Make the Merkle tree computation work in the wake of 7149f57 .

Fixes a bunch of downstream breakages:

bazelbuild/continuous-integration#1093
bazelbuild/continuous-integration#1094
bazelbuild/rules_nodejs#2464
bazelbuild/rules_python#419

Turns out, the assertion that "Merkle tree computation uses `ActionInput.getExecPath()`" was only mostly correct: there was a place where the key of the input map was used instead.

I'm somewhat surprised that this did not show up in our test battery, although, admittedly, "unsound directory as an input file in an external repository" doesn't sound like the most common use case.

RELNOTES: None.
PiperOrigin-RevId: 358366246

src/main/java/com/google/devtools/build/lib/remote/merkletree/DirectoryTreeBuilder.java

@@ -154,7 +154,7 @@ private static int buildFromActionInputs(
 
             case DIRECTORY:
               SortedMap<PathFragment, ActionInput> directoryInputs =
-                  explodeDirectory(path, execRoot);
+                  explodeDirectory(input.getExecPath(), execRoot);
               return buildFromActionInputs(
                   directoryInputs, metadataProvider, execRoot, digestUtil, tree);
 

src/test/shell/bazel/disk_cache_test.sh

@@ -66,4 +66,29 @@ EOF
   assert_equals "0" $(cat "${execution_file}")
 }
 
+function test_input_directories_in_external_repo_with_sibling_repository_layout() {
+  create_new_workspace
+  l=$TEST_TMPDIR/l
+  mkdir -p "$l/dir"
+  touch "$l/WORKSPACE"
+  touch "$l/dir/f"
+  cat > "$l/BUILD" <<'EOF'
+exports_files(["dir"])
+EOF
+
+  cat >> WORKSPACE <<EOF
+local_repository(name="l", path="$l")
+EOF
+
+  cat > BUILD <<'EOF'
+genrule(name="g", srcs=["@l//:dir"], outs=["go"], cmd="find $< > $@")
+EOF
+
+  bazel build \
+    --experimental_sibling_repository_layout  \
+    --disk_cache="$TEST_TMPDIR/cache" \
+    //:g || fail "build failed"
+
+}
+
 run_suite "local action cache test"