Support dependencies on environment variables

Ninja can't depend on environment variables, so modifying build
behavior based on environment variables requires coordinating
between the soong script that invokes ninja and the soong_build
manifest generator.

Allow any module to call Config.Getenv to get the contents of an
environment variable while registering a dependency on it.
After all modules have been processed write out the state of
all used environment variables to a JSON file called
.soong.environment.  During the next build the soong script
will use the soong_env tool to compare the contents of
.soong.environment to the current environment, and force a
build manifest regeneration by deleting the .soong.environment
file if any variables have changed.

Change-Id: Id0d81933a857bc2fc1cd7a393a3c6cec73dc4824

1 files changed, 17 insertions, 0 deletions

diff --git a/soong.bash b/soong.bash
index fc330d0b..fab15de5 100755
--- a/soong.bash
+++ b/soong.bash
@@ -27,4 +27,21 @@ fi
 # can regenerate the build manifest.
 export BLUEPRINT_NINJA_HAS_MULTIPASS=1
 
+# Ninja can't depend on environment variables, so do a manual comparison
+# of the relevant environment variables from the last build using the
+# soong_env tool and trigger a build manifest regeneration if necessary
+ENVFILE=${BUILDDIR}/.soong.environment
+ENVTOOL=${BUILDDIR}/.bootstrap/bin/soong_env
+if [ -f ${ENVFILE} ]; then
+    if [ -x ${ENVTOOL} ]; then
+        if ! ${ENVTOOL} ${ENVFILE}; then
+            echo "forcing build manifest regeneration"
+            rm -f ${ENVFILE}
+        fi
+    else
+        echo "Missing soong_env tool, forcing build manifest regeneration"
+        rm -f ${ENVFILE}
+    fi
+fi
+
 ${SRCDIR}/prebuilts/ninja/${PREBUILTOS}/ninja -C ${BUILDDIR} "$@"