fd)))
+    return -1;
+
+  fdstr = g_strdup_printf ("%d", dest);
+
+  sysprof_spawnable_setenv (self, envvar, fdstr);
+  sysprof_spawnable_take_fd (self, g_steal_fd (&dest), -1);
+
+  return g_steal_fd (&fd);
+}