[9front] git/merge: preserve exec bit correctly

[ori@eigenstate.org]

A while ago, qwx noticed that we clobbered the exec
bit when merging files. This is not what we want, so
we changed the operator precedence to avoid merging
dirty files implicitly.

But we do want to merge, because it's convenient for
maintaining permissions. So, instead, we should do a
3 way merge of the exec bit.

This patch does that, as well as reverting the rollback
of that change.

While we're here, we adjust the timestamps correctly
in git/branch.

This requires changes to git/fs, because without an open
handler, lib9p allows opening any file with any mode,
which confuses 'test -x'.

diff 061ec57021a7c813844582f6f1973dafae6e668b uncommitted
--- a//sys/lib/git/common.rc
+++ b//sys/lib/git/common.rc
@@ -29,13 +29,18 @@
 	' $*
 }
 
-fn present {
+fn mergeperm {
 	if(~ $1 /dev/null && cmp $2 $3>/dev/null)
 		status=gone
 	if not if (~ $3 /dev/null && cmp $1 $2>/dev/null)
 		status=gone
-	if not
+	if not {
+		if(test -x $2 && {! test -x $1 || ! test -x $3})
+			mergedperms='-x'
+		if not
+			mergedperms='+x'
 		status=()
+	}
 }
 
 fn whoami{
@@ -76,9 +81,10 @@
 	if(! ape/diff3 -3 -m $ours $base $theirs > $tmp)
 		echo merge needed: $out >[1=2]
 
-	if(present $ours $base $theirs){
+	if(mergeperm $ours $base $theirs){
 		mv $tmp $out
 		git/add $out
+		chmod $mergedperms $out
 	}
 	if not {
 		rm -f $tmp $out
--- a/sys/src/cmd/git/branch
+++ b/sys/src/cmd/git/branch
@@ -48,9 +48,12 @@
 modified=`$nl{git/query -c HEAD $base | grep '^[^-]' | subst '^..'}
 deleted=`$nl{git/query -c HEAD $base | grep '^-' | subst '^..'}
 
-if(! ~ $#modified 0 || {! ~ $#deleted 0 && ~ $#merge 0}){
-	git/walk -fRMA $modified $deleted || 
-		die 'uncommitted changes would be clobbered'
+# if we're not merging, don't clobber existing changes.
+if(~ $#merge 0){
+	if(! ~ $#modified 0 || ! ~ $#deleted 0){
+		git/walk -fRMA $modified $deleted || 
+			die 'uncommitted changes would be clobbered'
+	}
 }
 if(~ $delete 1){
 	rm -f .git/$new
@@ -97,10 +100,9 @@
 		rm -rf .git/index9/tracked/$m
 	}
 	if(~ $b file){
-		if(cp -x -- $basedir/tree/$m $m)
-			walk -eq $m > .git/index9/tracked/$m
-		if not
-			echo -n > .git/index9/tracked/$m
+		cp -x -- $basedir/tree/$m $m)
+		walk -eq $m > .git/index9/tracked/$m
+		touch $m
 	}
 }
 
--- a/sys/src/cmd/git/fs.c
+++ b/sys/src/cmd/git/fs.c
@@ -70,13 +70,14 @@
 	"ctl",
 };
 
-#define Eperm	"permission denied";
-#define Eexist	"does not exist";
-#define E2long	"path too long";
-#define Enodir	"not a directory";
-#define Erepo	"unable to read repo";
-#define Egreg	"wat";
-#define Ebadobj	"invalid object";
+#define Eperm	"permission denied"
+#define Eexist	"does not exist"
+#define E2long	"path too long"
+#define Enodir	"not a directory"
+#define Erepo	"unable to read repo"
+#define Eobject "invalid object"
+#define Egreg	"wat"
+#define Ebadobj	"invalid object"
 
 char	gitdir[512];
 char	*username;
@@ -624,9 +625,9 @@
 			e = objwalk1(q, o->obj, o, c, name, Qobject, aux);
 		}else{
 			if(hparse(&h, name) == -1)
-				return "invalid object name";
+				return Eobject;
 			if((c->obj = readobject(h)) == nil)
-				return "could not read object";
+				return Eobject;
 			if(c->obj->type == GBlob || c->obj->type == GTag){
 				c->mode = 0644;
 				q->type = 0;
@@ -805,6 +806,34 @@
 }
 
 static void
+gitopen(Req *r)
+{
+	Gitaux *aux;
+	Crumb *c;
+
+	aux = r->fid->aux;
+	c = crumb(aux, 0);
+	switch(r->ifcall.mode&3){
+	default:
+		respond(r, "botched mode");
+		break;
+	case OWRITE:
+		respond(r, Eperm);
+		break;
+	case OREAD:
+	case ORDWR:
+		respond(r, nil);
+		break;
+	case OEXEC:
+		if((c->mode & 0111) == 0)
+			respond(r, Eperm);
+		else
+			respond(r, nil);
+		break;
+	}
+}
+
+static void
 gitstat(Req *r)
 {
 	Gitaux *aux;
@@ -830,6 +859,7 @@
 	.attach=gitattach,
 	.walk1=gitwalk1,
 	.clone=gitclone,
+	.open=gitopen,
 	.read=gitread,
 	.stat=gitstat,
 	.destroyfid=gitdestroyfid,