fossil: import from plan 9

R=rsc
https://codereview.appspot.com/7988047

1 files changed, 1068 insertions, 0 deletions

diff --git a/src/cmd/fossil/source.c b/src/cmd/fossil/source.c
new file mode 100644
index 00000000..aa614192
--- /dev/null
+++ b/src/cmd/fossil/source.c
@@ -0,0 +1,1068 @@
+#include "stdinc.h"
+#include "dat.h"
+#include "fns.h"
+#include "error.h"
+#include "9.h"
+
+static int	sizeToDepth(uvlong s, int psize, int dsize);
+static u32int 	tagGen(void);
+static Block 	*sourceLoad(Source *r, Entry *e);
+static int	sourceShrinkDepth(Source*, Block*, Entry*, int);
+static int	sourceShrinkSize(Source*, Entry*, uvlong);
+static int	sourceGrowDepth(Source*, Block*, Entry*, int);
+
+#define sourceIsLocked(r)	((r)->b != nil)
+
+static Source *
+sourceAlloc(Fs *fs, Block *b, Source *p, u32int offset, int mode, int issnapshot)
+{
+	int epb;
+	u32int epoch;
+	char *pname = nil;
+	Source *r;
+	Entry e;
+
+	assert(p==nil || sourceIsLocked(p));
+
+	if(p == nil){
+		assert(offset == 0);
+		epb = 1;
+	}else
+		epb = p->dsize / VtEntrySize;
+
+	if(b->l.type != BtDir)
+		goto Bad;
+
+	/*
+	 * a non-active entry is the only thing that
+	 * can legitimately happen here. all the others
+	 * get prints.
+	 */
+	if(!entryUnpack(&e, b->data, offset % epb)){
+		pname = sourceName(p);
+		consPrint("%s: %s %V: sourceAlloc: entryUnpack failed\n",
+			fs->name, pname, b->score);
+		goto Bad;
+	}
+	if(!(e.flags & VtEntryActive)){
+		pname = sourceName(p);
+		if(0) consPrint("%s: %s %V: sourceAlloc: not active\n",
+			fs->name, pname, e.score);
+		goto Bad;
+	}
+	if(e.psize < 256 || e.dsize < 256){
+		pname = sourceName(p);
+		consPrint("%s: %s %V: sourceAlloc: psize %ud or dsize %ud < 256\n",
+			fs->name, pname, e.score, e.psize, e.dsize);
+		goto Bad;
+	}
+
+	if(e.depth < sizeToDepth(e.size, e.psize, e.dsize)){
+		pname = sourceName(p);
+		consPrint("%s: %s %V: sourceAlloc: depth %ud size %llud "
+			"psize %ud dsize %ud\n", fs->name, pname,
+			e.score, e.depth, e.size, e.psize, e.dsize);
+		goto Bad;
+	}
+
+	if((e.flags & VtEntryLocal) && e.tag == 0){
+		pname = sourceName(p);
+		consPrint("%s: %s %V: sourceAlloc: flags %#ux tag %#ux\n",
+			fs->name, pname, e.score, e.flags, e.tag);
+		goto Bad;
+	}
+
+	if(e.dsize > fs->blockSize || e.psize > fs->blockSize){
+		pname = sourceName(p);
+		consPrint("%s: %s %V: sourceAlloc: psize %ud or dsize %ud "
+			"> blocksize %ud\n", fs->name, pname, e.score,
+			e.psize, e.dsize, fs->blockSize);
+		goto Bad;
+	}
+
+	epoch = b->l.epoch;
+	if(mode == OReadWrite){
+		if(e.snap != 0){
+			vtSetError(ESnapRO);
+			return nil;
+		}
+	}else if(e.snap != 0){
+		if(e.snap < fs->elo){
+			vtSetError(ESnapOld);
+			return nil;
+		}
+		if(e.snap >= fs->ehi)
+			goto Bad;
+		epoch = e.snap;
+	}
+
+	r = vtMemAllocZ(sizeof(Source));
+	r->fs = fs;
+	r->mode = mode;
+	r->issnapshot = issnapshot;
+	r->dsize = e.dsize;
+	r->gen = e.gen;
+	r->dir = (e.flags & VtEntryDir) != 0;
+	r->lk = vtLockAlloc();
+	r->ref = 1;
+	r->parent = p;
+	if(p){
+		vtLock(p->lk);
+		assert(mode == OReadOnly || p->mode == OReadWrite);
+		p->ref++;
+		vtUnlock(p->lk);
+	}
+	r->epoch = epoch;
+//	consPrint("sourceAlloc: have %V be.%d fse.%d %s\n", b->score,
+//		b->l.epoch, r->fs->ehi, mode == OReadWrite? "rw": "ro");
