Be really explicit about needing to check for negative error codes with SDL_RWread()

docs/README-migration.md

@@ -531,35 +531,39 @@ Previously they looked more like stdio:
 
 ```c
 size_t SDL_RWread(SDL_RWops *context, void *ptr, size_t size, size_t maxnum);
+size_t SDL_RWwrite(SDL_RWops *context, const void *ptr, size_t size, size_t maxnum);
 ```
 
 But now they look more like POSIX:
 
 ```c
 Sint64 SDL_RWread(SDL_RWops *context, void *ptr, Sint64 size);
+Sint64 SDL_RWwrite(SDL_RWops *context, const void *ptr, Sint64 size);
 ```
 
-Previously they tried to read/write `size` objects of `maxnum` bytes each. Now they try to read/write `size` bytes, which solves concerns about what should happen to the file pointer if only a fraction of an object could be read, etc. The return value is different, too. For reading:
+SDL_RWread() previously returned 0 at end of file or other error. Now it returns the number of bytes read, 0 for end of file, -1 for another error, or -2 for data not ready (in the case of a non-blocking context).
 
-- SDL_RWread returns the number of bytes read, which will be less than requested on error or EOF.
-- If there was an error but some bytes were read, it will return the number of bytes read.
-- On error when no bytes were read, it returns -1.
-- For non-blocking RWops (new to SDL3!), if we are neither at an error or EOF but it would require blocking to read more data, it returns -2.
-
-For writing:
-
-- SDL_RWwrite returns the number of bytes written, which might be less on error or if the RWops is non-blocking.
-- If there was an error but some bytes were written, it will return the number of bytes written.
-- On error when no bytes were written, it returns -1.
-- For non-blocking RWops (new to SDL3!), if we are not at an error but it would require blocking to write more data, it returns -2.
-
-
-As you can see, RWops can now be non-blocking! There is no API in SDL to
-toggle a RWops to (non-)blocking mode, they must be created as such. The
-existing SDL_RWFrom* functions do not create non-blocking objects, so existing
-code (and much of the code you would care to write by default) does not have
-to contend with this behavior.
+Code that used to look like this:
+```
+    if (!SDL_RWread(context, ptr, size, 1)) {
+        ... handle error
+    }
+```
+should be changed to:
+```
+    if (SDL_RWread(context, ptr, size) != size) {
+        ... handle error
+    }
+```
+or, if you're using a custom non-blocking context or are handling variable size data:
+```
+    Sint64 amount = SDL_RWread(context, ptr, maxsize);
+    if (amount < 0) {
+        ... handle error
+    }
+```
 
+Similarly, SDL_RWwrite() can return -2 for data not ready in the case of a non-blocking context. There is currently no way to create a non-blocking context, we have simply defined the semantic for your own custom SDL_RWops object.
 
 SDL_RWFromFP has been removed from the API, due to issues when the SDL library uses a different C runtime from the application.
 

include/SDL3/SDL_rwops.h

@@ -434,7 +434,7 @@ extern DECLSPEC Sint64 SDLCALL SDL_RWtell(SDL_RWops *context);
  * \param context a pointer to an SDL_RWops structure
  * \param ptr a pointer to a buffer to read data into
  * \param size the number of bytes to read from the data source.
- * \returns the number of bytes read, or 0 at end of file, or -1 on error.
+ * \returns the number of bytes read, 0 at end of file, -1 on error, and -2 for data not ready with a non-blocking context.
  *
  * \since This function is available since SDL 3.0.0.
  *
@@ -445,8 +445,7 @@ extern DECLSPEC Sint64 SDLCALL SDL_RWtell(SDL_RWops *context);
  * \sa SDL_RWseek
  * \sa SDL_RWwrite
  */
-extern DECLSPEC Sint64 SDLCALL SDL_RWread(SDL_RWops *context,
-                                          void *ptr, Sint64 size);
+extern DECLSPEC Sint64 SDLCALL SDL_RWread(SDL_RWops *context, void *ptr, Sint64 size);
 
 /**
  * Write to an SDL_RWops data stream.