Offer to grant storage permissions if absent

* java/org/gnu/emacs/EmacsService.java (externalStorageAvailable)
(requestStorageAccess23, requestStorageAccess30)
(requestStorageAccess): New functions.

* lisp/startup.el (fancy-startup-tail, normal-splash-screen):
Call android-win functions for inserting the new storage
permission notice.

* lisp/term/android-win.el
(android-display-storage-permission-popup)
(android-after-splash-screen): New functions.

* src/android.c (android_init_emacs_service): Link to new Java
functions.
(android_external_storage_available_p)
(android_request_storage_access): New functions.

* src/android.h: Update prototypes.

* src/androidfns.c (Fandroid_external_storage_available_p)
(Fandroid_request_storage_access): New functions.
(syms_of_androidfns): Register new subrs.

3 files changed, 97 insertions, 0 deletions

diff --git a/src/android.c b/src/android.c
index e116426ca05..7ca5eab817c 100644
--- a/src/android.c
+++ b/src/android.c
@@ -1628,6 +1628,10 @@ android_init_emacs_service (void)
                "Ljava/lang/String;)Ljava/lang/String;");
   FIND_METHOD (valid_authority, "validAuthority",
                "(Ljava/lang/String;)Z");
+  FIND_METHOD (external_storage_available,
+               "externalStorageAvailable", "()Z");
+  FIND_METHOD (request_storage_access,
+               "requestStorageAccess", "()V");
 #undef FIND_METHOD
 }
 
@@ -6558,6 +6562,57 @@ android_request_directory_access (void)
   return rc;
 }
 
+/* Return whether Emacs is entitled to access external storage.
+
+   On Android 5.1 and earlier, such permissions as are declared within
+   an application's manifest are granted during installation and are
+   irrevocable.
+
+   On Android 6.0 through Android 10.0, the right to read external
+   storage is a regular permission granted from the Permissions
+   panel.
+
+   On Android 11.0 and later, that right must be granted through an
+   independent ``Special App Access'' settings panel.  */
+
+bool
+android_external_storage_available_p (void)
+{
+  jboolean rc;
+  jmethodID method;
+
+  if (android_api_level <= 22) /* LOLLIPOP_MR1 */
+    return true;
+
+  method = service_class.external_storage_available;
+  rc = (*android_java_env)->CallNonvirtualBooleanMethod (android_java_env,
+                                                         emacs_service,
+                                                         service_class.class,
+                                                         method);
+  android_exception_check ();
+
+  return rc;
+}
+
+/* Display a dialog from which the aforementioned rights can be
+   granted.  */
+
+void
+android_request_storage_access (void)
+{
+  jmethodID method;
+
+  if (android_api_level <= 22) /* LOLLIPOP_MR1 */
+    return;
+
+  method = service_class.request_storage_access;
+  (*android_java_env)->CallNonvirtualVoidMethod (android_java_env,
+                                                 emacs_service,
+                                                 service_class.class,
+                                                 method);
+  android_exception_check ();
+}
+
 
 
 /* The thread from which a query against a thread is currently being
diff --git a/src/android.h b/src/android.h
index 28d9d25930e..12f9472836f 100644
--- a/src/android.h
+++ b/src/android.h
@@ -123,6 +123,8 @@ extern void android_wait_event (void);
 extern void android_toggle_on_screen_keyboard (android_window, bool);
 extern _Noreturn void android_restart_emacs (void);
 extern int android_request_directory_access (void);
+extern bool android_external_storage_available_p (void);
+extern void android_request_storage_access (void);
 extern int android_get_current_api_level (void)
   __attribute__ ((pure));
 
@@ -289,6 +291,8 @@ struct android_emacs_service
   jmethodID rename_document;
   jmethodID move_document;
   jmethodID valid_authority;
+  jmethodID external_storage_available;
+  jmethodID request_storage_access;
 };
 
 extern JNIEnv *android_java_env;
diff --git a/src/androidfns.c b/src/androidfns.c
index 772a4f51e78..785587d9282 100644
--- a/src/androidfns.c
+++ b/src/androidfns.c
@@ -3096,6 +3096,42 @@ within the directory `/content/storage'.  */)
 
 
 
+/* Functions concerning storage permissions.  */
+
+DEFUN ("android-external-storage-available-p",
+       Fandroid_external_storage_available_p,
+       Sandroid_external_storage_available_p, 0, 0, 0,
+       doc: /* Return whether Emacs is entitled to access external storage.
+Return nil if the requisite permissions for external storage access
+have not been granted to Emacs, t otherwise.  Such permissions can be
+requested by means of the `android-request-storage-access'
+command.
+
+External storage on Android encompasses the `/sdcard' and
+`/storage/emulated' directories, access to which is denied to programs
+absent these permissions.  */)
+  (void)
+{
+  return android_external_storage_available_p () ? Qt : Qnil;
+}
+
+DEFUN ("android-request-storage-access", Fandroid_request_storage_access,
+       Sandroid_request_storage_access, 0, 0, "",
+       doc: /* Request rights to access external storage.
+
+Return nil whether access is accorded or not, immediately subsequent
+to displaying the permissions request dialog.
+
+`android-external-storage-available-p' (which see) ascertains if Emacs
+has received such rights.  */)
+  (void)
+{
+  android_request_storage_access ();
+  return Qnil;
+}
+
+
+
 /* Miscellaneous input method related stuff.  */
 
 /* Report X, Y, by the phys cursor width and height as the cursor
@@ -3302,6 +3338,8 @@ bell being rung.  */);
 #ifndef ANDROID_STUBIFY
   defsubr (&Sandroid_query_battery);
   defsubr (&Sandroid_request_directory_access);
+  defsubr (&Sandroid_external_storage_available_p);
+  defsubr (&Sandroid_request_storage_access);
 
   tip_timer = Qnil;
   staticpro (&tip_timer);