Fix relative paths for external repositories (#437)

Without this, using PGV as an external dependency whose rules then acted
on another external dependency was broken. This patch adds additional
logic to generate the correct paths even in that case.

Signed-off-by: Alex Konradi <akonradi@google.com>

bazel/protobuf.bzl

@@ -21,16 +21,41 @@ def _path_ignoring_repository(f):
         # before "external/workspace", so we need to add the starting index of "external/workspace"
         return f.path[f.path.find(f.owner.workspace_root) + len(f.owner.workspace_root) + 1:]
 
+def _common_prefix(a, b):
+    matching_parts = []
+    for (x, y) in zip(a, b):
+        if x != y:
+            break
+        matching_parts.append(x)
+    return matching_parts
+
+def _common_suffix(a, b):
+    return reversed(_common_prefix(reversed(a), reversed(b)))
+
 def _package_relative_path(ctx, file):
     # Remove the path prefix common to both the BUILD file that defines the
     # package and the input file.
-    path_parts = file.short_path.split("/")
-    for piece in ctx.build_file_path.split("/"):
-        if path_parts[0] != piece:
-            break
-        path_parts.pop(0)
+    #
+    # file.short_path includes the root and file.path is relative to the root. We remove the root
+    # and keep only the path relative to the workspace root.
+    #
+    # Examples from envoyproxy/envoy:
+    #   For package @envoy_api_canonical//envoy/annotations:
+    #     ctx.build_file_path = envoy_api_canonical/envoy/annotations/BUILD
+    #     file.short_path = ../envoy_api_canonical/envoy/annotations/resource.proto
+    #     file.path = external/envoy_api_canonical/envoy/annotations/resource.proto
+    #
+    #   For package @opentelemetry_proto//, where the files are in a subdirectory of the package directory:
+    #     ctx.build_file_path = opentelemetry_proto/BUILD.bazel
+    #     file.short_path = ../opentelemetry_proto/opentelemetry/proto/collector/logs/v1/logs_service.proto
+    #     file.path = external/opentelemetry_proto/opentelemetry/proto/collector/logs/v1/logs_service.proto
+
+    path_parts = _common_suffix(file.short_path.split("/"), file.path.split("/"))
+    common_prefix = _common_prefix(path_parts, ctx.build_file_path.split("/"))
 
-    return "/".join(path_parts)
+    # Return the path relative to the package directory by stripping off the common prefix of
+    # path_parts and the BUILD file path.
+    return "/".join(path_parts[len(common_prefix):])
 
 def _protoc_cc_output_files(ctx, proto_file_sources):
     cc_hdrs = []