Update Android port

* java/org/gnu/emacs/EmacsNative.java: Declare ftruncate.

* java/org/gnu/emacs/EmacsSafThread.java (openDocument1): If
initially opening with rwt, verify the file descriptor is really
writable; if not, resort to rw and truncating the file descriptor
by hand instead.

* src/androidvfs.c (NATIVE_NAME (ftruncate)): New function.
Truncate file descriptor and return whether that was successful.

ChangeLog.android

@@ -1,11 +1,20 @@
 2023-08-06  Po Lu  <luangruo@yahoo.com>
 
+	* java/org/gnu/emacs/EmacsSafThread.java (openDocument1): If
+	initially opening with rwt, verify the file descriptor is really
+	writable; if not, resort to rw and truncating the file descriptor
+	by hand instead.
+
+	* src/androidvfs.c (NATIVE_NAME (ftruncate)): New function.
+	Truncate file descriptor and return whether that was successful.
+
 	* src/androidvfs.c (android_saf_tree_chmod): Repair file access
 	permissions allowed within FLAGS.
 
 2023-08-05  Po Lu  <luangruo@yahoo.com>
 
 	* doc/lispref/commands.texi (Touchscreen Events): Fix typo.
+
 	* lisp/subr.el (y-or-n-p): Don't call set-text-conversion-style
 	when not present.
 

java/org/gnu/emacs/EmacsNative.java

@@ -274,6 +274,10 @@ public static native void blitRect (Bitmap src, Bitmap dest, int x1,
      operations.  */
   public static native void safPostRequest ();
 
+  /* Detect and return FD is writable.  FD may be truncated to 0 bytes
+     in the process.  */
+  public static native boolean ftruncate (int fd);
+
   static
   {
     /* Older versions of Android cannot link correctly with shared

java/org/gnu/emacs/EmacsSafThread.java

@@ -24,6 +24,7 @@
 import java.util.Iterator;
 
 import java.io.FileNotFoundException;
+import java.io.IOException;
 
 import android.content.ContentResolver;
 import android.database.Cursor;
@@ -1597,6 +1598,42 @@ In addition, arbitrary runtime exceptions (such as
       = resolver.openFileDescriptor (documentUri, mode,
 				     signal);
 
+    /* If a writable file descriptor is requested and TRUNCATE is set,
+       then probe the file descriptor to detect if it is actually
+       readable.  If not, close this file descriptor and reopen it
+       with MODE set to rw; some document providers granting access to
+       Samba shares don't implement rwt, but these document providers
+       invariably truncate the file opened even when the mode is
+       merely rw.
+
+       This may be ascribed to a mix-up in Android's documentation
+       regardin DocumentsProvider: the `openDocument' function is only
+       documented to accept r or rw, whereas the default
+       implementation of the `openFile' function (which documents rwt)
+       delegates to `openDocument'.  */
+
+    if (write && truncate && fileDescriptor != null
+	&& !EmacsNative.ftruncate (fileDescriptor.getFd ()))
+      {
+	try
+	  {
+	    fileDescriptor.closeWithError ("File descriptor requested"
+					   + " is not writable");
+	  }
+	catch (IOException e)
+	  {
+	    Log.w (TAG, "Leaking unclosed file descriptor " + e);
+	  }
+
+	fileDescriptor
+	  = resolver.openFileDescriptor (documentUri, "rw", signal);
+
+	/* Try to truncate fileDescriptor just to stay on the safe
+	   side.  */
+	if (fileDescriptor != null)
+	  EmacsNative.ftruncate (fileDescriptor.getFd ());
+      }
+
     /* Every time a document is opened, remove it from the file status
        cache.  */
     toplevel = getCache (treeUri);

src/androidvfs.c

@@ -5605,7 +5605,7 @@ android_saf_file_open (struct android_vnode *vnode, int flags,
   /* Open a parcel file descriptor according to flags.  */
 
   method = service_class.open_document;
-  trunc  = flags & O_TRUNC;
+  trunc  = (flags & O_TRUNC);
   write  = ((flags & O_RDWR) == O_RDWR || (flags & O_WRONLY));
   inside_saf_critical_section = true;
   descriptor
@@ -6121,6 +6121,12 @@ NATIVE_NAME (safPostRequest) (JNIEnv *env, jobject object)
   sem_post (&saf_completion_sem);
 }
 
+JNIEXPORT jboolean JNICALL
+NATIVE_NAME (ftruncate) (JNIEnv *env, jobject object, jint fd)
+{
+  return ftruncate (fd, 0) != -1;
+}
+
 #ifdef __clang__
 #pragma clang diagnostic pop
 #else /* GNUC */