Prevent full rebuild on every run of cmake

config.h is generated by CMake's configure_file(), which is normally
smart enough to not update the file's timestamp if the file is
unchanged. However, config.h.in #defines LCB_BUILD_TIMESTAMP which will
be different every time CMake is run. This means config.h will be
updated every time, and since most .cc files directly or indirectly
include config.h, this causes almost everything to be recompiled.

This change introduces a separate build-timestamp.h.in, which contains
only #define LCB_BUILD_TIMESTAMP. This in turn is only included by a new
file cbc-timestamp.cc, which defines a const global variable
lcb_build_timestamp with that value. Thus, only this tiny file will get
rebuilt every time CMake is run.

Change-Id: I67c78fed4c33be911427dfe1dfd97a258eb2e197
Reviewed-on: https://review.couchbase.org/c/libcouchbase/+/201943
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>

Author: ceejatec

cmake/Modules/GenerateConfigDotH.cmake

@@ -34,3 +34,6 @@ ENDIF()
 CONFIGURE_FILE(
     ${PROJECT_SOURCE_DIR}/cmake/config-cmake.h.in
     ${LCB_GENSRCDIR}/config.h)
+CONFIGURE_FILE(
+    ${PROJECT_SOURCE_DIR}/cmake/build-timestamp.h.in
+    ${LCB_GENSRCDIR}/build-timestamp.h)

cmake/build-timestamp.h.in

@@ -0,0 +1 @@
+#define LCB_BUILD_TIMESTAMP "${LCB_BUILD_TIMESTAMP}"

cmake/config-cmake.h.in

@@ -68,7 +68,6 @@
 #define LCB_CXX_FLAGS "${CMAKE_CXX_FLAGS}"
 #define LCB_C_COMPILER "${CMAKE_C_COMPILER_ID} ${CMAKE_C_COMPILER_VERSION}"
 #define LCB_C_FLAGS "${CMAKE_C_FLAGS}"
-#define LCB_BUILD_TIMESTAMP "${LCB_BUILD_TIMESTAMP}"
 
 #define LCB_CLIENT_ID                                                                                                  \
     "libcouchbase/" LCB_VERSION_STRING " (" LCB_SYSTEM "; " LCB_SYSTEM_PROCESSOR "; " LCB_C_COMPILER ")"

tools/CMakeLists.txt

@@ -2,7 +2,7 @@ INCLUDE_DIRECTORIES(${SOURCE_ROOT}/contrib/cliopts)
 FILE(GLOB T_COMMONSRC common/*.cc)
 ADD_LIBRARY(lcbtools OBJECT ${T_COMMONSRC})
 
-ADD_EXECUTABLE(cbc cbc.cc
+ADD_EXECUTABLE(cbc cbc.cc cbc-timestamp.cc
     $<TARGET_OBJECTS:lcbtools> $<TARGET_OBJECTS:cliopts> $<TARGET_OBJECTS:lcb_jsoncpp>)
 TARGET_LINK_LIBRARIES(cbc couchbase)
 

tools/cbc-timestamp.cc

@@ -0,0 +1,25 @@
+/* -*- Mode: C; tab-width: 4; c-basic-offset: 4; indent-tabs-mode: nil -*- */
+/*
+ *     Copyright 2011-2020 Couchbase, Inc.
+ *
+ *   Licensed under the Apache License, Version 2.0 (the "License");
+ *   you may not use this file except in compliance with the License.
+ *   You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *   Unless required by applicable law or agreed to in writing, software
+ *   distributed under the License is distributed on an "AS IS" BASIS,
+ *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *   See the License for the specific language governing permissions and
+ *   limitations under the License.
+ */
+
+#include "cbc-timestamp.h"
+#include "build-timestamp.h"
+
+namespace cbc {
+
+const char *const lcb_build_timestamp = LCB_BUILD_TIMESTAMP;
+
+} // namespace cbc

tools/cbc-timestamp.h

@@ -0,0 +1,27 @@
+/* -*- Mode: C; tab-width: 4; c-basic-offset: 4; indent-tabs-mode: nil -*- */
+/*
+ *     Copyright 2011-2020 Couchbase, Inc.
+ *
+ *   Licensed under the Apache License, Version 2.0 (the "License");
+ *   you may not use this file except in compliance with the License.
+ *   You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *   Unless required by applicable law or agreed to in writing, software
+ *   distributed under the License is distributed on an "AS IS" BASIS,
+ *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *   See the License for the specific language governing permissions and
+ *   limitations under the License.
+ */
+
+#ifndef CBC_TIMESTAMP_H
+#define CBC_TIMESTAMP_H
+
+namespace cbc {
+
+extern const char *const lcb_build_timestamp;
+
+} // namespace cbc
+
+#endif

tools/cbc.cc

@@ -35,6 +35,7 @@
 #include "common/options.h"
 #include "common/histogram.h"
 #include "cbc-handlers.h"
+#include "cbc-timestamp.h"
 #include "connspec.h"
 #include "rnd.h"
 #include "contrib/lcb-jsoncpp/lcb-jsoncpp.h"
@@ -1227,7 +1228,7 @@ void VersionHandler::run()
     printf("cbc:\n");
     printf("  Runtime: Version=%s, Changeset=%s\n", lcb_get_version(nullptr), changeset);
     printf("  Headers: Version=%s, Changeset=%s\n", LCB_VERSION_STRING, LCB_VERSION_CHANGESET);
-    printf("  Build Timestamp: %s\n", LCB_BUILD_TIMESTAMP);
+    printf("  Build Timestamp: %s\n", lcb_build_timestamp);
 #ifdef CMAKE_BUILD_TYPE
     printf("  CMake Build Type: %s\n", CMAKE_BUILD_TYPE);
 #endif