Replace shadowed android.system with custom androidx.system package

So that we don't shadow framework classes in our binary.
Update buffer position in OsCompat.write (fixed in Lollipop MR1).
Synthesize InterruptedIOException in Kitkat like in Lollipop.
OsCompat never throws EINTR errno.

Author: consp1racy

config/checkstyle/checkstyle.xml

@@ -154,10 +154,4 @@ page at http://checkstyle.sourceforge.net/config.html -->
     <module name="UpperEll" />
   </module>
 
-  <!-- Excludes android.system polyfill.                  -->
-  <!-- See https://checkstyle.org/config_filefilters.html -->
-  <module name="BeforeExecutionExclusionFileFilter">
-    <property name="fileNamePattern" value=".*[\\/]java[\\/]android[\\/]system[\\/].*$" />
-  </module>
-
 </module>

libcore/src/main/java/libcore/io/Os.java

@@ -19,7 +19,6 @@
 import java.nio.ByteBuffer;
 
 public interface Os {
-    public String strerror(int errno);
-
-    public int write(FileDescriptor fd, ByteBuffer buffer) throws ErrnoException;
+    String strerror(int errno);
+    int write(FileDescriptor fd, ByteBuffer buffer) throws ErrnoException;
 }

os-compat/build.gradle

@@ -0,0 +1,24 @@
+apply plugin: 'com.android.library'
+
+android {
+    compileSdkVersion 31
+    defaultConfig {
+        minSdkVersion 19
+        targetSdkVersion 31
+    }
+    androidResources {
+        namespace = "androidx.system"
+    }
+    buildFeatures {
+        aidl = false
+        androidResources = false
+        buildConfig = false
+    }
+}
+
+dependencies {
+    compileOnly rootProject.fileTree("thirdparty/androidx/annotation/1.3.0/annotation-1.3.0.jar")
+    compileOnly project(':libcore')
+}
+
+apply from: "$project.rootDir/config/android-checkstyle.gradle"

server/src/main/java/android/system/ErrnoException.java renamed to os-compat/src/main/java/androidx/system/ErrnoException.java

@@ -13,30 +13,19 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
-package android.system;
-
-import android.annotation.TargetApi;
+package androidx.system;
 
 import java.io.IOException;
 import java.net.SocketException;
 
-import libcore.io.Libcore;
-
 /**
- * A checked exception thrown when {@link Os} methods fail. This exception contains the native
- * errno value, for comparison against the constants in {@link OsConstants}, should sophisticated
+ * A checked exception thrown when {@link OsCompat} methods fail. This exception contains the native
+ * errno value, for comparison against the constants in {@link OsConstantsCompat}, should sophisticated
  * callers need to adjust their behavior based on the exact failure.
  */
-@SuppressWarnings("unused")
-@TargetApi(21)
 public final class ErrnoException extends Exception {
     private final String functionName;
-
-    /**
-     * The errno value, for comparison with the {@code E} constants in {@link OsConstants}.
-     */
-    public final int errno;
+    private final int errno;
 
     /**
      * Constructs an instance with the given function name and errno value.
@@ -55,17 +44,24 @@ public ErrnoException(String functionName, int errno, Throwable cause) {
         this.errno = errno;
     }
 
+    /**
+     * The errno value, for comparison with the {@code E} constants in {@link OsConstantsCompat}.
+     */
+    public int getErrno() {
+        return errno;
+    }
+
     /**
      * Converts the stashed function name and errno value to a human-readable string.
      * We do this here rather than in the constructor so that callers only pay for
      * this if they need it.
      */
     @Override public String getMessage() {
-        String errnoName = OsConstants.errnoName(errno);
+        String errnoName = OsConstantsCompat.errnoName(errno);
         if (errnoName == null) {
             errnoName = "errno " + errno;
         }
-        String description = Libcore.os.strerror(errno);
+        String description = OsCompat.strerror(errno);
         return functionName + " failed: " + errnoName + " (" + description + ")";
     }
 
@@ -77,9 +73,7 @@ public ErrnoException(String functionName, int errno, Throwable cause) {
      * {@code throw e.rethrowAsIOException()} to make that clear to the compiler.
      */
     public IOException rethrowAsIOException() throws IOException {
-        IOException newException = new IOException(getMessage());
-        newException.initCause(this);
-        throw newException;
+        throw new IOException(getMessage(), this);
     }
 
     /**

os-compat/src/main/java/androidx/system/Os.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2011 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package androidx.system;
+
+import java.io.FileDescriptor;
+import java.io.InterruptedIOException;
+import java.nio.ByteBuffer;
+
+interface Os {
+    String strerror(int errno);
+
+    int write(FileDescriptor fd, ByteBuffer buffer) throws ErrnoException, InterruptedIOException;
+
+    // https://android.googlesource.com/platform/libcore/+/lollipop-mr1-release/luni/src/main/java/libcore/io/Posix.java#253
+    static void maybeUpdateBufferPosition(ByteBuffer buffer, int originalPosition, int bytesReadOrWritten) {
+        if (bytesReadOrWritten > 0) {
+            buffer.position(bytesReadOrWritten + originalPosition);
+        }
+    }
+}

