Use compiler symbol visiblity features to hide internal functions

Previously we relied on libtool's -export-symbols-regex feature, but we
are hoping to drop GNU Autotools and GNU Libtool in soon in favour of
Meson.

Meson doesn't have an equivalent feature, instead the advice is to
control symbol visibility at compile time.

The approach taken in this patch is based on Pango's build system. Pango
tells the compiler to hide symbols by default (if possible), and then defines
a _PANGO_EXTERN variable at compile time which marks a single symbol as
public. In Pango's case there is then further machinary to hide symbols
based on deprecation policies but I have not copied that here, instead I
used _LIBMEDIAART_EXTERN directly.

If a compiler doesn't support hiding symbols then the library we build
makes all symbols available, which is exactly what would happen before
on platforms where the libtool didn't have an implementation for
-export-symbols-regex.

See also:

  http://mesonbuild.com/FAQ.html#how-do-i-do-the-equivalent-of-libtools-exportsymbol-and-exportregex
  https://git.gnome.org/browse/pango/
  https://git.gnome.org/browse/pango/tree/pango/pango-version-macros.h
  https://gcc.gnu.org/wiki/Visibility
  https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options (-fvisibility)

https://bugzilla.gnome.org/show_bug.cgi?id=783562

5 files changed, 64 insertions, 3 deletions

diff --git a/libmediaart/Makefile.am b/libmediaart/Makefile.am
index da5073a..a6f7bc2 100644
--- a/libmediaart/Makefile.am
+++ b/libmediaart/Makefile.am
@@ -1,6 +1,8 @@
 AM_CPPFLAGS = \
 	$(BUILD_CFLAGS) \
-	$(LIBMEDIAART_CFLAGS)
+	$(LIBMEDIAART_CFLAGS) \
+	$(LIBMEDIAART_HIDDEN_VISIBILITY_CFLAGS)
+
 
 #
 # Binaries
@@ -49,8 +51,7 @@ endif
 endif
 
 libmediaart_@LIBMEDIAART_API_VERSION@_la_LDFLAGS = \
-	-version-info $(LT_CURRENT):$(LT_REVISION):$(LT_AGE) \
-	-export-symbols-regex '^media_art_.*'
+	-version-info $(LT_CURRENT):$(LT_REVISION):$(LT_AGE)
 
 libmediaart_@LIBMEDIAART_API_VERSION@_la_LIBADD = \
 	$(BUILD_LIBS) \
diff --git a/libmediaart/cache.h b/libmediaart/cache.h
index 2bb711c..e656011 100644
--- a/libmediaart/cache.h
+++ b/libmediaart/cache.h
@@ -23,27 +23,34 @@
 #include <glib.h>
 #include <gio/gio.h>
 
+#include "mediaart-macros.h"
+
 #if !defined (__LIBMEDIAART_INSIDE__) && !defined (LIBMEDIAART_COMPILATION)
 #error "Only <libmediaart/mediaart.h> must be included directly."
 #endif
 
 G_BEGIN_DECLS
 
+_LIBMEDIAART_EXTERN
 gchar *  media_art_strip_invalid_entities (const gchar          *original);
 
+_LIBMEDIAART_EXTERN
 gboolean media_art_get_path               (const gchar          *artist,
                                            const gchar          *title,
                                            const gchar          *prefix,
                                            gchar               **cache_path);
+_LIBMEDIAART_EXTERN
 gboolean media_art_get_file               (const gchar          *artist,
                                            const gchar          *title,
                                            const gchar          *prefix,
                                            GFile               **cache_file);
 
+_LIBMEDIAART_EXTERN
 gboolean media_art_remove                 (const gchar          *artist,
                                            const gchar          *album,
                                            GCancellable         *cancellable,
                                            GError              **error);
+_LIBMEDIAART_EXTERN
 void     media_art_remove_async           (const gchar          *artist,
                                            const gchar          *album,
                                            gint                  io_priority,
@@ -51,6 +58,8 @@ void     media_art_remove_async           (const gchar          *artist,
                                            GCancellable         *cancellable,
                                            GAsyncReadyCallback   callback,
                                            gpointer              user_data);
+
+_LIBMEDIAART_EXTERN
 gboolean media_art_remove_finish          (GObject              *source_object,
                                            GAsyncResult         *result,
                                            GError              **error);
diff --git a/libmediaart/extract.h b/libmediaart/extract.h
index 8f6440a..460015d 100644
--- a/libmediaart/extract.h
+++ b/libmediaart/extract.h
@@ -23,6 +23,8 @@
 #include <glib.h>
 #include <gio/gio.h>
 
+#include "mediaart-macros.h"
+
 #if !defined (__LIBMEDIAART_INSIDE__) && !defined (LIBMEDIAART_COMPILATION)
 #error "Only <libmediaart/mediaart.h> must be included directly."
 #endif
@@ -88,6 +90,7 @@ typedef enum {
 } MediaArtError;
 
 
+_LIBMEDIAART_EXTERN
 GQuark media_art_error_quark (void) G_GNUC_CONST;
 
 
@@ -122,9 +125,12 @@ struct _MediaArtProcessClass {
 };
 
 
+_LIBMEDIAART_EXTERN
 GType            media_art_process_get_type      (void) G_GNUC_CONST;
 
