Allowing file symlink outputs. (#28)

* Allowing file symlink outputs.

This is a missing case from the existing API: symlink output directories
are already supported, because the SymlinkNodes are part of the
Directory message. Similarly, all file symlinks that are part of larger
output directories are already supported as well. However, if the output
is a file, then the current API does not distinguish between it being a
symlink or not, and instead just provides the content Digest.

A motivating case for distinguishing the symlink case is efficiency,
when the client did not want the action to create and download a copy of
an existing file.

TESTED=bazel build

	modified:   build/bazel/remote/execution/v2/remote_execution.proto

* Adding output_directory_symlinks field.

* Fixing OutputSymlink comment.

1 files changed, 54 insertions, 1 deletions

diff --git a/build/bazel/remote/execution/v2/remote_execution.proto b/build/bazel/remote/execution/v2/remote_execution.proto
index 800ebb9..715ee79 100644
--- a/build/bazel/remote/execution/v2/remote_execution.proto
+++ b/build/bazel/remote/execution/v2/remote_execution.proto
@@ -710,7 +710,9 @@ message ActionResult {
 
   // The output files of the action. For each output file requested in the
   // `output_files` field of the Action, if the corresponding file existed after
-  // the action completed, a single entry will be present in the output list.
+  // the action completed, a single entry will be present either in this field,
+  // or in the output_file_symlinks field, if the file was a symbolic link to
+  // another file.
   //
   // If the action does not produce the requested output, or produces a
   // directory where a regular file is expected or vice versa, then that output
@@ -718,6 +720,21 @@ message ActionResult {
   // list as desired; clients MUST NOT assume that the output list is sorted.
   repeated OutputFile output_files = 2;
 
+  // The output files of the action that are symbolic links to other files. Those
+  // may be links to other output files, or input files, or even absolute paths
+  // outside of the working directory, if the server supports
+  // [SymlinkAbsolutePathStrategy.ALLOWED][build.bazel.remote.execution.v2.SymlinkAbsolutePathStrategy].
+  // For each output file requested in the `output_files` field of the Action,
+  // if the corresponding file existed after
+  // the action completed, a single entry will be present either in this field,
+  // or in the `output_files` field, if the file was not a symbolic link.
+  //
+  // If the action does not produce the requested output, or produces a
+  // directory where a regular file is expected or vice versa, then that output
+  // will be omitted from the list. The server is free to arrange the output
+  // list as desired; clients MUST NOT assume that the output list is sorted.
+  repeated OutputSymlink output_file_symlinks = 10;
+
   // The output directories of the action. For each output directory requested
   // in the `output_directories` field of the Action, if the corresponding
   // directory existed after the action completed, a single entry will be
@@ -779,6 +796,22 @@ message ActionResult {
   // ```
   repeated OutputDirectory output_directories = 3;
 
+  // The output directories of the action that are symbolic links to other
+  // directories. Those may be links to other output directories, or input
+  // directories, or even absolute paths outside of the working directory,
+  // if the server supports
+  // [SymlinkAbsolutePathStrategy.ALLOWED][build.bazel.remote.execution.v2.SymlinkAbsolutePathStrategy].
+  // For each output directory requested in the `output_directories` field of
+  // the Action, if the directory file existed after
+  // the action completed, a single entry will be present either in this field,
+  // or in the `output_directories` field, if the directory was not a symbolic link.
+  //
+  // If the action does not produce the requested output, or produces a
+  // file where a directory is expected or vice versa, then that output
+  // will be omitted from the list. The server is free to arrange the output
+  // list as desired; clients MUST NOT assume that the output list is sorted.
+  repeated OutputSymlink output_directory_symlinks = 11;
+
   // The exit code of the command.
   int32 exit_code = 4;
 
@@ -866,6 +899,26 @@ message OutputDirectory {
   Digest tree_digest = 3;
 }
 
+// An `OutputSymlink` is similar to a
+// [Symlink][build.bazel.remote.execution.v2.SymlinkNode], but it is used as an
+// output in an `ActionResult`.
+//
+// `OutputSymlink` is binary-compatible with `SymlinkNode`.
+message OutputSymlink {
+  // The full path of the symlink relative to the working directory, including the
+  // filename. The path separator is a forward slash `/`. Since this is a
+  // relative path, it MUST NOT begin with a leading forward slash.
+  string path = 1;
+
+  // The target path of the symlink. The path separator is a forward slash `/`.
+  // The target path can be relative to the parent directory of the symlink or
+  // it can be an absolute path starting with `/`. Support for absolute paths
+  // can be checked using the [Capabilities][build.bazel.remote.execution.v2.Capabilities]
+  // API. The canonical form forbids the substrings `/./` and `//` in the target
+  // path. `..` components are allowed anywhere in the target path.
+  string target = 2;
+}
+
 // An `ExecutionPolicy` can be used to control the scheduling of the action.
 message ExecutionPolicy {
   // The priority (relative importance) of this action. Generally, a lower value