Add emacsclient desktop file equivalent on Android

* doc/emacs/android.texi (Android File System):
* java/AndroidManifest.xml.in: Update with new activity.  Remove
Android 10 restrictions through a special flag.

* java/org/gnu/emacs/EmacsNative.java (getProcName): New
function.
* java/org/gnu/emacs/EmacsOpenActivity.java (EmacsOpenActivity):
New file.
* java/org/gnu/emacs/EmacsService.java (getLibraryDirection):
Remove unused annotation.
* lib-src/emacsclient.c (decode_options): Set alt_display on
Android.
* src/android.c (android_proc_name): New function.
(NATIVE_NAME): Export via JNI.

7 files changed, 490 insertions, 13 deletions

diff --git a/doc/emacs/android.texi b/doc/emacs/android.texi
index cfdf77454eb..9dc6fddbb72 100644
--- a/doc/emacs/android.texi
+++ b/doc/emacs/android.texi
@@ -167,8 +167,8 @@ system settings.
   The external storage directory is found at @file{/sdcard}; the other
 directories are not found at any fixed location.
 
-@cindex file system limitations, Android 10
-  On Android 10 and later, the Android system restricts applications
+@cindex file system limitations, Android 11
+  On Android 11 and later, the Android system restricts applications
 from accessing files in the @file{/sdcard} directory using
 file-related system calls such as @code{open} and @code{readdir}.
 
@@ -177,16 +177,8 @@ makes the system more secure.  Unfortunately, it also means that Emacs
 cannot access files in those directories, despite holding the
 necessary permissions.  Thankfully, the Open Handset Alliance's
 version of Android allows this restriction to be disabled on a
-per-program basis; on Android 10, the corresponding option in the
-system settings panel is:
-
-@indentedblock
-System -> Developer Options -> App Compatibility Changes -> Emacs ->
-DEFAULT_SCOPED_STORAGE
-@end indentedblock
-
-  And on Android 11 and later, the corresponding option in the systems
-settings panel is:
+per-program basis; the corresponding option in the system settings
+panel is:
 
 @indentedblock
 System -> Apps -> Special App Access -> All files access -> Emacs
diff --git a/java/AndroidManifest.xml.in b/java/AndroidManifest.xml.in
index 544c87e1f1e..923c5a005d5 100644
--- a/java/AndroidManifest.xml.in
+++ b/java/AndroidManifest.xml.in
@@ -24,6 +24,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>. -->
           package="org.gnu.emacs"
           android:targetSandboxVersion="1"
           android:installLocation="auto"
+          android:requestLegacyExternalStorage="true"
           android:versionCode="@emacs_major_version@"
           android:versionName="@version@">
 
@@ -82,6 +83,84 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>. -->
       </intent-filter>
     </activity>
 