+_LIBMEDIAART_EXTERN
 MediaArtProcess *media_art_process_new           (GError               **error);
+_LIBMEDIAART_EXTERN
 gboolean         media_art_process_uri           (MediaArtProcess       *process,
                                                   MediaArtType           type,
                                                   MediaArtProcessFlags   flags,
@@ -133,6 +139,7 @@ gboolean         media_art_process_uri           (MediaArtProcess       *process
                                                   const gchar           *title,
                                                   GCancellable          *cancellable,
                                                   GError               **error);
+_LIBMEDIAART_EXTERN
 void             media_art_process_uri_async     (MediaArtProcess       *process,
                                                   MediaArtType           type,
                                                   MediaArtProcessFlags   flags,
@@ -143,9 +150,11 @@ void             media_art_process_uri_async     (MediaArtProcess       *process
                                                   GCancellable          *cancellable,
                                                   GAsyncReadyCallback    callback,
                                                   gpointer               user_data);
+_LIBMEDIAART_EXTERN
 gboolean         media_art_process_uri_finish    (MediaArtProcess       *process,
                                                   GAsyncResult          *result,
                                                   GError               **error);
+_LIBMEDIAART_EXTERN
 gboolean         media_art_process_file          (MediaArtProcess       *process,
                                                   MediaArtType           type,
                                                   MediaArtProcessFlags   flags,
@@ -154,6 +163,7 @@ gboolean         media_art_process_file          (MediaArtProcess       *process
                                                   const gchar           *title,
                                                   GCancellable          *cancellable,
                                                   GError               **error);
+_LIBMEDIAART_EXTERN
 void             media_art_process_file_async    (MediaArtProcess       *process,
                                                   MediaArtType           type,
                                                   MediaArtProcessFlags   flags,
@@ -164,9 +174,11 @@ void             media_art_process_file_async    (MediaArtProcess       *process
                                                   GCancellable          *cancellable,
                                                   GAsyncReadyCallback    callback,
                                                   gpointer               user_data);
+_LIBMEDIAART_EXTERN
 gboolean         media_art_process_file_finish   (MediaArtProcess       *process,
                                                   GAsyncResult          *result,
                                                   GError               **error);
+_LIBMEDIAART_EXTERN
 gboolean         media_art_process_buffer        (MediaArtProcess       *process,
                                                   MediaArtType           type,
                                                   MediaArtProcessFlags   flags,
@@ -178,6 +190,7 @@ gboolean         media_art_process_buffer        (MediaArtProcess       *process
                                                   const gchar           *title,
                                                   GCancellable          *cancellable,
                                                   GError               **error);
+_LIBMEDIAART_EXTERN
 void             media_art_process_buffer_async  (MediaArtProcess       *process,
                                                   MediaArtType           type,
                                                   MediaArtProcessFlags   flags,
@@ -191,6 +204,7 @@ void             media_art_process_buffer_async  (MediaArtProcess       *process
                                                   GCancellable          *cancellable,
                                                   GAsyncReadyCallback    callback,
                                                   gpointer               user_data);
+_LIBMEDIAART_EXTERN
 gboolean         media_art_process_buffer_finish (MediaArtProcess       *process,
                                                   GAsyncResult          *result,
                                                   GError               **error);
diff --git a/libmediaart/extractgeneric.h b/libmediaart/extractgeneric.h
index 8fa114b..b08fe2c 100644
--- a/libmediaart/extractgeneric.h
+++ b/libmediaart/extractgeneric.h
@@ -25,18 +25,24 @@
 
 #include <glib.h>
 
+#include "mediaart-macros.h"
+
 #if !defined (__LIBMEDIAART_INSIDE__) && !defined (LIBMEDIAART_COMPILATION)
 #error "Only <libmediaart/mediaart.h> must be included directly."
 #endif
 
 G_BEGIN_DECLS
 
+_LIBMEDIAART_EXTERN
 void      media_art_plugin_init     (gint                  max_width);
+_LIBMEDIAART_EXTERN
 void      media_art_plugin_shutdown (void);
 
+_LIBMEDIAART_EXTERN
 gboolean  media_art_file_to_jpeg    (const gchar          *filename,
                                      const gchar          *target,
                                      GError              **error);
+_LIBMEDIAART_EXTERN
 gboolean  media_art_buffer_to_jpeg  (const unsigned char  *buffer,
                                      size_t                len,
                                      const gchar          *buffer_mime,
diff --git a/libmediaart/mediaart-macros.h b/libmediaart/mediaart-macros.h
new file mode 100644
index 0000000..84b6d79
--- /dev/null
+++ b/libmediaart/mediaart-macros.h
@@ -0,0 +1,31 @@
+/*
+ * Copyright 2017, Sam Thursfield <sam.thursfield@codethink.co.uk>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA  02110-1301, USA.
+ */
+
+#ifndef __LIBMEDIAART_MACROS_H__
+#define __LIBMEDIAART_MACROS_H__
+
+#if !defined (__LIBMEDIAART_INSIDE__) && !defined (LIBMEDIAART_COMPILATION)
+#error "Only <libmediaart/mediaart.h> must be included directly."
+#endif
+
+#ifndef _LIBMEDIAART_EXTERN
+#define _LIBMEDIAART_EXTERN extern
+#endif
+
+#endif /* __LIBMEDIAART_MACROS_H__ */