+	memmove(r->score, b->score, VtScoreSize);
+	r->scoreEpoch = b->l.epoch;
+	r->offset = offset;
+	r->epb = epb;
+	r->tag = b->l.tag;
+
+//	consPrint("%s: sourceAlloc: %p -> %V %d\n", r, r->score, r->offset);
+
+	return r;
+Bad:
+	free(pname);
+	vtSetError(EBadEntry);
+	return nil;
+}
+
+Source *
+sourceRoot(Fs *fs, u32int addr, int mode)
+{
+	Source *r;
+	Block *b;
+
+	b = cacheLocalData(fs->cache, addr, BtDir, RootTag, mode, 0);
+	if(b == nil)
+		return nil;
+
+	if(mode == OReadWrite && b->l.epoch != fs->ehi){
+		consPrint("sourceRoot: fs->ehi = %ud, b->l = %L\n",
+			fs->ehi, &b->l);
+		blockPut(b);
+		vtSetError(EBadRoot);
+		return nil;
+	}
+
+	r = sourceAlloc(fs, b, nil, 0, mode, 0);
+	blockPut(b);
+	return r;
+}
+
+Source *
+sourceOpen(Source *r, ulong offset, int mode, int issnapshot)
+{
+	ulong bn;
+	Block *b;
+
+	assert(sourceIsLocked(r));
+	if(r->mode == OReadWrite)
+		assert(r->epoch == r->b->l.epoch);
+	if(!r->dir){
+		vtSetError(ENotDir);
+		return nil;
+	}
+
+	bn = offset/(r->dsize/VtEntrySize);
+
+	b = sourceBlock(r, bn, mode);
+	if(b == nil)
+		return nil;
+	r = sourceAlloc(r->fs, b, r, offset, mode, issnapshot);
+	blockPut(b);
+	return r;
+}
+
+Source *
+sourceCreate(Source *r, int dsize, int dir, u32int offset)
+{
+	int i, epb, psize;
+	u32int bn, size;
+	Block *b;
+	Entry e;
+	Source *rr;
+
+	assert(sourceIsLocked(r));
+
+	if(!r->dir){
+		vtSetError(ENotDir);
+		return nil;
+	}
+
+	epb = r->dsize/VtEntrySize;
+	psize = (dsize/VtScoreSize)*VtScoreSize;
+
+	size = sourceGetDirSize(r);
+	if(offset == 0){
+		/*
+		 * look at a random block to see if we can find an empty entry
+		 */
+		offset = lnrand(size+1);
+		offset -= offset % epb;
+	}
+
+	/* try the given block and then try the last block */
+	for(;;){
+		bn = offset/epb;
+		b = sourceBlock(r, bn, OReadWrite);
+		if(b == nil)
+			return nil;
+		for(i=offset%r->epb; i<epb; i++){
+			entryUnpack(&e, b->data, i);
+			if((e.flags&VtEntryActive) == 0 && e.gen != ~0)
+				goto Found;
+		}
+		blockPut(b);
+		if(offset == size){
+			fprint(2, "sourceCreate: cannot happen\n");
+			vtSetError("sourceCreate: cannot happen");
+			return nil;
+		}
+		offset = size;
+	}
+
+Found:
+	/* found an entry - gen already set */
+	e.psize = psize;
+	e.dsize = dsize;
+	assert(psize && dsize);
+	e.flags = VtEntryActive;
+	if(dir)
+		e.flags |= VtEntryDir;
+	e.depth = 0;
+	e.size = 0;
+	memmove(e.score, vtZeroScore, VtScoreSize);
+	e.tag = 0;
+	e.snap = 0;
+	e.archive = 0;
+	entryPack(&e, b->data, i);
+	blockDirty(b);
+
+	offset = bn*epb + i;
+	if(offset+1 > size){
+		if(!sourceSetDirSize(r, offset+1)){
+			blockPut(b);
+			return nil;
+		}
+	}
+
+	rr = sourceAlloc(r->fs, b, r, offset, OReadWrite, 0);
+	blockPut(b);
+	return rr;
+}
+
+static int
+sourceKill(Source *r, int doremove)
+{
+	Entry e;
+	Block *b;
+	u32int addr;
+	u32int tag;
+	int type;
+
+	assert(sourceIsLocked(r));
+	b = sourceLoad(r, &e);
+	if(b == nil)
+		return 0;
+
+	assert(b->l.epoch == r->fs->ehi);
+
+	if(doremove==0 && e.size == 0){
+		/* already truncated */
+		blockPut(b);
+		return 1;
+	}
+
+	/* remember info on link we are removing */
+	addr = globalToLocal(e.score);
+	type = entryType(&e);
+	tag = e.tag;
+
+	if(doremove){
+		if(e.gen != ~0)
+			e.gen++;
+		e.dsize = 0;
+		e.psize = 0;