+    <activity android:name="org.gnu.emacs.EmacsOpenActivity"
+              android:exported="true">
+
+      <!-- Allow Emacs to open all kinds of files known to Android.  -->
+
+      <intent-filter>
+        <action android:name="android.intent.action.VIEW"/>
+        <action android:name="android.intent.action.EDIT"/>
+        <action android:name="android.intent.action.PICK"/>
+
+        <category android:name="android.intent.category.DEFAULT"/>
+
+        <data android:mimeType="image/aces"/>
+        <data android:mimeType="image/avci"/>
+        <data android:mimeType="image/avcs"/>
+        <data android:mimeType="image/avif"/>
+        <data android:mimeType="image/bmp"/>
+        <data android:mimeType="image/cgm"/>
+        <data android:mimeType="image/dicom-rle"/>
+        <data android:mimeType="image/dpx"/>
+        <data android:mimeType="image/emf"/>
+        <data android:mimeType="image/example"/>
+        <data android:mimeType="image/fits"/>
+        <data android:mimeType="image/g3fax"/>
+        <data android:mimeType="image/heic"/>
+        <data android:mimeType="image/heic-sequence"/>
+        <data android:mimeType="image/heif"/>
+        <data android:mimeType="image/heif-sequence"/>
+        <data android:mimeType="image/hej2k"/>
+        <data android:mimeType="image/hsj2"/>
+        <data android:mimeType="image/jls"/>
+        <data android:mimeType="image/jp2"/>
+        <data android:mimeType="image/jph"/>
+        <data android:mimeType="image/jphc"/>
+        <data android:mimeType="image/jpm"/>
+        <data android:mimeType="image/jpx"/>
+        <data android:mimeType="image/jxr"/>
+        <data android:mimeType="image/jxrA"/>
+        <data android:mimeType="image/jxrS"/>
+        <data android:mimeType="image/jxs"/>
+        <data android:mimeType="image/jxsc"/>
+        <data android:mimeType="image/jxsi"/>
+        <data android:mimeType="image/jxss"/>
+        <data android:mimeType="image/ktx"/>
+        <data android:mimeType="image/ktx2"/>
+        <data android:mimeType="image/naplps"/>
+        <data android:mimeType="image/png"/>
+        <data android:mimeType="image/prs.btif"/>
+        <data android:mimeType="image/prs.pti"/>
+        <data android:mimeType="image/pwg-raster"/>
+        <data android:mimeType="image/svg+xml"/>
+        <data android:mimeType="image/t38"/>
+        <data android:mimeType="image/tiff"/>
+        <data android:mimeType="image/tiff-fx"/>
+        <data android:mimeType="text/*"/>
+        <data android:mimeType="application/*xml"/>
+        <data android:mimeType="application/atom+xml"/>
+        <data android:mimeType="application/dxf"/>
+        <data android:mimeType="application/ecmascript"/>
+        <data android:mimeType="application/javascript"/>
+        <data android:mimeType="application/json"/>
+        <data android:mimeType="application/*log*"/>
+        <data android:mimeType="application/octet-stream"/>
+        <data android:mimeType="application/soap+xm"/>
+        <data android:mimeType="application/x-caramel"/>
+        <data android:mimeType="application/x-klaunch"/>
+        <data android:mimeType="application/x-latex"/>
+        <data android:mimeType="application/x-sh"/>
+        <data android:mimeType="application/x-tcl"/>
+        <data android:mimeType="application/x-tex*"/>
+        <data android:mimeType="application/x-troff*"/>
+        <data android:mimeType="application/xhtml+xml"/>
+        <data android:mimeType="application/xml*"/>
+        <data android:mimeType="application/zip"/>
+        <data android:mimeType="application/x-zip-compressed"/>
+      </intent-filter>
+    </activity>
+
     <activity android:name="org.gnu.emacs.EmacsMultitaskActivity"
               android:windowSoftInputMode="adjustResize"
               android:exported="true"
diff --git a/java/org/gnu/emacs/EmacsNative.java b/java/org/gnu/emacs/EmacsNative.java
index 4e91a7be322..aba356051cd 100644
--- a/java/org/gnu/emacs/EmacsNative.java
+++ b/java/org/gnu/emacs/EmacsNative.java
@@ -153,6 +153,10 @@ public class EmacsNative
   public static native long sendExpose (short window, int x, int y,
                                         int width, int height);
 
