Look for backend implementations in a specific directory

Make the backend loader search for implementations in the
<libdir>/wpe-<apiversion> directory (typically /usr/lib/wpe-1.0)
when a relative path is passed to wpe_loader_init().

Fixes #59

CMakeLists.txt

@@ -19,7 +19,7 @@ CALCULATE_LIBRARY_VERSIONS_FROM_LIBTOOL_TRIPLE(LIBWPE 4 0 3)
 project(libwpe VERSION "${PROJECT_VERSION}")
 
 set(WPE_BACKEND "" CACHE STRING
-    "Name of the backend library to load, instead of libWPEBackend-default.so")
+    "Path of a fixed backend library to load, instead of libWPEBackend-default.so")
 
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
@@ -86,6 +86,7 @@ target_include_directories(wpe PRIVATE
 )
 target_compile_definitions(wpe PRIVATE
     WPE_COMPILATION
+    WPE_BACKENDS_DIR=\"${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/wpe-${WPE_API_VERSION}\"
     $<TARGET_PROPERTY:GL::egl,INTERFACE_COMPILE_DEFINITIONS>
 )
 if (WPE_BACKEND)

docs/backend-implementing.md

@@ -0,0 +1,24 @@
+# Implementing Backends
+
+## Installation
+
+Backend implementations must be installed to a specific directory. The
+location is determined at build time to be `<prefix>/lib/wpe-<apiversion>`,
+with `<prefix>` being the base directory where *libwpe* is installed, and
+`<apiversion>` the public API version.
+
+The `pkg-config` tool can be used to query the `backendsdir` variable,
+which contains the location where backend implementations will be searched
+for:
+
+```sh
+pkg-config wpe-1.0 --variable=backendsdir
+```
+
+For example, the following Make snippet will install a backend in the
+correct location:
+
+```make
+install: libMyBackend.so
+	install -Dm755 -t "$$(pkg-config wpe-1.0 --variable=backendsdir)" $<
+```

docs/sitemap.txt

@@ -1,2 +1,3 @@
 index.md
+	backend-implementing.md
 	c-index

include/wpe/loader.h

@@ -69,7 +69,15 @@ struct wpe_loader_interface {
  * @impl_library_name: (transfer none): Name of the shared library object
  *     to load as WPE backend implementation.
  *
- * Initializes the `libwpe` object loader
+ * Initializes the `libwpe` object loader. If the @impl_library_name is a
+ * full path it will be used as-is, while a relative path will cause the
+ * backend implementation library to be searched for in a fixed location.
+ *
+ * The backends location is determined at build time, and typically will
+ * be `<prefix>/lib/wpe-<apiversion>`, where `<prefix>` is the installation
+ * prefix and `<apiversion>` is the libwpe API version being used. See the
+ * [backend implementation
+ * documentation](backend-implementing.md#Installation) for more.
  *
  * Returns: Whether initialization succeeded.
  */
@@ -81,10 +89,11 @@ wpe_loader_init(const char* impl_library_name);
  * wpe_loader_get_loaded_implementation_library_name:
  *
  * Obtain the name of the shared library object loaded as WPE backend
- * implementation. Note that in general this will return the value passed
- * to wpe_loader_init(), but that is not guaranteed.
+ * implementation. Note that in general this will return the actual location
+ * of the backend being used, which may not be the same value passed to
+ * wpe_loader_init().
  *
- * Returns: (transfer none): Name of the shared library object for the
+ * Returns: (transfer none): Path of the shared library object for the
  *     backend implementation.
  */
 WPE_EXPORT

src/loader.c

@@ -31,68 +31,67 @@
 #include <stdlib.h>
 #include <string.h>
 
+#define LENGTHOF(array) (sizeof(array) / sizeof(array[0]))
+
 static void* s_impl_library = 0;
 static struct wpe_loader_interface* s_impl_loader = 0;
 
 #ifndef WPE_BACKEND
-#define IMPL_LIBRARY_NAME_BUFFER_SIZE 512
-static char* s_impl_library_name;
-static char s_impl_library_name_buffer[IMPL_LIBRARY_NAME_BUFFER_SIZE];
+static char* s_impl_library_path = NULL;
+static char s_impl_library_path_buffer[512];
 #endif
 
 #ifndef WPE_BACKEND
 static void
-wpe_loader_set_impl_library_name(const char* impl_library_name)
+wpe_loader_set_impl_library_path(const char* impl_library_path)
 {
-    size_t len;
-
-    if (!impl_library_name)
+    if (!impl_library_path)
         return;
 
-    len = strlen(impl_library_name) + 1;
+    size_t len = strlen(impl_library_path) + 1;
     if (len == 1)
         return;
 
