glib-compat: add g_unix_get_passwd_entry_qemu()

The glib function was introduced in 2.64. It's a safer version of
getpwnam, and also simpler to use than getpwnam_r.

Currently, it's only use by the next patch in qemu-ga, which doesn't
(well well...) need the thread safety guarantees. Since the fallback
version is still unsafe, I would rather keep the _qemu postfix, to make
sure it's not being misused by mistake. When/if necessary, we can
implement a safer fallback and drop the _qemu suffix.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
*fix checkpatch warnings about newlines before/after block comments
Signed-off-by: Michael Roth <michael.roth@amd.com>

include/glib-compat.h

@@ -30,6 +30,11 @@
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 
 #include <glib.h>
+#if defined(G_OS_UNIX)
+#include <glib-unix.h>
+#include <sys/types.h>
+#include <pwd.h>
+#endif
 
 /*
  * Note that because of the GLIB_VERSION_MAX_ALLOWED constant above, allowing
@@ -72,6 +77,29 @@
 gint g_poll_fixed(GPollFD *fds, guint nfds, gint timeout);
 #endif
 
+#if defined(G_OS_UNIX)
+/*
+ * Note: The fallback implementation is not MT-safe, and it returns a copy of
+ * the libc passwd (must be g_free() after use) but not the content. Because of
+ * these important differences the caller must be aware of, it's not #define for
+ * GLib API substitution.
+ */
+static inline struct passwd *
+g_unix_get_passwd_entry_qemu(const gchar *user_name, GError **error)
+{
+#if GLIB_CHECK_VERSION(2, 64, 0)
+    return g_unix_get_passwd_entry(user_name, error);
+#else
+    struct passwd *p = getpwnam(user_name);
+    if (!p) {
+        g_set_error_literal(error, G_UNIX_ERROR, 0, g_strerror(errno));
+        return NULL;
+    }
+    return (struct passwd *)g_memdup(p, sizeof(*p));
+#endif
+}
+#endif /* G_OS_UNIX */
+
 #pragma GCC diagnostic pop
 
 #endif