Fix the genrule for cvd version output

The build runs in a sandbox and not in the working directory the `bazel`
command is executed in, so the previous version never had an output to
populate the `cvd version` field.  Also, `git describe` was returning an
old tag.  I updated to return the latest HEAD commit hash instead.

If the
`--working_status_command=path/to/tools/buildutils/stamp_helper.sh` flag
is added to a build, the output goes into a file intended for embedding
outside information for use inside the build sandbox.

The commit hash should not change "often" (like a timestamp would be
different every single build), so we mark it as a stable key.  Then, if
the value changes, every affected action (marked `stamp = 1`) is
triggered.  Volatile keys do not trigger actions when they change.

base/cvd/BUILD.bazel

@@ -13,7 +13,8 @@ genrule(
     name = "build_version_header",
     srcs = ["build_version.h.in"],
     outs = ["build/version.h"],
-    cmd = "sed -e \"s|@VCS_TAG@|`git describe`|\" $< > $@",
+    cmd = "sed -e \"s|@VCS_TAG@|`cat bazel-out/*-status.txt | grep \"STABLE_HEAD_COMMIT\" | cut -f 2 -d ' '`|\" $< > $@",
+    stamp = 1,
 )
 
 filegroup(

base/debian/rules

@@ -20,7 +20,8 @@ override_dh_installinit:
 	dh_installinit --name=cuttlefish-host-resources
 	dh_installinit
 
+# the `--workspace_status_command` flag path depends on the current working directory of base/cvd
 override_dh_auto_build:
-	cd cvd && bazel build --linkopt="-Wl,--build-id=sha1" cuttlefish/host/commands/cvd:cvd --spawn_strategy=local
+	cd cvd && bazel build --linkopt="-Wl,--build-id=sha1" cuttlefish/host/commands/cvd:cvd --spawn_strategy=local --workspace_status_command=../../tools/buildutils/stamp_helper.sh
 	dh_auto_build
 

tools/buildutils/stamp_helper.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+#
+# https://bazel.build/docs/user-manual#workspace-status
+#
+# append `--workspace_status_command=path/to/stamp_helper.sh` to build command
+# to "stamp" this output to `bazel-out/stable_status.txt``
+# 
+# NOTES: 
+# - output must be a "key value" format on a single line
+# - output key must begin with "STABLE_" or the key and value end up in
+#   `volatile_status.txt`
+# - changes to `stable_status.txt` values triggers a re-run of the affected
+#   actions
+
+echo STABLE_HEAD_COMMIT `git rev-parse HEAD`
+