Use add_custom_command instead of execute_process for building protos.

This more closely matches how CMake is supposed to be used. It adds the
commands as build steps instead of running them as part of analyzing the
build script itself. This allows the build steps to be parallelized,
produce output in the correct place, and run only when libicing is
actually being built.

Bug: 149853706
Test: ./gradlew external:icing:assemble
Change-Id: I548cf82d5d5ed6377fd65fa8f5ed04e90bd0d4f8

1 files changed, 20 insertions, 34 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f41890a..398b35e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -26,31 +26,23 @@ set(CMAKE_HOST_ARGS
 set(Protobuf_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../protobuf")
 set(Protobuf_HOST_BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}/protobuf-host")
 set(Protobuf_TARGET_BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}/protobuf-target")
+add_custom_command(
+    OUTPUT "${Protobuf_HOST_BINARY_DIR}/protoc"
 
-# Run another cmake invocation to configure the protobuf project
-execute_process(
+    # Run another cmake invocation to configure the protobuf project
     COMMAND "${CMAKE_COMMAND}"
-    ${CMAKE_HOST_ARGS}
-    -H${Protobuf_SOURCE_DIR}/cmake
-    -B${Protobuf_HOST_BINARY_DIR}
-    -Dprotobuf_BUILD_TESTS:BOOL=OFF
-    RESULT_VARIABLE exec_value
-    OUTPUT_VARIABLE exec_output
-    ERROR_VARIABLE exec_output
-)
-message("Result of proto configuration: ${exec_value}. Output: ${exec_output}")
+      ${CMAKE_HOST_ARGS}
+      -H${Protobuf_SOURCE_DIR}/cmake
+      -B${Protobuf_HOST_BINARY_DIR}
+      -Dprotobuf_BUILD_TESTS:BOOL=OFF
 
-# Run the actual build tool (ninja) to compile protoc for the host
-execute_process(
+    # Run the actual build tool (ninja) to compile protoc for the host
     COMMAND "${CMAKE_MAKE_PROGRAM}" protoc
     WORKING_DIRECTORY ${Protobuf_HOST_BINARY_DIR}
-    RESULT_VARIABLE exec_value
-    OUTPUT_VARIABLE exec_output
-    ERROR_VARIABLE exec_output
 )
-message("Result of proto build: ${exec_value}. Output: ${exec_output}")
+message(STATUS "Building protoc at: ${Protobuf_HOST_BINARY_DIR}/protoc")
 
-# Glob Icing proto sources
+# Glob Icing proto sources. Results look like this: icing/proto/document.proto
 file(
     GLOB_RECURSE
     Icing_PROTO_FILES
@@ -64,26 +56,20 @@ file(MAKE_DIRECTORY ${Icing_PROTO_GEN_DIR})
 foreach(FILE ${Icing_PROTO_FILES})
     # Find the name of the proto file without the .proto extension
     string(REGEX REPLACE "\.proto$" "" FILE_NOEXT ${FILE})
-    execute_process(
+    list(APPEND Icing_PROTO_SOURCES
+      "${Icing_PROTO_GEN_DIR}/${FILE_NOEXT}.pb.cc"
+      "${Icing_PROTO_GEN_DIR}/${FILE_NOEXT}.pb.h")
+    add_custom_command(
+        OUTPUT "${Icing_PROTO_GEN_DIR}/${FILE_NOEXT}.pb.cc"
+          "${Icing_PROTO_GEN_DIR}/${FILE_NOEXT}.pb.h"
         COMMAND "${Protobuf_HOST_BINARY_DIR}/protoc"
-        --proto_path ${CMAKE_CURRENT_SOURCE_DIR}
-        --cpp_out ${Icing_PROTO_GEN_DIR}
-        ${FILE}
+          --proto_path ${CMAKE_CURRENT_SOURCE_DIR}
+          --cpp_out ${Icing_PROTO_GEN_DIR}
+          ${FILE}
         WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
-        RESULT_VARIABLE exec_value
-        OUTPUT_VARIABLE exec_output
-        ERROR_VARIABLE exec_output
+        DEPENDS "${Protobuf_HOST_BINARY_DIR}/protoc"
     )
-    message("Result of protoc ${FILE}: ${exec_value}. Output: ${exec_output}")
 endforeach()
-
-# Glob generated source files from running protoc
-file(
-    GLOB_RECURSE
-    Icing_PROTO_SOURCES
-    "${Icing_PROTO_GEN_DIR}/*.pb.cc"
-    "${Icing_PROTO_GEN_DIR}/*.pb.h"
-)
 message(STATUS "Icing_PROTO_SOURCES=${Icing_PROTO_SOURCES}")
 
 # Compile libprotobuf