+  /* Return the file name associated with the specified file
+     descriptor, or NULL if there is none.  */
+  public static native byte[] getProcName (int fd);
+
   static
   {
     System.loadLibrary ("emacs");
diff --git a/java/org/gnu/emacs/EmacsOpenActivity.java b/java/org/gnu/emacs/EmacsOpenActivity.java
new file mode 100644
index 00000000000..268a9abd7b1
--- /dev/null
+++ b/java/org/gnu/emacs/EmacsOpenActivity.java
@@ -0,0 +1,357 @@
+/* Communication module for Android terminals.  -*- c-file-style: "GNU" -*-
+
+Copyright (C) 2023 Free Software Foundation, Inc.
+
+This file is part of GNU Emacs.
+
+GNU Emacs is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or (at
+your option) any later version.
+
+GNU Emacs is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
+
+package org.gnu.emacs;
+
+/* This class makes the Emacs server work reasonably on Android.
+
+   There is no way to make the Unix socket publicly available on
+   Android.
+
+   Instead, this activity tries to connect to the Emacs server, to
+   make it open files the system asks Emacs to open, and to emulate
+   some reasonable behavior when Emacs has not yet started.
+
+   First, Emacs registers itself as an application that can open text
+   and image files.
+
+   Then, when the user is asked to open a file and selects ``Emacs''
+   as the application that will open the file, the system pops up a
+   window, this activity, and calls the `onCreate' function.
+
+   `onCreate' then tries very to find the file name of the file that
+   was selected, and give it to emacsclient.
+
+   If emacsclient successfully opens the file, then this activity
+   starts EmacsActivity (to bring it on to the screen); otherwise, it
+   displays the output of emacsclient or any error message that occurs
+   and exits.  */
+
+import android.app.AlertDialog;
+import android.app.Activity;
+
+import android.content.Context;
+import android.content.ContentResolver;
+import android.content.DialogInterface;
+import android.content.Intent;
+
+import android.net.Uri;
+
+import android.os.Build;
+import android.os.Bundle;
+import android.os.ParcelFileDescriptor;
+
+import java.io.File;
+import java.io.FileReader;
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.UnsupportedEncodingException;
+
+public class EmacsOpenActivity extends Activity
+  implements DialogInterface.OnClickListener
+{
+  private class EmacsClientThread extends Thread
+  {
+    private ProcessBuilder builder;
+
+    public
+    EmacsClientThread (ProcessBuilder processBuilder)
+    {
+      builder = processBuilder;
+    }
+
+    @Override
+    public void
+    run ()
+    {
+      Process process;
+      InputStream error;
+      String errorText;
+
+      try
+        {
+          /* Start emacsclient.  */
+          process = builder.start ();
+          process.waitFor ();
+
+          /* Now figure out whether or not starting the process was
+             successful.  */
+          if (process.exitValue () == 0)
+            finishSuccess ();
+          else
+            finishFailure ("Error opening file", null);
+        }
+      catch (IOException exception)
+        {
+          finishFailure ("Internal error", exception.toString ());
+        }
+      catch (InterruptedException exception)
+        {
+          finishFailure ("Internal error", exception.toString ());
+        }
+    }
+  }
+
+  @Override
+  public void
+  onClick (DialogInterface dialog, int which)
+  {
+    finish ();
+  }
+
+  public String
+  readEmacsClientLog ()
+  {
+    File file, cache;
+    FileReader reader;
+    char[] buffer;
+    int rc;
+    String what;
+
+    cache = getCacheDir ();
+    file = new File (cache, "emacsclient.log");
+    what = "";
+
+    try
+      {
+        reader = new FileReader (file);
+        buffer = new char[2048];
+
+        while ((rc = reader.read (buffer, 0, 2048)) != -1)
+          what += String.valueOf (buffer, 0, 2048);
+
+        reader.close ();
+        return what;
+      }
+    catch (IOException exception)
+      {
+        return ("Couldn't read emacsclient.log: "
+                + exception.toString ());
+      }
+  }
+
+  private void
+  displayFailureDialog (String title, String text)
+  {
+    AlertDialog.Builder builder;
+    AlertDialog dialog;
+
+    builder = new AlertDialog.Builder (this);
+    dialog = builder.create ();
+    dialog.setTitle (title);
+
+    if (text == null)
+      /* Read in emacsclient.log instead.  */
+      text = readEmacsClientLog ();
+
+    dialog.setMessage (text);
+    dialog.setButton (DialogInterface.BUTTON_POSITIVE, "OK", this);
+    dialog.show ();
+  }
+
+  /* Finish this activity in response to emacsclient having
+     successfully opened a file.
+
+     In the main thread, close this window, and open a window
+     belonging to an Emacs frame.  */
+
+  public void
+  finishSuccess ()
+  {
+    runOnUiThread (new Runnable () {
+        @Override
+        public void
+        run ()
+        {
+          Intent intent;
+
+          intent = new Intent (EmacsOpenActivity.this,
+                               EmacsActivity.class);
