Old env vars as fallback for SDL_VIDEO_DRIVER + SDL_AUDIO_DRIVER #11115

especially SDL_VIDEODRIVER is commonly used to use the native Wayland
backend, so I think it's a good idea to keep supporting the old name
instead of forcing users to find out that they now have to add an
underscore..
Not sure how popular SDL_AUDIODRIVER is, but with all the audio backends
that exist on Linux alone I'm sure some people use it to work around
sound issues.

Note: Doing this in the SDL_hints implementation instead of the
call-sites of SDL_GetHint(SDL_HINT_VIDEO_DRIVER) etc ensures that
1. Hint priorities work (env var overriding hint set by application with normal
   priority, but not when application used SDL_HINT_OVERRIDE)
2. SDL_ResetHint() (called by user code) respects the fallback
   environment variable

docs/README-migration.md

@@ -798,7 +798,7 @@ The following functions have been removed:
 
 Calling SDL_GetHint() with the name of the hint being changed from within a hint callback will now return the new value rather than the old value. The old value is still passed as a parameter to the hint callback.
 
-The environment variables SDL_VIDEODRIVER and SDL_AUDIODRIVER have been renamed to SDL_VIDEO_DRIVER and SDL_AUDIO_DRIVER.
+The environment variables SDL_VIDEODRIVER and SDL_AUDIODRIVER have been renamed to SDL_VIDEO_DRIVER and SDL_AUDIO_DRIVER, but the old names are still supported as a fallback.
 
 The environment variables SDL_VIDEO_X11_WMCLASS and SDL_VIDEO_WAYLAND_WMCLASS have been removed and replaced by either using the appindentifier param to SDL_SetAppMetadata() or setting SDL_PROP_APP_METADATA_IDENTIFIER_STRING with SDL_SetAppMetadataProperty()
 

src/SDL_hints.c

@@ -83,13 +83,28 @@ static void SDLCALL CleanupHintProperty(void *userdata, void *value)
     SDL_free(hint);
 }
 
+static const char* GetHintEnvironmentVariable(const char *name)
+{
+    const char *result = SDL_getenv(name);
+    if (!result && name && *name) {
+        // fall back to old (SDL2) names of environment variables that
+        // are important to users (e.g. many use SDL_VIDEODRIVER=wayland)
+        if (SDL_strcmp(name, SDL_HINT_VIDEO_DRIVER) == 0) {
+            result = SDL_getenv("SDL_VIDEODRIVER");
+        } else if (SDL_strcmp(name, SDL_HINT_AUDIO_DRIVER) == 0) {
+            result = SDL_getenv("SDL_AUDIODRIVER");
+        }
+    }
+    return result;
+}
+
 bool SDL_SetHintWithPriority(const char *name, const char *value, SDL_HintPriority priority)
 {
     if (!name || !*name) {
         return SDL_InvalidParamError("name");
     }
 
-    const char *env = SDL_getenv(name);
+    const char *env = GetHintEnvironmentVariable(name);
     if (env && (priority < SDL_HINT_OVERRIDE)) {
         return SDL_SetError("An environment variable is taking priority");
     }
@@ -143,7 +158,7 @@ bool SDL_ResetHint(const char *name)
         return SDL_InvalidParamError("name");
     }
 
-    const char *env = SDL_getenv(name);
+    const char *env = GetHintEnvironmentVariable(name);
 
     const SDL_PropertiesID hints = GetHintProperties(false);
     if (!hints) {
@@ -182,7 +197,7 @@ static void SDLCALL ResetHintsCallback(void *userdata, SDL_PropertiesID hints, c
         return;  // uh...okay.
     }
 
-    const char *env = SDL_getenv(name);
+    const char *env = GetHintEnvironmentVariable(name);
     if ((!env && hint->value) || (env && !hint->value) || (env && SDL_strcmp(env, hint->value) != 0)) {
         SDL_HintWatch *entry = hint->callbacks;
         while (entry) {
@@ -213,7 +228,7 @@ const char *SDL_GetHint(const char *name)
         return NULL;
     }
 
-    const char *result = SDL_getenv(name);
+    const char *result = GetHintEnvironmentVariable(name);
 
     const SDL_PropertiesID hints = GetHintProperties(false);
     if (hints) {