tg: initial Git sha256 hash algorithm support

The primary issue that arises when supporting a different hash
algorithm is that the "constant" hash values used for different
items such as the "null" object and the "empty" blob and tree all
change.

Update the code to automatically use the correct "constant" hash
values for the hash algorithm that's currently in use.

Also make sure the wayback machine preserves the correct hash
algorithm when activating wayback mode.

Signed-off-by: Kyle J. McKay <[email protected]>

tg--index-merge-one-file.sh

@@ -1,8 +1,8 @@
 #!/bin/sh
 
 # tg--index-merge-one-file -- auto merge a file without touching working tree
-# Copyright (C) 2017 Kyle J. McKay
-# All rights reserved.
+# Copyright (C) 2017,2021 Kyle J. McKay
+# All rights reserved
 # License GPLv2+
 
 # $TG_TMP_DIR => location to store temporary files
@@ -15,7 +15,6 @@
 # $6          => stage 2 mode
 # $7          => stage 3 mode
 
-nullsha="0000000000000000000000000000000000000000"
 tab='	'
 
 if [ "$1" = "-h" ] && [ $# -eq 1 ]; then
@@ -31,6 +30,11 @@ esac; fi
 
 [ $# -eq 7 ] || exit 1
 
+mtblob="$(git hash-object -t blob --stdin </dev/null 2>/dev/null)" || :
+nullsha="0000000000000000000000000000000000000000"
+test "${#mtblob}" != "64" ||
+nullsha="0000000000000000000000000000000000000000000000000000000000000000"
+
 # The read-tree --aggressive option handles three cases that may end up in
 # here:
 #

tg.sh

@@ -16,8 +16,6 @@ octet='[0-9a-f][0-9a-f]'
 octet4="$octet$octet$octet$octet"
 octet19="$octet4$octet4$octet4$octet4$octet$octet$octet"
 octet20="$octet4$octet4$octet4$octet4$octet4"
-nullsha="0000000000000000000000000000000000000000" # :|git mktree|tr 0-9a-f 0
-mtblob="e69de29bb2d1d6434b8b29ae775ad8c2e48c5391" # :|git hash-object --stdin -w
 tab='	'
 lf='
 '
@@ -2099,6 +2097,46 @@ setup_git_dirs()
 	fi
 	[ -n "$git_dir" ] && [ -n "$git_common_dir" ] &&
 	[ -d "$git_dir" ] && [ -d "$git_common_dir" ] || die "Not a git repository"
+	activate_awksome $1
+}
+
+mtblob=
+setup_hashalgo_vars()
+{
+	test -z "$mtblob" || return 0
+
+	# the empty blob will match the hash algorithm of the repository (if any)
+
+	mtblob="$(git hash-object -t blob --stdin </dev/null 2>/dev/null)" ||
+		die "git hash-object failed"
+	case "${#mtblob}" in
+	40)
+		nullsha="0000000000000000000000000000000000000000"
+		;;
+	64)
+		nullsha="0000000000000000000000000000000000000000000000000000000000000000"
+		;;
+	*)
+		die "git hash-object failed"
+	esac
+}
+
+## Include awk scripts and their utility functions (separated for easier debugging)
+awksome_loaded=
+activate_awksome()
+{
+	test -z "$awksome_loaded" || return 0
+
+	# tg--awksome requires that mtblob be set before sourcing it;
+	# mtblob is configured by setup_hashalgo_vars;
+	# therefore run setup_hashalgo_vars first.
+
+	setup_hashalgo_vars $1
+	[ -f "$TG_INST_CMDDIR/tg--awksome" ] && [ -r "$TG_INST_CMDDIR/tg--awksome" ] ||
+		die "Missing awk scripts: '$TG_INST_CMDDIR/tg--awksome'"
+	. "$TG_INST_CMDDIR/tg--awksome"
+
+	awksome_loaded=1
 }
 
 basic_setup_remote()
@@ -2340,14 +2378,20 @@ activate_wayback_machine()
 	qpesc="$(printf '%s\n' "$git_common_dir" | sed -e 's/\([\\""]\)/\\\1/g' -e '$!s/$/\\n/' | tr -d '\n')"
 	laru="false"
 	[ -z "$2" ] || laru="true"
+	rfv=0
+	ofmt=
+	if [ "${#mtblob}" = 64 ]; then
+		rfv=1
+		ofmt="$lf${tab}objectFormat = sha256"
+	fi
 	printf '%s' "\
 [include]
 	path = \"$qpesc/config\"
 [core]
 	bare = false
 	logAllRefUpdates = $laru
-	repositoryFormatVersion = 0
-[extensions]
+	repositoryFormatVersion = $rfv
+[extensions]$ofmt
 	preciousObjects = true
 [gc]
 	auto = 0
@@ -2417,6 +2461,7 @@ set_topbases()
 
 	[ -z "$tg_topbases_set" ] || return 0
 
+	activate_awksome # may not have been loaded yet
 	topbases_implicit_default=1
 	# See if topgit.top-bases is set to heads or refs
 	tgtb="$(git config "topgit.top-bases" 2>/dev/null)" || :
@@ -2556,12 +2601,6 @@ v_get_abs_path()
 [ -d "$TG_INST_CMDDIR" ] ||
 	die "No command directory: '$TG_INST_CMDDIR'"
 
-## Include awk scripts and their utility functions (separated for easier debugging)
-
-[ -f "$TG_INST_CMDDIR/tg--awksome" ] && [ -r "$TG_INST_CMDDIR/tg--awksome" ] ||
-	die "Missing awk scripts: '$TG_INST_CMDDIR/tg--awksome'"
-. "$TG_INST_CMDDIR/tg--awksome"
-
 if [ -n "$tg__include" ]; then
 
 	# We were sourced from another script for our utility functions;