ResourceManager::findPath(): validate; accept absolute, read-allowed paths

- Perform the SGPath::validate() test for read access before returning a
  path obtained with a non-null second argument ("aContext").

- If SGPath(aContext, aResource) is an absolute path for which read
  access is authorized by SGPath::validate(), return it. This restores
  the possibility of using the 'play-audio-sample' FGCommand with an
  absolute, read-allowed path (was lost in FG commit 8853fded2953959).

- Because a fair number of the existing uses of
  ResourceManager::findPath() are not quite correct IMO, we execute the
  final part where all providers are tried in turn even if
  SGPath(aContext, aResource) is an absolute path (otherwise, the sim
  wouldn't start because it couldn't load materials.xml---see [1]).

- The SG_LOG call will spot a few errors in calling code/data, such as
  access tried for
  '/Aircraft/Generic/flightrecorder/generic-piston-propeller-4.xml' and
  '/Textures/Sky/cl_cumulus2.png'; since the function does not return at
  this point, these incorrect absolute paths which should be relative
  will still be given a chance with the BasePathProvider that has its
  base path set to $FG_ROOT; thus, they will be found as before this
  commit despite the new "access refused" warning (but please fix
  them!).

This commit requires FlightGear commit e7594f46876fc6b0b.

[1] https://sourceforge.net/p/flightgear/mailman/message/37697516/

simgear/embedded_resources/embedded_resources_test.cxx

@@ -314,7 +314,7 @@ void test_addAlreadyExistingResource()
     "add an already existing resource" << endl;
   const auto& resMgr = EmbeddedResourceManager::instance();
 
-  for (const string& locale: {"", "fr", "fr_FR"}) {
+  for (const string locale: {"", "fr", "fr_FR"}) {
     // For these tests, we don't care about the resource contents -> no need
     // to substract 1 from the result of sizeof() as we did above.
     unique_ptr<const RawEmbeddedResource> someRes(
@@ -390,7 +390,7 @@ void test_getLocaleAndSelectLocale()
     "EmbeddedResourceManager" << endl;
   const auto& resMgr = EmbeddedResourceManager::instance();
 
-  for (const string& locale: {"", "fr", "fr_FR", "de_DE"}) {
+  for (const string locale: {"", "fr", "fr_FR", "de_DE"}) {
     // The important effects of setLocale() are tested in
     // test_localeDependencyOfResourceFetching()
     resMgr->selectLocale(locale);

simgear/misc/ResourceManager.cxx

@@ -127,20 +127,32 @@ void ResourceManager::removeProvider(ResourceProvider* aProvider)
 
 SGPath ResourceManager::findPath(const std::string& aResource, SGPath aContext)
 {
-    if (!aContext.isNull()) {
-        SGPath r(aContext, aResource);
-        if (r.exists()) {
-            return r;
+    const SGPath completePath(aContext, aResource);
+
+    if (!aContext.isNull() && completePath.exists()) {
+        return completePath;
+    }
+
+    // Absolute, existing path and SGPath::validate() grants read access -> OK
+    if (completePath.isAbsolute()) {
+        const auto authorizedPath = completePath.validate(false);
+        if (!authorizedPath.isNull() && authorizedPath.exists()) {
+            return authorizedPath;
         }
     }
-    
-    for (auto provider : _providers) {
-      SGPath path = provider->resolve(aResource, aContext);
-      if (!path.isNull()) {
-        return path;
-      }
+
+    // Loop over the resource providers even if 'completePath' is absolute.
+    // For instance, when readProperties() processes 'include' attributes, it
+    // is expected that 'aResource' be interpreted relatively to 'aContext'
+    // or, if this doesn't lead to an existing file, a data path like
+    // $FG_ROOT. In the latter case, BasePathProvider will do the job.
+    for (const auto& provider : _providers) {
+        const SGPath path = provider->resolve(aResource, aContext);
+        if (!path.isNull()) {
+            return path;
+        }
     }
-    
+
     return SGPath();
 }