build: do not relink every single binary just for a timestamp

Summary:
Prior to this change, "make check" would always waste a lot of
time relinking 60+ binaries. With this change, it does that
only when the generated file, util/build_version.cc, changes,
and that happens only when the date changes or when the
current git SHA changes.

This change makes some other improvements: before, there was no
rule to build a deleted util/build_version.cc. If it was somehow
removed, any attempt to link a program would fail.
There is no longer any need for the separate file,
build_tools/build_detect_version.  Its functionality is
now in the Makefile.

* Makefile (DEPFILES): Don't filter-out util/build_version.cc.
No need, and besides, removing that dependency was wrong.
(date, git_sha, gen_build_version): New helper variables.
(util/build_version.cc): New rule, to create this file
and update it only if it would contain new information.
* build_tools/build_detect_platform: Remove file.
* db/db_impl.cc: Now, print only date (not the time).
* util/build_version.h (rocksdb_build_compile_time): Remove
declaration.  No longer used.

Test Plan:
- Run "make check" twice, and note that the second time no linking is performed.
- Remove util/build_version.cc and ensure that any "make"
command regenerates it before doing anything else.
- Run this: strings librocksdb.a|grep _build_.
That prints output including the following:

  rocksdb_build_git_date:2015-02-19
  rocksdb_build_git_sha:2.8.fb-1792-g3cb6cc0

Reviewers: ljin, sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D33591

Makefile

@@ -130,6 +130,27 @@ CXXFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverl
 
 LDFLAGS += $(PLATFORM_LDFLAGS)
 
+date := $(shell date +%F)
+git_sha := $(shell git describe HEAD 2>/dev/null)
+gen_build_version =							\
+  printf '%s\n'								\
+    '\#include "build_version.h"'					\
+    'const char* rocksdb_build_git_sha =				\
+      "rocksdb_build_git_sha:$(git_sha)";'				\
+    'const char* rocksdb_build_git_date =				\
+      "rocksdb_build_git_date:$(date)";'				\
+    'const char* rocksdb_build_compile_date = __DATE__;'
+
+# Record the version of the source that we are compiling.
+# We keep a record of the git revision in this file.  It is then built
+# as a regular source file as part of the compilation process.
+# One can run "strings executable_filename | grep _build_" to find
+# the version of the source that we used to build the executable file.
+util/build_version.cc:
+	$(AM_V_GEN)$(gen_build_version) > $@.tmp
+	$(AM_V_at)if test -f $@; then \
+	  cmp -s $@.tmp $@ && : || mv -f $@.tmp $@; else mv -f $@.tmp $@; fi
+
 LIBOBJECTS = $(SOURCES:.cc=.o)
 MOCKOBJECTS = $(MOCK_SOURCES:.cc=.o)
 
@@ -751,7 +772,7 @@ endif
 	@$(CXX) $(CXXFLAGS) $(PLATFORM_SHARED_CFLAGS) \
 	  -MM -MT'$@' -MT'$(<:.cc=.o)' "$<" -o '$@'
 
-DEPFILES = $(filter-out util/build_version.d,$(SOURCES:.cc=.d))
+DEPFILES = $(SOURCES:.cc=.d)
 
 depend: $(DEPFILES)
 

build_tools/build_detect_platform

@@ -165,11 +165,7 @@ esac
 
 JAVA_LDFLAGS="$PLATFORM_LDFLAGS"
 
-if test -z "$DO_NOT_RUN_BUILD_DETECT_VERSION"; then
+# We want to make a list of all cc files in $DIRS,
-  "$PWD/build_tools/build_detect_version"
-fi
-
-# We want to make a list of all cc files within util, db and table
 # except for the test and benchmark files. By default, find will output a list
 # of all files matching either rule, so we need to append -print to make the
 # prune take effect.
@@ -183,6 +179,12 @@ PORTABLE_FILES=`cd "$ROCKSDB_ROOT"; find $DIRS $PRUNE_TEST -o $PRUNE_BENCH -o $P
 MOCK_SOURCES=`cd "$ROCKSDB_ROOT"; find $DIRS -name '*mock*.cc' -print | grep -v "test" | sort | tr "\n" " "`
 set +f # re-enable globbing
 
+# If the generated util/build_version.cc is not on the list, add it:
+case $PORTABLE_FILES in
+    *util/build_version.cc*) ;;
+    *) PORTABLE_FILES="$PORTABLE_FILES util/build_version.cc" ;;
+esac
+
 # The sources consist of the portable files, plus the platform-specific port
 # file.
 echo "SOURCES=$PORTABLE_FILES $GENERIC_PORT_FILES $PORT_FILES" >> "$OUTPUT"

build_tools/build_detect_version

@@ -1,22 +0,0 @@
-#!/bin/sh
-#
-# Record the version of the source that we are compiling.
-# We keep a record of the git revision in util/version.cc. This source file
-# is then built as a regular source file as part of the compilation process.
-# One can run "strings executable_filename | grep _build_" to find the version of
-# the source that we used to build the executable file.
-
-OUTFILE="$PWD/util/build_version.cc"
-
-GIT_SHA=""
-if command -v git >/dev/null 2>&1; then
-    GIT_SHA=$(git rev-parse HEAD 2>/dev/null)
-fi
-
-cat > "${OUTFILE}" <<EOF
-#include "build_version.h"
-const char* rocksdb_build_git_sha = "rocksdb_build_git_sha:${GIT_SHA}";
-const char* rocksdb_build_git_datetime = "rocksdb_build_git_datetime:$(date)";
-const char* rocksdb_build_compile_date = __DATE__;
-const char* rocksdb_build_compile_time = __TIME__;
-EOF

db/db_impl.cc

@@ -4047,8 +4047,8 @@ void DumpRocksDBBuildVersion(Logger * log) {
       "RocksDB version: %d.%d.%d\n", ROCKSDB_MAJOR, ROCKSDB_MINOR,
       ROCKSDB_PATCH);
   Log(InfoLogLevel::INFO_LEVEL, log, "Git sha %s", rocksdb_build_git_sha);
-  Log(InfoLogLevel::INFO_LEVEL, log, "Compile time %s %s",
+  Log(InfoLogLevel::INFO_LEVEL, log, "Compile date %s",
-      rocksdb_build_compile_time, rocksdb_build_compile_date);
+      rocksdb_build_compile_date);
 #endif
 }
 

util/build_version.h

@@ -7,10 +7,9 @@
 #if !defined(IOS_CROSS_COMPILE)
 // if we compile with Xcode, we don't run build_detect_vesion, so we don't
 // generate these variables
-// these variables tell us about the git config and time
+// this variable tells us about the git revision
 extern const char* rocksdb_build_git_sha;
 
-// these variables tell us when the compilation occurred
+// Date on which the code was compiled:
-extern const char* rocksdb_build_compile_time;
 extern const char* rocksdb_build_compile_date;
 #endif