build: regenerate config-list.h when Documentation changes

The Meson-based build doesn't know when to rebuild config-list.h, so the
header is sometimes stale.

For example, an old build directory might have config-list.h from before
4173df5 (submodule: introduce extensions.submodulePathConfig,
2026-01-12), which added submodule.<name>.gitdir to the list. Without
it, t9902-completion.sh fails. Regenerating the config-list.h artifact
from sources fixes the artifact and the test.

Since Meson does not have (or want) builtin support for globbing like
Make, teach generate-configlist.sh to also generate a list of
Documentation files its output depends on, and incorporate that into the
Meson build. We honor the undocumented GCC/Clang contract of outputting
empty targets for all the dependencies (like they do with -MP). That is,
generate lines like

    build/config-list.h: $SOURCE_DIR/Documentation/config.adoc
    $SOURCE_DIR/Documentation/config.adoc:

We assume that if a user adds a new file under
Documentation/config then they will also edit one of the existing files
to include that new file, and that will trigger a rebuild. Also mark the
generator script as a dependency.

While we're at it, teach the Makefile to use the same "the script knows
it's dependencies" logic.

For Meson, combining the following commands helps debug dependencies:

    ninja -C <builddir> -t deps config-list.h
    ninja -C <builddir> -t browse config-list.h

The former lists all the dependencies discovered from our output ".d"
file (the config documentation) and the latter shows the dependency on
the script itself, among other useful edges in the dependency graph.

Helped-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: benknoble

Makefile

@@ -2687,9 +2687,10 @@ $(BUILT_INS): git$X
 	cp $< $@
 
 config-list.h: generate-configlist.sh
+	@mkdir -p .depend
+	$(QUIET_GEN)$(SHELL_PATH) ./generate-configlist.sh . $@ .depend/config-list.h.d
 
-config-list.h: Documentation/*config.adoc Documentation/config/*.adoc
-	$(QUIET_GEN)$(SHELL_PATH) ./generate-configlist.sh . $@
+-include .depend/config-list.h.d
 
 command-list.h: generate-cmdlist.sh command-list.txt
 

generate-configlist.sh

@@ -2,10 +2,11 @@
 
 SOURCE_DIR="$1"
 OUTPUT="$2"
+DEPFILE="$3"
 
 if test -z "$SOURCE_DIR" || ! test -d "$SOURCE_DIR" || test -z "$OUTPUT"
 then
-	echo >&2 "USAGE: $0 <SOURCE_DIR> <OUTPUT>"
+	echo >&2 "USAGE: $0 <SOURCE_DIR> <OUTPUT> [<DEPFILE>]"
 	exit 1
 fi
 
@@ -36,3 +37,16 @@ EOF
 	echo
 	print_config_list
 } >"$OUTPUT"
+
+if test -n "$DEPFILE"
+then
+	QUOTED_OUTPUT="$(printf '%s\n' "$OUTPUT" | sed 's,[&/\],\\&,g')"
+	{
+		printf '%s\n' "$SOURCE_DIR"/Documentation/*config.adoc \
+			"$SOURCE_DIR"/Documentation/config/*.adoc |
+			sed -e 's/[# ]/\\&/g' -e "s/^/$QUOTED_OUTPUT: /"
+		printf '%s:\n' "$SOURCE_DIR"/Documentation/*config.adoc \
+			"$SOURCE_DIR"/Documentation/config/*.adoc |
+			sed -e 's/[# ]/\\&/g'
+	} >"$DEPFILE"
+fi

meson.build

@@ -720,11 +720,14 @@ endif
 
 builtin_sources += custom_target(
   output: 'config-list.h',
+  depfile: 'config-list.h.d',
+  depend_files: [ 'generate-configlist.sh' ],
   command: [
     shell,
-    meson.current_source_dir() + '/generate-configlist.sh',
+    meson.current_source_dir() / 'generate-configlist.sh',
     meson.current_source_dir(),
     '@OUTPUT@',
+    '@DEPFILE@',
   ],
   env: script_environment,
 )