Divide gRPC-Core in Interface & Impl subspecs

This works around the Cocoapods' linter header_mapping_dir restrictions.
Incidentally, it also removes the need to set build settings in the user
target.

src/objective-c/tests/Podfile

@@ -44,18 +44,15 @@ pre_install do |installer|
   grpc_core_spec = installer.pod_targets.find{|t| t.name == 'gRPC-Core'}.root_spec
 
   # Copied from gRPC-Core.podspec, except for the adjusted src_root:
-
   src_root = "$(PODS_ROOT)/../#{GRPC_LOCAL_SRC}"
-
-  public_build_settings = {
+  grpc_core_spec.pod_target_xcconfig = {
     'GRPC_SRC_ROOT' => src_root,
     'HEADER_SEARCH_PATHS' => '"$(inherited)" "$(GRPC_SRC_ROOT)/include"',
-  }
-  private_build_settings = public_build_settings.merge({
+    'USER_HEADER_SEARCH_PATHS' => '"$(GRPC_SRC_ROOT)"',
+    # If we don't set these two settings, `include/grpc/support/time.h` and
+    # `src/core/lib/support/string.h` shadow the system `<time.h>` and `<string.h>`, breaking the
+    # build.
     'USE_HEADERMAP' => 'NO',
     'ALWAYS_SEARCH_USER_PATHS' => 'NO',
-    'USER_HEADER_SEARCH_PATHS' => '"$(GRPC_SRC_ROOT)"',
-  })
-  grpc_core_spec.user_target_xcconfig = public_build_settings
-  grpc_core_spec.pod_target_xcconfig = private_build_settings
+  }
 end

templates/gRPC-Core.podspec.template

@@ -35,21 +35,30 @@
   # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
   <%!
-  def grpc_files(libs):
+  def grpc_private_files(libs):
     out = []
     for lib in libs:
       if lib.name in ("grpc", "gpr"):
         out += lib.get('headers', [])
-        out += lib.get('public_headers', [])
         out += lib.get('src', [])
     return out;
-  
+
+  def grpc_public_headers(libs):
+    out = []
+    for lib in libs:
+      if lib.name in ("grpc", "gpr"):
+        out += lib.get('public_headers', [])
+    return out
+
   def grpc_private_headers(libs):
     out = []
     for lib in libs:
       if lib.name in ("grpc", "gpr"):
         out += lib.get('headers', [])
     return out
+
+  def ruby_multiline_list(files, indent):
+    return (',\n' + indent*' ').join('\'%s\'' % f for f in files)
   %>
   Pod::Spec.new do |s|
     s.name     = 'gRPC-Core'
@@ -63,6 +72,7 @@
     s.source = {
       :git => 'https://github.com/grpc/grpc.git',
       :tag => "release-#{version.gsub(/\./, '_')}-objectivec-#{version}",
+      # TODO(jcanizales): Depend explicitly on the nanopb pod, and disable submodules.
       :submodules => true,
     }
 
@@ -71,42 +81,77 @@
     s.requires_arc = false
 
     name = 'grpc'
+
+    # When creating a dynamic framework, name it grpc.framework instead of gRPC-Core.framework.
+    # This lets users write their includes like `#include <grpc/grpc.h>` as opposed to `#include
+    # <gRPC-Core/grpc.h>`.
     s.module_name = name
-    s.header_dir = name
 
+    # When creating a dynamic framework, copy the headers under `include/grpc/` into the root of
+    # the `Headers/` directory of the framework (i.e., not under `Headers/include/grpc`).
+    #
+    # TODO(jcanizales): Debug why this doesn't work on macOS.
     s.header_mappings_dir = 'include/grpc'
 
