From: Johannes Schindelin <redacted>

In 7f5397a07c6c (cmake: support for testing git when building out of the
source tree, 2020-06-26), we implemented support for running Git's test
scripts even after building Git in a different directory than the source
directory.

The way we did this was to edit the file `t/test-lib.sh` to override
`GIT_BUILD_DIR` to point somewhere else than the parent of the `t/`
directory.

This is unideal because it always leaves a tracked file marked as
modified, and it is all too easy to commit that change by mistake.

Let's change the strategy by teaching `t/test-lib.sh` to detect the
presence of a file called `GIT-BUILD-DIR` in the source directory. If it
exists, the contents are interpreted as the location to the _actual_
build directory. We then write this file as part of the CTest
definition.

To support building Git via a regular `make` invocation after building
it using CMake, we ensure that the `GIT-BUILD-DIR` file is deleted (for
convenience, this is done as part of the Makefile rule that is already
run with every `make` invocation to ensure that `GIT-BUILD-OPTIONS` is
up to date).

Signed-off-by: Johannes Schindelin <redacted>
---
 .gitignore                          |  1 +
 Makefile                            |  1 +
 contrib/buildsystems/CMakeLists.txt |  7 +------
 t/test-lib.sh                       | 11 ++++++++++-
 4 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/.gitignore b/.gitignore
index a4522157641..b72ddf09346 100644
--- a/.gitignore
+++ b/.gitignore

@@ -2,6 +2,7 @@
 /fuzz_corpora
 /fuzz-pack-headers
 /fuzz-pack-idx
+/GIT-BUILD-DIR
 /GIT-BUILD-OPTIONS
 /GIT-CFLAGS
 /GIT-LDFLAGS

diff --git a/Makefile b/Makefile
index 04d0fd1fe60..9347ed90da7 100644
--- a/Makefile
+++ b/Makefile

@@ -3028,6 +3028,7 @@ else
 	@echo RUNTIME_PREFIX=\'false\' >>$@+
 endif
 	@if cmp $@+ $@ >/dev/null 2>&1; then $(RM) $@+; else mv $@+ $@; fi
+	@if test -f GIT-BUILD-DIR; then rm GIT-BUILD-DIR; fi

 ### Detect Python interpreter path changes
 ifndef NO_PYTHON

diff --git a/contrib/buildsystems/CMakeLists.txt b/contrib/buildsystems/CMakeLists.txt
index fe606c179f7..29d7e236ae1 100644
--- a/contrib/buildsystems/CMakeLists.txt
+++ b/contrib/buildsystems/CMakeLists.txt

@@ -1067,14 +1067,9 @@ endif()
 #Make the tests work when building out of the source tree
 get_filename_component(CACHE_PATH ${CMAKE_CURRENT_LIST_DIR}/../../CMakeCache.txt ABSOLUTE)
 if(NOT ${CMAKE_BINARY_DIR}/CMakeCache.txt STREQUAL ${CACHE_PATH})
-	file(RELATIVE_PATH BUILD_DIR_RELATIVE ${CMAKE_SOURCE_DIR} ${CMAKE_BINARY_DIR}/CMakeCache.txt)
-	string(REPLACE "/CMakeCache.txt" "" BUILD_DIR_RELATIVE ${BUILD_DIR_RELATIVE})
 	#Setting the build directory in test-lib.sh before running tests
 	file(WRITE ${CMAKE_BINARY_DIR}/CTestCustom.cmake
-		"file(STRINGS ${CMAKE_SOURCE_DIR}/t/test-lib.sh GIT_BUILD_DIR_REPL REGEX \"GIT_BUILD_DIR=(.*)\")\n"
-		"file(STRINGS ${CMAKE_SOURCE_DIR}/t/test-lib.sh content NEWLINE_CONSUME)\n"
-		"string(REPLACE \"\${GIT_BUILD_DIR_REPL}\" \"GIT_BUILD_DIR=\\\"$TEST_DIRECTORY/../${BUILD_DIR_RELATIVE}\\\"\" content \"\${content}\")\n"
-		"file(WRITE ${CMAKE_SOURCE_DIR}/t/test-lib.sh \${content})")
+		"file(WRITE ${CMAKE_SOURCE_DIR}/GIT-BUILD-DIR \"${CMAKE_BINARY_DIR}\")")
 	#misc copies
 	file(COPY ${CMAKE_SOURCE_DIR}/t/chainlint.sed DESTINATION ${CMAKE_BINARY_DIR}/t/)
 	file(COPY ${CMAKE_SOURCE_DIR}/po/is.po DESTINATION ${CMAKE_BINARY_DIR}/po/)

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 55857af601b..4468ac51f25 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh

@@ -42,7 +42,16 @@ then
 	TEST_OUTPUT_DIRECTORY=$TEST_DIRECTORY
 fi
 GIT_BUILD_DIR="${TEST_DIRECTORY%/t}"
-if test "$TEST_DIRECTORY" = "$GIT_BUILD_DIR"
+if test -f "$GIT_BUILD_DIR/GIT-BUILD-DIR"
+then
+	GIT_BUILD_DIR="$(cat "$GIT_BUILD_DIR/GIT-BUILD-DIR")" || exit 1
+	# On Windows, we must convert Windows paths lest they contain a colon
+	case "$(uname -s)" in
+	*MINGW*)
+		GIT_BUILD_DIR="$(cygpath -au "$GIT_BUILD_DIR")"
+		;;
+	esac
+elif test "$TEST_DIRECTORY" = "$GIT_BUILD_DIR"
 then
 	echo "PANIC: Running in a $TEST_DIRECTORY that doesn't end in '/t'?" >&2
 	exit 1