Build error + fix when using capnp/ez-rpc with bazel

[salil-enf]

We use bazel to build and started using capnproto bazel rules directly (since last week). Earlier we were doing a configure_make step to pull in the repo. Using bazel-rules directly was must faster/easier however we ran into a build issue on the server side when we use ez-rpc. Including ez-rpc.h header leads to below build failure

bazel-out/k8-fastbuild/bin/external/capnp-cpp/src/capnp/_virtual_includes/capnp-rpc/capnp/ez-rpc.h:25:10: fatal error: message.h: No such file or directory
   25 | #include "message.h"
      |          ^~~~~~~~~~~

The straight forward fix here of adding an include path did not seem to work. Because that started re-ordering include paths for all builds leading to problems like this below

In file included from bazel-out/k8-fastbuild/bin/external/capnp-cpp/src/kj/_virtual_includes/kj/kj/string.h:24,
                 from bazel-out/k8-fastbuild/bin/external/capnp-cpp/src/capnp/_virtual_includes/capnp/capnp/common.h:29,
                 from bazel-out/k8-fastbuild/bin/external/capnp-cpp/src/capnp/_virtual_includes/capnp/capnp/endian.h:24,
                 from /usr/include/x86_64-linux-gnu/sys/types.h:176,
                 from /usr/include/stdlib.h:394,
                 from /usr/include/c++/10/cstdlib:75,
                 from /usr/include/c++/10/bits/stl_algo.h:59,
                 from /usr/include/c++/10/string:52

I tried this patch below and it seems to resolve the build issues for us. Before I deploy, wanted to make sure I wasn't missing something obvious or if this is wrong because it could lead to future issues etc. Does this look okay?

@@ -0,0 +1,14 @@
+--- a/src/capnp/ez-rpc.h      2024-02-13 16:08:37.598140934 +0000
++++ b/src/capnp/ez-rpc.h      2024-02-13 16:08:49.558176863 +0000
+@@ -21,8 +21,8 @@
+
+ #pragma once
+
+-#include "rpc.h"
+-#include "message.h"
++#include "capnp/rpc.h"
++#include "capnp/message.h"
+
+ CAPNP_BEGIN_HEADER

[kentonv]

The correct change would be:

-#include "message.h"
+#include <capnp/message.h>

This is needed since message.h is in a separate bazel library, and bazel doesn't like relative includes cross-library. I guess it's surprising this doesn't cause ez-rpc.c++ itself to fail to build.

(Actually, bazel prefers that all includes are absolute includes using quoted syntax. This is a weird Google-ism that I very much disagree with. In capnp we follow the rule that quoted includes are relative, <> includes are absolute.)

[salil-enf]

Thanks for following up on this. I am inclined to leave my patch as is for now since that is consistent with our include style. Is this change planned from your side anytime soon?

[kentonv]

I'm happy to accept a PR fixing this, but I don't have the bandwidth to work on it myself.