2 files changed, 41 insertions, 0 deletions

diff --git a/java/org/gnu/emacs/EmacsNative.java b/java/org/gnu/emacs/EmacsNative.java
index 7d72a9f192e..fae0ba98f86 100644
--- a/java/org/gnu/emacs/EmacsNative.java
+++ b/java/org/gnu/emacs/EmacsNative.java
@@ -274,6 +274,10 @@ public final class EmacsNative
      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
diff --git a/java/org/gnu/emacs/EmacsSafThread.java b/java/org/gnu/emacs/EmacsSafThread.java
index 29cd3fa6bc7..3ae3c0839ce 100644
--- a/java/org/gnu/emacs/EmacsSafThread.java
+++ b/java/org/gnu/emacs/EmacsSafThread.java
@@ -24,6 +24,7 @@ import java.util.HashMap;
 import java.util.Iterator;
 
 import java.io.FileNotFoundException;
+import java.io.IOException;
 
 import android.content.ContentResolver;
 import android.database.Cursor;
@@ -1597,6 +1598,42 @@ public final class EmacsSafThread extends HandlerThread
       = 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);