The current scheme for getting build-time variables into a
shell script is to munge the script with sed, and stick the
munged variable into a special sentinel file so that "make"
knows about the dependency.

Instead, we can combine both functions by generating a shell
snippet with our value, and then "building" shell scripts by
concatenating their snippets. "make" then handles the
dependency automatically, and it's easy to generate tighter
dependencies.

We demonstrate here by moving the "DIFF" substitution into
its own snippet, which lets us rebuild only the single
affected file when it changes.

Signed-off-by: Jeff King <p...@peff.net>
---
The astute reader will notice that:

  MAKE_DIFF='diff'
  $MAKE_DIFF ...

that we end up with is not _quite_ the same as replacing "$MAKE_DIFF" in
the actual script text with "diff" at build-time. In particular, the way
that whitespace and quotes are treated is different. I'm not sure what
we would want to do here. Calling "eval" would work. Or we could have
the snippet produce a function, rather than a variable, like:

  DIFF() {
      diff "$@"
  }

 Makefile        | 18 +++++++++++++++---
 git-sh-setup.sh |  2 +-
 script/mksh     |  4 ++++
 3 files changed, 20 insertions(+), 4 deletions(-)
 create mode 100644 script/mksh

diff --git a/Makefile b/Makefile
index 203171d..ad3100d 100644
--- a/Makefile
+++ b/Makefile
@@ -1598,6 +1598,11 @@ MAKE/%-string.h: MAKE/% script/mkcstring
                $(subst -,_,$*) <$< >$@+ && \
                mv $@+ $@
 
+MAKE/%.sh: MAKE/% script/mksh
+       $(QUIET_GEN)$(SHELL_PATH) script/mksh \
+               $(subst -,_,$*) <$< >$@+ && \
+               mv $@+ $@
+
 LIBS = $(GITLIBS) $(EXTLIBS)
 
 BASIC_CFLAGS += -DSHA1_HEADER=$(call sq,$(SHA1_HEADER)) \
@@ -1734,7 +1739,6 @@ common-cmds.h: $(wildcard Documentation/git-*.txt)
 
 $(eval $(call make-var,SCRIPT-DEFINES,script parameters,\
        :$(SHELL_PATH)\
-       :$(DIFF)\
        :$(GIT_VERSION)\
        :$(localedir)\
        :$(NO_CURL)\
@@ -1743,18 +1747,24 @@ $(eval $(call make-var,SCRIPT-DEFINES,script 
parameters,\
        :$(gitwebdir)\
        :$(PERL_PATH)\
 ))
+$(eval $(call make-var,DIFF,diff command,$(DIFF)))
 define cmd_munge_script
 $(RM) $@ $@+ && \
+{ \
+includes="$(filter MAKE/%.sh,$^)"; \
+if ! test -z "$$includes"; then \
+       cat $$includes; \
+fi && \
 sed -e '1s|#!.*/sh|#!$(call sqi,$(SHELL_PATH))|' \
     -e 's|@SHELL_PATH@|$(call sqi,$(SHELL_PATH))|' \
-    -e 's|@@DIFF@@|$(call sqi,$(DIFF))|' \
     -e 's|@@LOCALEDIR@@|$(call sqi,$(localedir))|g' \
     -e 's/@@NO_CURL@@/$(NO_CURL)/g' \
     -e 's/@@USE_GETTEXT_SCHEME@@/$(USE_GETTEXT_SCHEME)/g' \
     -e $(BROKEN_PATH_FIX) \
     -e 's|@@GITWEBDIR@@|$(call sqi,$(gitwebdir))|g' \
     -e 's|@@PERL@@|$(call sqi,$(PERL_PATH))|g' \
-    $@.sh >$@+
+    $@.sh; \
+} >$@+
 endef
 
 $(patsubst %.sh,%,$(SCRIPT_SH)) : % : %.sh MAKE/SCRIPT-DEFINES
@@ -1766,6 +1776,8 @@ $(SCRIPT_LIB) : % : %.sh MAKE/SCRIPT-DEFINES
        $(QUIET_GEN)$(cmd_munge_script) && \
        mv $@+ $@
 
+git-sh-setup: MAKE/DIFF.sh
+
 git.res: git.rc GIT-VERSION-FILE
        $(QUIET_RC)$(RC) \
          $(join -DMAJOR= -DMINOR=, $(wordlist 1,2,$(subst -, ,$(subst ., 
,$(GIT_VERSION))))) \
diff --git a/git-sh-setup.sh b/git-sh-setup.sh
index fffa3c7..627d289 100644
--- a/git-sh-setup.sh
+++ b/git-sh-setup.sh
@@ -285,7 +285,7 @@ clear_local_git_env() {
 # remove lines from $1 that are not in $2, leaving only common lines.
 create_virtual_base() {
        sz0=$(wc -c <"$1")
-       @@DIFF@@ -u -La/"$1" -Lb/"$1" "$1" "$2" | git apply --no-add
+       $MAKE_DIFF -u -La/"$1" -Lb/"$1" "$1" "$2" | git apply --no-add
        sz1=$(wc -c <"$1")
 
        # If we do not have enough common material, it is not
diff --git a/script/mksh b/script/mksh
new file mode 100644
index 0000000..d41e77a
--- /dev/null
+++ b/script/mksh
@@ -0,0 +1,4 @@
+#!/bin/sh
+
+name=$1; shift
+printf "MAKE_%s='%s'\n" "$name" "$(sed "s/'/'\\''/g")"
-- 
1.8.5.2.500.g8060133

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