+          intent.addFlags (Intent.FLAG_ACTIVITY_NEW_TASK);
+          startActivity (intent);
+
+          EmacsOpenActivity.this.finish ();
+        }
+      });
+  }
+
+  /* Finish this activity after displaying a dialog associated with
+     failure to open a file.
+
+     Use TITLE as the title of the dialog.  If TEXT is non-NULL,
+     display that text in the dialog.  Otherwise, use the contents of
+     emacsclient.log in the cache directory instead.  */
+
+  public void
+  finishFailure (final String title, final String text)
+  {
+    runOnUiThread (new Runnable () {
+        @Override
+        public void
+        run ()
+        {
+          displayFailureDialog (title, text);
+        }
+      });
+  }
+
+  public String
+  getLibraryDirectory ()
+  {
+    int apiLevel;
+    Context context;
+
+    context = getApplicationContext ();
+    apiLevel = Build.VERSION.SDK_INT;
+
+    if (apiLevel >= Build.VERSION_CODES.GINGERBREAD)
+      return context.getApplicationInfo().nativeLibraryDir;
+    else if (apiLevel >= Build.VERSION_CODES.DONUT)
+      return context.getApplicationInfo().dataDir + "/lib";
+
+    return "/data/data/" + context.getPackageName() + "/lib";
+  }
+
+  public void
+  startEmacsClient (String fileName)
+  {
+    String libDir;
+    ProcessBuilder builder;
+    Process process;
+    EmacsClientThread thread;
+    File file;
+
+    file = new File (getCacheDir (), "emacsclient.log");
+
+    libDir = getLibraryDirectory ();
+    builder = new ProcessBuilder (libDir + "/libemacsclient.so",
+                                  fileName, "--reuse-frame",
+                                  "--timeout=10", "--no-wait");
+
+    /* Redirect standard error to a file so that errors can be
+       meaningfully reported.  */
+
+    if (file.exists ())
+      file.delete ();
+
+    builder.redirectError (file);
+
+    /* Track process output in a new thread, since this is the UI
+       thread and doing so here can cause deadlocks when EmacsService
+       decides to wait for something.  */
+
+    thread = new EmacsClientThread (builder);
+    thread.start ();
+  }
+
+  @Override
+  public void
+  onCreate (Bundle savedInstanceState)
+  {
+    String action, fileName;
+    Intent intent;
+    Uri uri;
+    ContentResolver resolver;
+    ParcelFileDescriptor fd;
+    byte[] names;
+    String errorBlurb;
+
+    super.onCreate (savedInstanceState);
+
+    /* Obtain the intent that started Emacs.  */
+    intent = getIntent ();
+    action = intent.getAction ();
+
+    if (action == null)
+      {
+        finish ();
+        return;
+      }
+
+    /* Now see if the action specified is supported by Emacs.  */
+
+    if (action.equals ("android.intent.action.VIEW")
+        || action.equals ("android.intent.action.EDIT")
+        || action.equals ("android.intent.action.PICK"))
+      {
+        /* Obtain the URI of the action.  */
+        uri = intent.getData ();
+
+        if (uri == null)
+          {
+            finish ();
+            return;
+          }
+
+        /* Now, try to get the file name.  */
+
+        if (uri.getScheme ().equals ("file"))
+          fileName = uri.getPath ();
+        else
+          {
+            fileName = null;
+
+            if (uri.getScheme ().equals ("content"))
+              {
+                /* This is one of the annoying Android ``content''
+                   URIs.  Most of the time, there is actually an
+                   underlying file, but it cannot be found without
+                   opening the file and doing readlink on its file
+                   descriptor in /proc/self/fd.  */
+                resolver = getContentResolver ();
+
+                try
+                  {
+                    fd = resolver.openFileDescriptor (uri, "r");
+                    names = EmacsNative.getProcName (fd.getFd ());
+                    fd.close ();
+
+                    /* What is the right encoding here? */
+
+                    if (names != null)
+                      fileName = new String (names, "UTF-8");
+                  }
+                catch (FileNotFoundException exception)
+                  {
+                    /* Do nothing.  */
+                  }
+                catch (IOException exception)
+                  {
+                    /* Do nothing.  */
+                  }
+              }
+
+            if (fileName == null)
+              {
+                errorBlurb = ("The URI: " + uri + " could not be opened"
+                              + ", as it does not encode file name inform"
+                              + "ation.");
+                displayFailureDialog ("Error opening file", errorBlurb);
+                return;
+              }
+          }
+
+        /* And start emacsclient.  */
+        startEmacsClient (fileName);
+      }
+    else
+      finish ();
+  }
+}
diff --git a/java/org/gnu/emacs/EmacsService.java b/java/org/gnu/emacs/EmacsService.java
index d17f6d1286c..2ec2ddf9bda 100644
--- a/java/org/gnu/emacs/EmacsService.java
+++ b/java/org/gnu/emacs/EmacsService.java
@@ -152,7 +152,6 @@ public class EmacsService extends Service
       }
   }
 
