Force output checking for incremental run commands without the bytes.

When building without the bytes, outputs are only materialized when either
(1) an action prefetches them, or (2) they've been explicitly requested. When
both --remote_download_minimal and --noexperimental_check_output_files are
set, this presents a problem for a build command followed by a run command
for the same target: the build command won't download the outputs and the run
command won't check for their presence, proceeding without them. A
non-incremental run command works, because the outputs are considered
top-level even in minimal mode.

This is only a problem for a run command because it exists outside of action
execution, and doesn't get a chance to prefetch inputs; it would have been
better to implement the run command by injecting a specially crafted action
into the build graph, but that will have to wait for another day. The present
fix might theoretically slow things down if output checking is truly
unnecessary, but I doubt that would cause a significant regression in
practice.

Fixes #20843.

src/main/java/com/google/devtools/build/lib/buildtool/ExecutionTool.java

@@ -519,11 +519,15 @@ private ModifiedFileSet startBuildAndDetermineModifiedOutputFiles(
         startLocalOutputBuild();
       }
     }
-    if (!request.getPackageOptions().checkOutputFiles
-        // Do not skip invalidation in case the output tree is empty -- this can happen
-        // after it's cleaned or corrupted.
-        && !modifiedOutputFiles.treatEverythingAsDeleted()) {
-      modifiedOutputFiles = ModifiedFileSet.NOTHING_MODIFIED;
+    if (!request.getPackageOptions().checkOutputFiles) {
+      // Do not skip output invalidation in the following cases:
+      // 1. If the output tree is empty: this can happen after it's cleaned or corrupted.
+      // 2. For a run command: so that outputs are downloaded even if they were previously built
+      //    with --remote_download_minimal. See https://github.com/bazelbuild/bazel/issues/20843.
+      if (!modifiedOutputFiles.treatEverythingAsDeleted() && !request.getCommandName()
+          .equals("run")) {
+        return ModifiedFileSet.NOTHING_MODIFIED;
+      }
     }
     return modifiedOutputFiles;
   }

src/test/shell/bazel/remote/build_without_the_bytes_test.sh

@@ -2210,4 +2210,42 @@ EOF
     //:foo >& $TEST_log || fail "Failed to build //:foo"
 }
 
+function test_incremental_run_command_with_no_check_output_files() {
+  # Regression test for https://github.com/bazelbuild/bazel/issues/20843.
+  cat > BUILD <<'EOF'
+genrule(
+  name = "gen",
+  outs = ["out.txt"],
+  cmd = "touch $@",
+)
+sh_binary(
+  name = "foo",
+  srcs = ["foo.sh"],
+  data = ["out.txt"],
+)
+EOF
+  cat > foo.sh <<'EOF'
+#!/bin/bash
+if ! [[ -f "$0.runfiles/_main/out.txt" ]]; then
+  echo "runfile $0.runfiles/_main/out.txt not found" 1>&2
+  exit 1
+fi
+EOF
+  chmod +x foo.sh
+
+  CACHEDIR=$(mktemp -d)
+  FLAGS=(--disk_cache="$CACHEDIR" --remote_download_minimal --noexperimental_check_output_files)
+
+  # Populate the disk cache.
+  bazel build "${FLAGS[@]}" //:foo >& $TEST_log || fail "Failed to build //:foo"
+
+  # Clean build. No outputs are considered top-level, so nothing is downloaded.
+  bazel clean "${FLAGS[@]}"
+  bazel build "${FLAGS[@]}" //:foo >& $TEST_log || fail "Failed to build //:foo"
+
+  # Incremental run. Even though output checking is disabled, invalidation must
+  # must still occur to force them to be downloaded.
+  bazel run "${FLAGS[@]}" //:foo >& $TEST_log || fail "Failed to run //:foo"
+}
+
 run_suite "Build without the Bytes tests"