-    if (len > IMPL_LIBRARY_NAME_BUFFER_SIZE)
-        s_impl_library_name = (char *)malloc(len);
+    if (len > LENGTHOF(s_impl_library_path_buffer))
+        s_impl_library_path = malloc(len);
     else
-        s_impl_library_name = s_impl_library_name_buffer;
-    memcpy(s_impl_library_name, impl_library_name, len);
+        s_impl_library_path = s_impl_library_path_buffer;
+    memcpy(s_impl_library_path, impl_library_path, len);
 }
-#endif
+#endif /* !WPE_BACKEND */
 
 void
-load_impl_library()
+load_impl_library(void)
 {
 #ifdef WPE_BACKEND
     s_impl_library = dlopen(WPE_BACKEND, RTLD_NOW);
     if (!s_impl_library) {
         fprintf(stderr, "wpe: could not load compile-time defined WPE_BACKEND: %s\n", dlerror());
         abort();
     }
-#else
+#else /* !WPE_BACKEND */
 #ifndef NDEBUG
     // Get the impl library from an environment variable, if available.
-    char* env_library_name = getenv("WPE_BACKEND_LIBRARY");
-    if (env_library_name) {
-        s_impl_library = dlopen(env_library_name, RTLD_NOW);
+    const char* env_library_path = getenv("WPE_BACKEND_LIBRARY");
+    if (env_library_path) {
+        s_impl_library = dlopen(env_library_path, RTLD_NOW);
         if (!s_impl_library) {
             fprintf(stderr, "wpe: could not load specified WPE_BACKEND_LIBRARY: %s\n", dlerror());
             abort();
         }
-        wpe_loader_set_impl_library_name(env_library_name);
+        wpe_loader_set_impl_library_path(env_library_path);
     }
-#endif
+#endif /* !NDEBUG */
     if (!s_impl_library) {
         // Load libWPEBackend-default.so by ... default.
-        s_impl_library = dlopen("libWPEBackend-default.so", RTLD_NOW);
+        s_impl_library = dlopen(WPE_BACKENDS_DIR "/libWPEBackend-default.so", RTLD_NOW);
         if (!s_impl_library) {
             fprintf(stderr, "wpe: could not load the impl library. Is there any backend installed?: %s\n", dlerror());
             abort();
         }
-        wpe_loader_set_impl_library_name("libWPEBackend-default.so");
+        wpe_loader_set_impl_library_path(WPE_BACKENDS_DIR "/libWPEBackend-default.so");
     }
-#endif
+#endif /* WPE_BACKEND */
 
     s_impl_loader = dlsym(s_impl_library, "_wpe_loader_interface");
 }
@@ -101,41 +100,51 @@ bool
 wpe_loader_init(const char* impl_library_name)
 {
 #ifndef WPE_BACKEND
-    if (!impl_library_name) {
+    if (!(impl_library_name && impl_library_name[0] != '\0')) {
         fprintf(stderr, "wpe_loader_init: invalid implementation library name\n");
         abort();
     }
 
+    const bool relative_path = (impl_library_name[0] != '/');
+
+    size_t len = strlen(impl_library_name) + 1 + (relative_path ? LENGTHOF(WPE_BACKENDS_DIR) : 0);
+    char impl_library_path[len];
+
+    if (relative_path)
+        snprintf(impl_library_path, len, WPE_BACKENDS_DIR "/%s", impl_library_name);
+    else
+        strncpy(impl_library_path, impl_library_name, len);
+
     if (s_impl_library) {
-        if (!s_impl_library_name || strcmp(s_impl_library_name, impl_library_name) != 0) {
+        if (!s_impl_library_path || strcmp(s_impl_library_path, impl_library_path) != 0) {
             fprintf(stderr, "wpe_loader_init: already initialized\n");
             return false;
         }
         return true;
     }
 
-    s_impl_library = dlopen(impl_library_name, RTLD_NOW);
+    s_impl_library = dlopen(impl_library_path, RTLD_NOW);
     if (!s_impl_library) {
-        fprintf(stderr, "wpe_loader_init could not load the library '%s': %s\n", impl_library_name, dlerror());
+        fprintf(stderr, "wpe_loader_init could not load the library '%s': %s\n", impl_library_path, dlerror());
         return false;
     }
-    wpe_loader_set_impl_library_name(impl_library_name);
+    wpe_loader_set_impl_library_path(impl_library_path);
 
     s_impl_loader = dlsym(s_impl_library, "_wpe_loader_interface");
     return true;
-#else
+#else /* WPE_BACKEND */
     return false;
-#endif
+#endif /* !WPE_BACKEND */
 }
 
 const char*
 wpe_loader_get_loaded_implementation_library_name(void)
 {
 #ifdef WPE_BACKEND
     return s_impl_library ? WPE_BACKEND : NULL;
-#else
-    return s_impl_library_name;
-#endif
+#else /* !WPE_BACKEND */
+    return s_impl_library_path;
+#endif /* WPE_BACKEND */
 }
 
 void*

wpe.pc.cmake

@@ -2,6 +2,7 @@ prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
 includedir=${prefix}/include
 libdir=${exec_prefix}/lib
+backendsdir=${libdir}/wpe-@WPE_API_VERSION@
 
 Name: wpe-@WPE_API_VERSION@
 Description: The wpe library