[PATCH 6/6] Makefile: add a INSTALL_FALLBACK_LN_CP mode

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Change the default behavior on "make install" where we fallback
through a chain of "ln || ln -s || cp" to instead error out when we
can't symlink or hardlink, and not then fallback on a "cp" (or from a
symlink to hardlink etc.).

The fallback behavior was introduced in 3e073dc5611 (Makefile: always
provide a fallback when hardlinks fail, 2008-08-25), since then we've
gained the ability to specify e.g. that we'd like symlinks via the
INSTALL_SYMLINKS setting.

It doesn't make sense as a default to silently fall back if we can't
satisfy those settings. We should instead error out, at which point
the developer/builder/sysadmin can set one or more of the relevant
hardlink or symlink settings.

This also changes the behavior for the build (not install!) to always
use the new strict mode. This will theoretically break things for
someone who can't make symlinks or hardlinks in their git checkout
when building.

That part of this change would break building on Windows and other
platforms that don't support symlinks if INSTALL_SYMLINKS were to
become the default, but it's not on by default, so we should be fine
here. If and when INSTALL_SYMLINKS ever becomes the default the right
way to deal with this would be to tweak config.mak.uname
appropriately, not to silently fall back on a copy.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Makefile    | 11 +++++++++++
 ln-or-cp.sh | 29 ++++++++++++++++++++++++++++-
 2 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 466df1a8e90..ccbded79093 100644
--- a/Makefile
+++ b/Makefile
@@ -350,6 +350,13 @@ all::
 # or if that fails fall back on a "cp" instead of a "ln". Useful for
 # when you don't want hardlinks at all.
 #
+# Define INSTALL_FALLBACK_LN_CP if you'd like your
+# "INSTALL_SYMLINKS=Y" to fall back on hardlinks if we can't run "ln
+# -s", and for "ln" to fall back on "NO_INSTALL_HARDLINKS=Y" if we
+# can't perform a "ln" and need to fall-back to a "cp". This used to
+# be the default behavior, but we'll now error if we can't satisfy
+# your INSTALL_SYMLINKS, NO_INSTALL_HARDLINKS etc. settings.
+#
 # Define SKIP_DASHED_BUILT_INS if you do not need the dashed versions of the
 # built-ins to be linked/copied at all.
 #
@@ -3020,6 +3027,7 @@ endif
 	{ test "$$bindir/" = "$$execdir/" || \
 	  for p in git$X $(filter $(install_bindir_programs),$(ALL_PROGRAMS)); do \
 		./ln-or-cp.sh \
+			--install-fallback-ln-cp "$(INSTALL_FALLBACK_LN_CP)" \
 			--install-symlinks "$(INSTALL_SYMLINKS)" \
 			--no-install-hardlinks "$(NO_INSTALL_HARDLINKS)" \
 			--no-cross-directory-hardlinks "$(NO_CROSS_DIRECTORY_HARDLINKS)" \
@@ -3029,6 +3037,7 @@ endif
 	} && \
 	for p in $(filter $(install_bindir_programs),$(BUILT_INS)); do \
 		./ln-or-cp.sh \
+			--install-fallback-ln-cp "$(INSTALL_FALLBACK_LN_CP)" \
 			--install-symlinks "$(INSTALL_SYMLINKS)" \
 			--no-install-hardlinks "$(NO_INSTALL_HARDLINKS)" \
 			--symlink-target "git$X" \
@@ -3038,6 +3047,7 @@ endif
 		if test -z "$(SKIP_DASHED_BUILT_INS)"; \
 		then \
 			./ln-or-cp.sh \
+				--install-fallback-ln-cp "$(INSTALL_FALLBACK_LN_CP)" \
 				--install-symlinks "$(INSTALL_SYMLINKS)" \
 				--no-install-hardlinks "$(NO_INSTALL_HARDLINKS)" \
 				--symlink-target "$$destdir_from_execdir_SQ/$(bindir_relative_SQ)/git$X" \
@@ -3047,6 +3057,7 @@ endif
 	remote_curl_aliases="$(REMOTE_CURL_ALIASES)" && \
 	for p in $$remote_curl_aliases; do \
 		./ln-or-cp.sh \
+			--install-fallback-ln-cp "$(INSTALL_FALLBACK_LN_CP)" \
 			--install-symlinks "$(INSTALL_SYMLINKS)" \
 			--no-install-hardlinks "$(NO_INSTALL_HARDLINKS)" \
 			--symlink-target "git-remote-http$X" \
diff --git a/ln-or-cp.sh b/ln-or-cp.sh
index 37380993c64..f77dad71bdb 100755
--- a/ln-or-cp.sh
+++ b/ln-or-cp.sh
@@ -1,5 +1,6 @@
 #!/bin/sh
 
+install_fallback_ln_cp=
 install_symlinks=
 no_install_hardlinks=
 no_cross_directory_hardlinks=
@@ -7,6 +8,10 @@ symlink_target=
 while test $# != 0
 do
 	case "$1" in
+	--install-fallback-ln-cp)
+		install_fallback_ln_cp="$2"
+		shift
+		;;
 	--install-symlinks)
 		install_symlinks="$2"
 		shift
@@ -61,4 +66,26 @@ main_with_fallbacks () {
 	fi
 }
 
-main_with_fallbacks
+main_no_fallbacks () {
+	if test -n "$no_install_hardlinks" -a -z "$install_symlinks"
+	then
+		cp "$target" "$link"
+	elif test -n "$install_symlinks" -o -n "$no_cross_directory_hardlinks"
+	then
+		ln -f -s "$symlink_target" "$link"
+	elif test -n "$no_install_hardlinks"
+	then
+		cp "$target" "$link"
+	else
+		ln -f "$target" "$link"
+	fi
+}
+
+if test -z "$install_fallback_ln_cp"
+then
+	# The stricter mode, where we know what we want
+	main_no_fallbacks
+else
+	main_with_fallbacks
+
+fi
-- 
2.31.1.461.gd47399f6574