os-compat/src/main/java/androidx/system/OsApi21.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright (C) 2011 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package androidx.system;
+
+import androidx.annotation.RequiresApi;
+
+import java.io.FileDescriptor;
+import java.io.InterruptedIOException;
+import java.nio.ByteBuffer;
+
+import static android.os.Build.VERSION.SDK_INT;
+import static androidx.system.Os.maybeUpdateBufferPosition;
+
+@RequiresApi(21)
+final class OsApi21 implements Os {
+    @Override
+    public String strerror(int errno) {
+        return android.system.Os.strerror(errno);
+    }
+
+    @Override
+    public int write(FileDescriptor fd, ByteBuffer buffer) throws ErrnoException, InterruptedIOException {
+        try {
+            final int position = buffer.position();
+            final int bytesWritten = android.system.Os.write(fd, buffer);
+            if (SDK_INT < 22) {
+                maybeUpdateBufferPosition(buffer, position, bytesWritten);
+            }
+            return bytesWritten;
+        } catch (android.system.ErrnoException e) {
+            throw new ErrnoException("write", e.errno);
+        }
+    }
+}

server/src/main/java/android/system/Os.java renamed to os-compat/src/main/java/androidx/system/OsCompat.java

@@ -13,41 +13,46 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
-package android.system;
-
-import android.annotation.TargetApi;
+package androidx.system;
 
 import java.io.FileDescriptor;
 import java.io.InterruptedIOException;
-import java.net.InetAddress;
-import java.net.InetSocketAddress;
-import java.net.SocketAddress;
-import java.net.SocketException;
 import java.nio.ByteBuffer;
 
-import libcore.io.Libcore;
+import static android.os.Build.VERSION.SDK_INT;
 
 /**
  * Access to low-level system functionality. Most of these are system calls. Most users will want
  * to use higher-level APIs where available, but this class provides access to the underlying
  * primitives used to implement the higher-level APIs.
  *
- * <p>The corresponding constants can be found in {@link OsConstants}.
+ * <p>The corresponding constants can be found in {@link OsConstantsCompat}.
  */
-@SuppressWarnings("unused")
-@TargetApi(21)
-public final class Os {
-  private Os() {}
+public final class OsCompat {
+    private OsCompat() {
+    }
+
+    private static final Os IMPL;
+
+    static {
+        if (SDK_INT >= 21) {
+            IMPL = new OsApi21();
+        } else {
+            IMPL = new OsLibcore();
+        }
+    }
+
+    /**
+     * See <a href="http://man7.org/linux/man-pages/man3/strerror.3.html">strerror(2)</a>.
+     */
+    public static String strerror(int errno) {
+        return IMPL.strerror(errno);
+    }
 
-  /**
-   * See <a href="http://man7.org/linux/man-pages/man2/write.2.html">write(2)</a>.
-   */
-  public static int write(FileDescriptor fd, ByteBuffer buffer) throws ErrnoException, InterruptedIOException {
-    try {
-      return Libcore.os.write(fd, buffer);
-    } catch (libcore.io.ErrnoException e) {
-      throw new ErrnoException("write", e.errno);
+    /**
+     * See <a href="http://man7.org/linux/man-pages/man2/write.2.html">write(2)</a>.
+     */
+    public static int write(FileDescriptor fd, ByteBuffer buffer) throws ErrnoException, InterruptedIOException {
+        return IMPL.write(fd, buffer);
     }
-  }
 }

os-compat/src/main/java/androidx/system/OsConstants.java

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2011 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package androidx.system;
+
+interface OsConstants {
+
+    String errnoName(int errno);
+}

os-compat/src/main/java/androidx/system/OsConstantsApi21.java

@@ -0,0 +1,11 @@
+package androidx.system;
+
+import androidx.annotation.RequiresApi;
+
+@RequiresApi(21)
+final class OsConstantsApi21 implements OsConstants {
+    @Override
+    public String errnoName(int errno) {
+        return android.system.OsConstants.errnoName(errno);
+    }
+}

server/src/main/java/android/system/OsConstants.java renamed to os-compat/src/main/java/androidx/system/OsConstantsCompat.java

@@ -13,23 +13,31 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package android.system;
+package androidx.system;
 
-import android.annotation.TargetApi;
+import android.system.Os;
 
-@SuppressWarnings("unused")
-@TargetApi(21)
-public final class OsConstants {
+import static android.os.Build.VERSION.SDK_INT;
 
-    private OsConstants() { }
+public final class OsConstantsCompat {
+    private OsConstantsCompat() {
+    }
+
+    private static final OsConstants IMPL;
 
-    public static final int EINTR = libcore.io.OsConstants.EINTR;
+    static {
+        if (SDK_INT >= 21) {
+            IMPL = new OsConstantsApi21();
+        } else {
+            IMPL = new OsConstantsLibcore();
+        }
+    }
 
     /**
      * Returns the string name of an errno value.
      * For example, "EACCES". See {@link Os#strerror} for human-readable errno descriptions.
      */
     public static String errnoName(int errno) {
-        return libcore.io.OsConstants.errnoName(errno);
+        return IMPL.errnoName(errno);
     }
 }