Revert "Revert "Modify symlink_forest to rerun when soong_build has changed.""

This reverts commit aa5cc2cd6a.

Reason for revert: Fixed the bug in the original draft.
There was an error in how we compared the file's MTime to the
file's contents.

Bug: 300288299
Test: presubmits
Test: build/soong/tests/run_integration_tests.sh

Change-Id: I9e53432c0842c0b9fc13fe20d30ce9af37640c7f

tests/run_integration_tests.sh

@@ -10,6 +10,7 @@ TOP="$(readlink -f "$(dirname "$0")"/../../..)"
 "$TOP/build/soong/tests/persistent_bazel_test.sh"
 "$TOP/build/soong/tests/soong_test.sh"
 "$TOP/build/soong/tests/stale_metrics_files_test.sh"
+"$TOP/build/soong/tests/symlink_forest_rerun_test.sh"
 "$TOP/prebuilts/build-tools/linux-x86/bin/py3-cmd" "$TOP/build/bazel/ci/rbc_dashboard.py" aosp_arm64-userdebug
 
 # The following tests build against the full source tree and don't rely on the

tests/symlink_forest_rerun_test.sh

@@ -0,0 +1,43 @@
+#!/bin/bash -eu
+
+set -o pipefail
+
+# Tests that symlink forest will replant if soong_build has changed
+# Any change to the build system should trigger a rerun
+
+source "$(dirname "$0")/lib.sh"
+
+function test_symlink_forest_reruns {
+  setup
+
+  mkdir -p a
+  touch a/g.txt
+  cat > a/Android.bp <<'EOF'
+filegroup {
+    name: "g",
+    srcs: ["g.txt"],
+  }
+EOF
+
+  run_soong g
+
+  mtime=`cat out/soong/workspace/soong_build_mtime`
+  # rerun with no changes - ensure that it hasn't changed
+  run_soong g
+  newmtime=`cat out/soong/workspace/soong_build_mtime`
+  if [[ ! "$mtime" == "$mtime" ]]; then
+     fail "symlink forest reran when it shouldn't have"
+  fi
+
+  # change exit codes to force a soong_build rebuild.
+  sed -i 's/os.Exit(1)/os.Exit(2)/g' build/soong/bp2build/symlink_forest.go
+
+  run_soong g
+  newmtime=`cat out/soong/workspace/soong_build_mtime`
+  if [[ "$mtime" == "$newmtime" ]]; then
+     fail "symlink forest did not rerun when it should have"
+  fi
+
+}
+
+scan_and_run_tests