cabal: Add support for empty libraries

Previously, an empty library that only exported modules from another
library failed to build. Bazel would error out when the .a file wasn't
created. These libraries are becoming more common in the Haskell
ecosystem, so we need to support them.

There is a straightforward way to handle these libraries. We make the
package database the default output. We avoid creating an output file
for any libraries if the library is marked as empty. This is all
straightforward bazel dependency/output management.

Adjust the cabal wrapper to fix up the package database even in the case
that there is no library. Otherwise we end up with an empty package
database, which isn't what we're looking for.

Author: iteratee

haskell/cabal.bzl

@@ -483,7 +483,7 @@ def _haskell_cabal_library_impl(ctx):
         "_install/{}_data".format(package_id),
         sibling = cabal,
     )
-    with_haddock = ctx.attr.haddock and hs.tools_config.supports_haddock
+    with_haddock = ctx.attr.haddock and hs.tools_config.supports_haddock and not ctx.attr.empty_library
     if with_haddock:
         haddock_file = hs.actions.declare_file(
             "_install/{}_haddock/{}.haddock".format(package_id, package_name),
@@ -496,30 +496,36 @@ def _haskell_cabal_library_impl(ctx):
     else:
         haddock_file = None
         haddock_html_dir = None
-    vanilla_library = hs.actions.declare_file(
-        "_install/lib/libHS{}.a".format(package_id),
-        sibling = cabal,
-    )
-    if with_profiling:
-        profiling_library = hs.actions.declare_file(
-            "_install/lib/libHS{}_p.a".format(package_id),
-            sibling = cabal,
-        )
-        static_library = profiling_library
-    else:
+    if ctx.attr.empty_library:
+        vanilla_library = None
+        static_library = None
         profiling_library = None
-        static_library = vanilla_library
-    if hs.toolchain.static_runtime:
         dynamic_library = None
     else:
-        dynamic_library = hs.actions.declare_file(
-            "_install/lib/libHS{}-ghc{}.{}".format(
-                package_id,
-                hs.toolchain.version,
-                _so_extension(hs),
-            ),
+        vanilla_library = hs.actions.declare_file(
+            "_install/lib/libHS{}.a".format(package_id),
             sibling = cabal,
         )
+        if with_profiling:
+            profiling_library = hs.actions.declare_file(
+                "_install/lib/libHS{}_p.a".format(package_id),
+                sibling = cabal,
+            )
+            static_library = profiling_library
+        else:
+            profiling_library = None
+            static_library = vanilla_library
+        if hs.toolchain.static_runtime:
+            dynamic_library = None
+        else:
+            dynamic_library = hs.actions.declare_file(
+                "_install/lib/libHS{}-ghc{}.{}".format(
+                    package_id,
+                    hs.toolchain.version,
+                    _so_extension(hs),
+                ),
+                sibling = cabal,
+            )
     (tool_inputs, tool_input_manifests) = ctx.resolve_tools(tools = ctx.attr.tools)
     c = _prepare_cabal_inputs(
         hs,
@@ -553,11 +559,12 @@ def _haskell_cabal_library_impl(ctx):
     outputs = [
         package_database,
         interfaces_dir,
-        vanilla_library,
         data_dir,
     ]
     if with_haddock:
         outputs.extend([haddock_file, haddock_html_dir])
+    if vanilla_library != None:
+        outputs.append(vanilla_library)
     if dynamic_library != None:
         outputs.append(dynamic_library)
     if with_profiling:
@@ -578,8 +585,13 @@ def _haskell_cabal_library_impl(ctx):
         progress_message = "HaskellCabalLibrary {}".format(hs.label),
     )
 
+    if not ctx.attr.empty_library:
+      default_info_libs = depset([static_library] + ([dynamic_library] if dynamic_library != None else []))
+    else:
+      default_info_libs = depset([package_database])
+
     default_info = DefaultInfo(
-        files = depset([static_library] + ([dynamic_library] if dynamic_library != None else [])),
+        files = default_info_libs,
         runfiles = ctx.runfiles(
             files = [data_dir],
             collect_default = True,
@@ -628,7 +640,7 @@ def _haskell_cabal_library_impl(ctx):
     )
     linker_input = cc_common.create_linker_input(
         owner = ctx.label,
-        libraries = depset(direct = [
+        libraries = depset(direct = ([] if ctx.attr.empty_library else [
             cc_common.create_library_to_link(
                 actions = ctx.actions,
                 feature_configuration = feature_configuration,
@@ -638,7 +650,7 @@ def _haskell_cabal_library_impl(ctx):
                 static_library = static_library,
                 cc_toolchain = cc_toolchain,
             ),
-        ]),
+        ])),
     )
     compilation_context = cc_common.create_compilation_context()
     linking_context = cc_common.create_linking_context(
@@ -747,6 +759,11 @@ haskell_cabal_library = rule(
             library symlink underneath `_solib_<cpu>` will be shortened to
             avoid exceeding the MACH-O header size limit on MacOS.""",
         ),
+        "empty_library": attr.bool(
+            default = False,
+            doc = """Whether the main cabal library is empty and merely re-exports from other sub libraries.
+            It is necessary to set this, otherwise bazel will complain about missing "*libHS.a" files.""",
+        ),
     },
     toolchains = use_cc_toolchain() + [
         "@rules_haskell//haskell:toolchain",

haskell/private/cabal_wrapper.py

@@ -392,7 +392,7 @@ def make_relative_to_pkgroot(matchobj):
     line = re.sub(re.escape(cfg_execroot) + r'\S*', make_relative_to_pkgroot, line)
     return line
 
-if libraries != [] and os.path.isfile(package_conf_file):
+if os.path.isfile(package_conf_file):
     for lib in libraries:
         os.rename(lib, os.path.join(dynlibdir, os.path.basename(lib)))