Inject unresolved symlinks with BwoB

Creating unresolved symlinks locally when building with `--remote_download_outputs=minimal` results in unnecessary I/O and may not even be supported by the host (e.g. on Windows). Instead, only create it in the remote action file system.

src/main/java/com/google/devtools/build/lib/remote/AbstractActionInputPrefetcher.java

@@ -40,6 +40,7 @@
 import com.google.devtools.build.lib.actions.Artifact.SpecialArtifact;
 import com.google.devtools.build.lib.actions.Artifact.TreeFileArtifact;
 import com.google.devtools.build.lib.actions.FileArtifactValue;
+import com.google.devtools.build.lib.actions.FileArtifactValue.UnresolvedSymlinkArtifactValue;
 import com.google.devtools.build.lib.actions.InputMetadataProvider;
 import com.google.devtools.build.lib.actions.cache.OutputMetadataStore;
 import com.google.devtools.build.lib.actions.cache.VirtualActionInput;
@@ -374,7 +375,10 @@ private ListenableFuture<Void> prefetchFile(
       PathFragment execPath = input.getExecPath();
 
       FileArtifactValue metadata = metadataSupplier.getMetadata(input);
-      if (metadata == null || !canDownloadFile(execRoot.getRelative(execPath), metadata)) {
+      if (metadata instanceof UnresolvedSymlinkArtifactValue unresolvedSymlink) {
+        return toListenableFuture(
+            plantRelativeSymlink(execPath, unresolvedSymlink.getSymlinkTarget()));
+      } else if (metadata == null || !canDownloadFile(execRoot.getRelative(execPath), metadata)) {
         return immediateVoidFuture();
       }
 
@@ -399,7 +403,7 @@ private ListenableFuture<Void> prefetchFile(
               priority);
 
       if (symlink != null) {
-        result = result.andThen(plantSymlink(symlink));
+        result = result.andThen(plantAbsoluteSymlink(symlink));
       }
 
       return toListenableFuture(result);
@@ -651,7 +655,21 @@ private void deletePartialDownload(Path path) {
     }
   }
 
-  private Completable plantSymlink(Symlink symlink) {
+  private Completable plantRelativeSymlink(PathFragment linkPath, String target) {
+    return downloadCache.executeIfNot(
+        execRoot.getRelative(linkPath),
+        Completable.defer(
+            () -> {
+              Path link = execRoot.getRelative(linkPath);
+              // Delete the link path if it already exists. This is the case for tree artifacts,
+              // whose root directory is created before the action runs.
+              link.delete();
+              link.createSymbolicLink(PathFragment.create(target));
+              return Completable.complete();
+            }));
+  }
+
+  private Completable plantAbsoluteSymlink(Symlink symlink) {
     return downloadCache.executeIfNot(
         execRoot.getRelative(symlink.linkExecPath()),
         Completable.defer(

src/main/java/com/google/devtools/build/lib/remote/RemoteActionFileSystem.java

@@ -309,6 +309,10 @@ void injectRemoteFile(PathFragment path, byte[] digest, long size, long expireAt
     remoteOutputTree.injectFile(path, metadata);
   }
 
+  void injectUnresolvedSymlink(PathFragment symlink, PathFragment target) throws IOException {
+    remoteOutputTree.injectSymlink(symlink, target);
+  }
+
   @Override
   public String getFileSystemType(PathFragment path) {
     return "remoteActionFS";
@@ -937,6 +941,11 @@ protected void injectFile(PathFragment path, FileArtifactValue metadata) throws
       remoteInMemoryFileInfo.set(metadata);
     }
 
+    protected void injectSymlink(PathFragment path, PathFragment target) throws IOException {
+      createDirectoryAndParents(path.getParentDirectory());
+      createSymbolicLink(path, target);
+    }
+
     // Override for access within this class
     @Nullable
     @Override

src/main/java/com/google/devtools/build/lib/remote/RemoteExecutionService.java

@@ -1006,20 +1006,29 @@ private void moveOutputsToFinalLocation(
     }
   }
 
-  private void createSymlinks(Iterable<SymlinkMetadata> symlinks) throws IOException {
+  private void createSymlinks(
+      RemoteActionResult result,
+      @Nullable RemoteActionFileSystem remoteActionFileSystem,
+      Iterable<SymlinkMetadata> symlinks)
+      throws IOException {
     for (SymlinkMetadata symlink : symlinks) {
-      Preconditions.checkNotNull(
-              symlink.path().getParentDirectory(),
-              "Failed creating directory and parents for %s",
-              symlink.path())
-          .createDirectoryAndParents();
-      // If a directory output is being materialized as a symlink, we must first delete the
-      // preexisting empty directory.
-      if (symlink.path().exists(Symlinks.NOFOLLOW)
-          && symlink.path().isDirectory(Symlinks.NOFOLLOW)) {
-        symlink.path().delete();
+      if (shouldDownload(result, symlink.path().relativeTo(execRoot))) {
+        Preconditions.checkNotNull(
+                symlink.path().getParentDirectory(),
+                "Failed creating directory and parents for %s",
+                symlink.path())
+            .createDirectoryAndParents();
+        // If a directory output is being materialized as a symlink, we must first delete the
+        // preexisting empty directory.
+        if (symlink.path().exists(Symlinks.NOFOLLOW)
+            && symlink.path().isDirectory(Symlinks.NOFOLLOW)) {
+          symlink.path().delete();
+        }
+        symlink.path().createSymbolicLink(symlink.target());
+      } else if (!(outputService instanceof BazelOutputService)) {
+        checkNotNull(remoteActionFileSystem)
+            .injectUnresolvedSymlink(symlink.path().asFragment(), symlink.target());
       }
-      symlink.path().createSymbolicLink(symlink.target());
     }
   }
 
@@ -1439,7 +1448,7 @@ public InMemoryOutput downloadOutputs(RemoteAction action, RemoteActionResult re
 
     // Create the symbolic links after all downloads are finished, because dangling symlinks
     // might not be supported on all platforms.
-    createSymlinks(symlinks);
+    createSymlinks(result, remoteActionFileSystem, symlinks);
 
     if (result.success()) {
       // Check that all mandatory outputs are created.