Fix #507 - Always run from WORKSPACE root in compile_pip_requirements update exe (#564)

python/pip_install/pip_compile.py

diff --git a/python/pip_install/pip_compile.py b/python/pip_install/pip_compile.py
index 278e848..ce40285 100644
--- a/python/pip_install/pip_compile.py
+++ b/python/pip_install/pip_compile.py
@@ -41,11 +41,22 @@
     )
     copyfile(requirements_txt, requirements_out)
 
-elif "BUILD_WORKING_DIRECTORY" in os.environ:
-    os.chdir(os.environ['BUILD_WORKING_DIRECTORY'])
+elif "BUILD_WORKSPACE_DIRECTORY" in os.environ:
+    # This value, populated when running under `bazel run`, is a path to the
+    # "root of the workspace where the build was run."
+    # This matches up with the values passed in via the macro using the 'rootpath' Make variable,
+    # which for source files provides a path "relative to your workspace root."
+    #
+    # Changing to the WORKSPACE root avoids 'file not found' errors when the `.update` target is run
+    # from different directories within the WORKSPACE.
+    os.chdir(os.environ['BUILD_WORKSPACE_DIRECTORY'])
 else:
+    err_msg = (
+        "Expected to find BUILD_WORKSPACE_DIRECTORY (running under `bazel run`) or "
+        "TEST_TMPDIR (running under `bazel test`) in environment."
+    )
     print(
-        "Expected to find BUILD_WORKING_DIRECTORY in environment",
+        err_msg,
         file=sys.stderr,
     )
     sys.exit(1)