-    src_root = '$(PODS_ROOT)/gRPC-Core'
-    # This isn't officially supported in Cocoapods. We've asked for an alternative:
-    # https://github.com/CocoaPods/CocoaPods/issues/4386
+    # The above has an undesired effect when creating a static library: It forces users to write
+    # includes like `#include <gRPC-Core/grpc.h>`. `s.header_dir` adds a path prefix to that, and
+    # because Cocoapods lets omit the pod name when including headers of static libraries, the
+    # following lets users write `#include <grpc/grpc.h>`.
+    s.header_dir = name
+
+    # The module map created automatically by Cocoapods doesn't work for C libraries like gRPC-Core.
+    s.module_map = 'include/grpc/module.modulemap'
+
+    # To compile the library, we need the user headers search path (quoted includes) to point to the
+    # root of the repo, and the system headers search path (angled includes) to point to `include/`.
+    # Cocoapods effectively clones the repo under `<Podfile dir>/Pods/gRPC-Core/`, and sets a build
+    # variable called `$(PODS_ROOT)` to `<Podfile dir>/Pods/`, so we use that.
+    #
+    # Relying on the file structure under $(PODS_ROOT) isn't officially supported in Cocoapods, as it
+    # is taken as an implementation detail. We've asked for an alternative, and have been told that
+    # what we're doing should keep working: https://github.com/CocoaPods/CocoaPods/issues/4386
     #
-    # The src_root value of $(PODS_ROOT)/gRPC-Core assumes Cocoapods is installing this pod from its
-    # remote repo. For local development of this library, enabled by using ":path" in the Podfile,
+    # The `src_root` value of `$(PODS_ROOT)/gRPC-Core` assumes Cocoapods is installing this pod from
+    # its remote repo. For local development of this library, enabled by using `:path` in the Podfile,
     # that assumption is wrong. In such case, the following settings need to be reset with the
-    # appropriate value of src_root. This can be accomplished in the pre_install hook of the Podfile;
-    # see src/objective-c/tests/Podfile for an example.
-    public_build_settings = {
+    # appropriate value of `src_root`. This can be accomplished in the `pre_install` hook of the
+    # Podfile; see `src/objective-c/tests/Podfile` for an example.
+    src_root = '$(PODS_ROOT)/gRPC-Core'
+    s.pod_target_xcconfig = {
       'GRPC_SRC_ROOT' => src_root,
       'HEADER_SEARCH_PATHS' => '"$(inherited)" "$(GRPC_SRC_ROOT)/include"',
-    }
-    private_build_settings = public_build_settings.merge({
+      'USER_HEADER_SEARCH_PATHS' => '"$(GRPC_SRC_ROOT)"',
+      # If we don't set these two settings, `include/grpc/support/time.h` and
+      # `src/core/lib/support/string.h` shadow the system `<time.h>` and `<string.h>`, breaking the
+      # build.
       'USE_HEADERMAP' => 'NO',
       'ALWAYS_SEARCH_USER_PATHS' => 'NO',
-      'USER_HEADER_SEARCH_PATHS' => '"$(GRPC_SRC_ROOT)"',
-    })
-    s.user_target_xcconfig = public_build_settings
-    s.pod_target_xcconfig = private_build_settings
+    }
 
-    s.libraries = 'z'
-    s.dependency 'BoringSSL', '~> 4.0'
+    # Like many other C libraries, gRPC-Core has its public headers under `include/<libname>/` and its
+    # sources and private headers in other directories outside `include/`. Cocoapods' linter doesn't
+    # allow any header to be listed outside the `header_mappings_dir` (even though doing so works in
+    # practice). Because we need our `header_mappings_dir` to be `include/grpc/` for the reason
+    # mentioned above, we work around the linter limitation by dividing the pod into two subspecs, one
+    # for public headers and the other for implementation. Each gets its own `header_mappings_dir`,
+    # making the linter happy.
+    #
+    # The list of source files is generated by a template: `templates/gRPC-Core.podspec.template`. It
+    # can be regenerated from the template by running `tools/buildgen/generate_projects.sh`.
+    s.subspec 'Interface' do |ss|
+      ss.header_mappings_dir = 'include/grpc'
 
-    # A module map is necessary for a dynamic framework to be correctly created by Cocoapods.
-    s.module_map = 'include/grpc/module.modulemap'
+      ss.source_files = ${ruby_multiline_list(grpc_public_headers(libs), 22)}
+    end
+    s.subspec 'Implementation' do |ss|
+      ss.header_mappings_dir = '.'
+      ss.libraries = 'z'
+      ss.dependency "#{s.name}/Interface", version
+      ss.dependency 'BoringSSL', '~> 4.0'
 
-    # List of source files generated by a template: `templates/gRPC-Core.podspec.template`. It can be
-    # regenerated from the template by running `tools/buildgen/generate_projects.sh`.
-    # To save you from scrolling, this is the last part of the podspec.
-    s.source_files = ${(',\n' + 19*' ').join('\'%s\'' % f for f in grpc_files(libs))}
+      # To save you from scrolling, this is the last part of the podspec.
+      ss.source_files = ${ruby_multiline_list(grpc_private_files(libs), 22)}
 
-    s.private_header_files = ${(',\n' + 27*' ').join('\'%s\'' % f for f in grpc_private_headers(libs))}
+      ss.private_header_files = ${ruby_multiline_list(grpc_private_headers(libs), 30)}
+    end
   end