[9front] git: cache size in bytes

[ori@eigenstate.org]

I was looking at syzkaller recently,
and cloning it made git9 unhappy. This
is because our cache size is in object
counts, rather than bytes -- so lots
of large objects can blow away our size.

This change makes git9 track cache sizes
in bytes, which prevents a lot of large
objects from blowing away our memory.

It defaults to 512 MiB, but I'm happy to
tweak that.

diff facb0e757ac63f763bd942a2714f979538b99eb0 uncommitted
--- a/sys/src/cmd/git/git.h
+++ b/sys/src/cmd/git/git.h
@@ -4,6 +4,7 @@
 #include <flate.h>
 #include <regexp.h>
 
+typedef struct Capset	Capset;
 typedef struct Conn	Conn;
 typedef struct Hash	Hash;
 typedef struct Delta	Delta;
@@ -26,6 +27,9 @@
 	Npackcache	= 32,
 	Hashsz		= 20,
 	Pktmax		= 65536,
+	KiB		= 1024,
+	MiB		= 1024*KiB,
+	GiB		= 1024*MiB,
 };
 
 enum {
--- a/sys/src/cmd/git/pack.c
+++ b/sys/src/cmd/git/pack.c
@@ -66,7 +66,7 @@
 Object *lruhead;
 Object *lrutail;
 int	ncache;
-int	cachemax = 4096;
+int	cachemax = 512*MiB;
 Packf	*packf;
 int	npackf;
 int	openpacks;
@@ -158,7 +158,7 @@
 	if(!(o->flag & Ccache)){
 		o->flag |= Ccache;
 		ref(o);
-		ncache++;
+		ncache += o->size;
 	}
 	while(ncache > cachemax && lrutail != nil){
 		p = lrutail;
@@ -168,8 +168,8 @@
 		p->flag &= ~Ccache;
 		p->prev = nil;
 		p->next = nil;
+		ncache -= p->size;
 		unref(p);
-		ncache--;
 	}		
 }