2 files changed, 54 insertions, 8 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ActionFileSystem.java b/src/main/java/com/google/devtools/build/lib/skyframe/ActionFileSystem.java
index bb4904283c..604f1c3dae 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ActionFileSystem.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ActionFileSystem.java
@@ -24,6 +24,7 @@ import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Streams;
 import com.google.common.hash.Hashing;
 import com.google.common.io.BaseEncoding;
+import com.google.common.io.ByteStreams;
 import com.google.devtools.build.lib.actions.ActionInput;
 import com.google.devtools.build.lib.actions.ActionInputMap;
 import com.google.devtools.build.lib.actions.Artifact;
@@ -421,6 +422,16 @@ final class ActionFileSystem extends AbstractFileSystemWithCustomStat
   @Override
   protected InputStream getInputStream(Path path) throws IOException {
     FileArtifactValue metadata = getMetadataChecked(asExecPath(path));
+    InputStream inputStream = getInputStreamFromMetadata(metadata, path);
+    if (inputStream != null) {
+      return inputStream;
+    }
+    return getSourcePath(path.asFragment()).getInputStream();
+  }
+
+  @Nullable
+  private InputStream getInputStreamFromMetadata(FileArtifactValue metadata, Path path)
+      throws IOException {
     if (metadata instanceof InlineFileArtifactValue) {
       return ((InlineFileArtifactValue) metadata).getInputStream();
     }
@@ -430,13 +441,12 @@ final class ActionFileSystem extends AbstractFileSystemWithCustomStat
     if (metadata instanceof RemoteFileArtifactValue) {
       throw new IOException("ActionFileSystem cannot read remote file: " + path);
     }
-    return getSourcePath(path.asFragment()).getInputStream();
+    return null;
   }
 
   @Override
   protected OutputStream getOutputStream(Path path, boolean append) {
-    Preconditions.checkArgument(!append, "ActionFileSystem doesn't support append.");
-    return outputs.getUnchecked(asExecPath(path)).getOutputStream();
+    return outputs.getUnchecked(asExecPath(path)).getOutputStream(append, path);
   }
 
   @Override
@@ -634,23 +644,38 @@ final class ActionFileSystem extends AbstractFileSystemWithCustomStat
     }
 
     /** Callers are expected to close the returned stream. */
-    public ByteArrayOutputStream getOutputStream() {
-      Preconditions.checkState(metadata == null, "getOutputStream called twice for: %s", artifact);
-      return new ByteArrayOutputStream() {
+    public ByteArrayOutputStream getOutputStream(boolean append, Path path) {
+      ByteArrayOutputStream baos = new ByteArrayOutputStream() {
         @Override
         public void close() throws IOException {
-          flush();
+          flushInternal(true);
           super.close();
         }
 
         @Override
         public void flush() throws IOException {
+          flushInternal(false);
+        }
+
+        private void flushInternal(boolean notify) throws IOException {
           super.flush();
           byte[] data = toByteArray();
           set(new InlineFileArtifactValue(data, Hashing.md5().hashBytes(data).asBytes()),
-              /*notifyConsumer=*/ true);
+              /*notifyConsumer=*/ notify);
         }
       };
+
+      if (append && metadata != null) {
+        try (InputStream in = getInputStreamFromMetadata(metadata, path)) {
+          if (in == null) {
+            throw new IOException("Unable to read file " + path + " with metadata " + metadata);
+          }
+          ByteStreams.copy(in, baos);
+        } catch (IOException e) {
+          throw new IllegalStateException(e);
+        }
+      }
+      return baos;
     }
   }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/ActionFileSystemTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/ActionFileSystemTest.java
index d7b5e78127..ee90fe69d0 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/ActionFileSystemTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/ActionFileSystemTest.java
@@ -23,6 +23,7 @@ import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.PathFragment;
 import com.google.devtools.build.lib.vfs.inmemoryfs.InMemoryFileSystem;
 import java.io.OutputStream;
+import java.io.PrintStream;
 import java.nio.charset.StandardCharsets;
 import org.junit.Before;
 import org.junit.Test;
@@ -64,6 +65,26 @@ public class ActionFileSystemTest {
   }
 
   @Test
+  public void testAppendLocalFile() throws Exception {
+    String testData = "abc";
+
+    Path file = outputPath.getRelative("foo/bar");
+    FileSystemUtils.writeContentAsLatin1(file, testData);
+    assertThat(new String(FileSystemUtils.readContentAsLatin1(file))).isEqualTo(testData);
+
+    try (OutputStream out = file.getOutputStream(true)) {
+      PrintStream printStream = new PrintStream(out);
+      printStream.append("defg");
+      printStream.flush();
+    }
+    assertThat(new String(FileSystemUtils.readContentAsLatin1(file))).isEqualTo("abcdefg");
+
+    // Now make sure we can still overwrite the file in non-append mode.
+    FileSystemUtils.writeContentAsLatin1(file, "cheesy");
+    assertThat(new String(FileSystemUtils.readContentAsLatin1(file))).isEqualTo("cheesy");
+  }
+
+  @Test
   public void testFlushedButNotClosedFileWrite() throws Exception {
     String testData = "abc19";