-  @TargetApi (Build.VERSION_CODES.GINGERBREAD)
   private String
   getLibraryDirectory ()
   {
diff --git a/lib-src/emacsclient.c b/lib-src/emacsclient.c
index 698bf9b50ae..a72fced1bf2 100644
--- a/lib-src/emacsclient.c
+++ b/lib-src/emacsclient.c
@@ -626,6 +626,8 @@ decode_options (int argc, char **argv)
       alt_display = "w32";
 #elif defined (HAVE_HAIKU)
       alt_display = "be";
+#elif defined (HAVE_ANDROID)
+      alt_display = "android";
 #endif
 
 #ifdef HAVE_PGTK
diff --git a/src/android.c b/src/android.c
index 57a95bcd4f9..a0e64471a05 100644
--- a/src/android.c
+++ b/src/android.c
@@ -1369,6 +1369,27 @@ android_get_home_directory (void)
   return android_files_dir;
 }
 
+/* Return the name of the file behind a file descriptor FD by reading
+   /proc/self/fd/.  Place the name in BUFFER, which should be able to
+   hold size bytes.  Value is 0 upon success, and 1 upon failure.  */
+
+static int
+android_proc_name (int fd, char *buffer, size_t size)
+{
+  char format[sizeof "/proc/self/fd/"
+              + INT_STRLEN_BOUND (int)];
+  ssize_t read;
+
+  sprintf (format, "/proc/self/fd/%d", fd);
+  read = readlink (format, buffer, size - 1);
+
+  if (read == -1)
+    return 1;
+
+  buffer[read] = '\0';
+  return 0;
+}
+
 
 
 /* JNI functions called by Java.  */
@@ -1598,6 +1619,29 @@ NATIVE_NAME (setEmacsParams) (JNIEnv *env, jobject object,
      now.  */
 }
 
+JNIEXPORT jobject JNICALL
+NATIVE_NAME (getProcName) (JNIEnv *env, jobject object, jint fd)
+{
+  char buffer[PATH_MAX + 1];
+  size_t length;
+  jbyteArray array;
+
+  if (android_proc_name (fd, buffer, PATH_MAX + 1))
+    return NULL;
+
+  /* Return a byte array, as Java strings cannot always encode file
+     names.  */
+  length = strlen (buffer);
+  array = (*env)->NewByteArray (env, length);
+  if (!array)
+    return NULL;
+
+  (*env)->SetByteArrayRegion (env, array, 0, length,
+                              (jbyte *) buffer);
+
+  return array;
+}
+
 /* Initialize service_class, aborting if something goes wrong.  */
 
 static void