+		e.flags = 0;
+	}else{
+		e.flags &= ~VtEntryLocal;
+	}
+	e.depth = 0;
+	e.size = 0;
+	e.tag = 0;
+	memmove(e.score, vtZeroScore, VtScoreSize);
+	entryPack(&e, b->data, r->offset % r->epb);
+	blockDirty(b);
+	if(addr != NilBlock)
+		blockRemoveLink(b, addr, type, tag, 1);
+	blockPut(b);
+
+	if(doremove){
+		sourceUnlock(r);
+		sourceClose(r);
+	}
+
+	return 1;
+}
+
+int
+sourceRemove(Source *r)
+{
+	return sourceKill(r, 1);
+}
+
+int
+sourceTruncate(Source *r)
+{
+	return sourceKill(r, 0);
+}
+
+uvlong
+sourceGetSize(Source *r)
+{
+	Entry e;
+	Block *b;
+
+	assert(sourceIsLocked(r));
+	b = sourceLoad(r, &e);
+	if(b == nil)
+		return 0;
+	blockPut(b);
+
+	return e.size;
+}
+
+static int
+sourceShrinkSize(Source *r, Entry *e, uvlong size)
+{
+	int i, type, ppb;
+	uvlong ptrsz;
+	u32int addr;
+	uchar score[VtScoreSize];
+	Block *b;
+
+	type = entryType(e);
+	b = cacheGlobal(r->fs->cache, e->score, type, e->tag, OReadWrite);
+	if(b == nil)
+		return 0;
+
+	ptrsz = e->dsize;
+	ppb = e->psize/VtScoreSize;
+	for(i=0; i+1<e->depth; i++)
+		ptrsz *= ppb;
+
+	while(type&BtLevelMask){
+		if(b->addr == NilBlock || b->l.epoch != r->fs->ehi){
+			/* not worth copying the block just so we can zero some of it */
+			blockPut(b);
+			return 0;
+		}
+
+		/*
+		 * invariant: each pointer in the tree rooted at b accounts for ptrsz bytes
+		 */
+
+		/* zero the pointers to unnecessary blocks */
+		i = (size+ptrsz-1)/ptrsz;
+		for(; i<ppb; i++){
+			addr = globalToLocal(b->data+i*VtScoreSize);
+			memmove(b->data+i*VtScoreSize, vtZeroScore, VtScoreSize);
+			blockDirty(b);
+			if(addr != NilBlock)
+				blockRemoveLink(b, addr, type-1, e->tag, 1);
+		}
+
+		/* recurse (go around again) on the partially necessary block */
+		i = size/ptrsz;
+		size = size%ptrsz;
+		if(size == 0){
+			blockPut(b);
+			return 1;
+		}
+		ptrsz /= ppb;
+		type--;
+		memmove(score, b->data+i*VtScoreSize, VtScoreSize);
+		blockPut(b);
+		b = cacheGlobal(r->fs->cache, score, type, e->tag, OReadWrite);
+		if(b == nil)
+			return 0;
+	}
+
+	if(b->addr == NilBlock || b->l.epoch != r->fs->ehi){
+		blockPut(b);
+		return 0;
+	}
+
+	/*
+	 * No one ever truncates BtDir blocks.
+	 */
+	if(type == BtData && e->dsize > size){
+		memset(b->data+size, 0, e->dsize-size);
+		blockDirty(b);
+	}
+	blockPut(b);
+	return 1;
+}
+
+int
+sourceSetSize(Source *r, uvlong size)
+{
+	int depth;
+	Entry e;
+	Block *b;
+
+	assert(sourceIsLocked(r));
+	if(size == 0)
+		return sourceTruncate(r);
+
+	if(size > VtMaxFileSize || size > ((uvlong)MaxBlock)*r->dsize){
+		vtSetError(ETooBig);
+		return 0;
+	}
+
+	b = sourceLoad(r, &e);
+	if(b == nil)
+		return 0;
+
+	/* quick out */
+	if(e.size == size){
+		blockPut(b);
+		return 1;
+	}
+
+	depth = sizeToDepth(size, e.psize, e.dsize);
+
+	if(depth < e.depth){
+		if(!sourceShrinkDepth(r, b, &e, depth)){
+			blockPut(b);
+			return 0;
+		}
+	}else if(depth > e.depth){
+		if(!sourceGrowDepth(r, b, &e, depth)){
+			blockPut(b);
+			return 0;
+		}
+	}
+
+	if(size < e.size)
+		sourceShrinkSize(r, &e, size);
+
+	e.size = size;
+	entryPack(&e, b->data, r->offset % r->epb);
+	blockDirty(b);
+	blockPut(b);
+
+	return 1;
+}
+
+int
+sourceSetDirSize(Source *r, ulong ds)
+{
+	uvlong size;
+	int epb;
+
+	assert(sourceIsLocked(r));
+	epb = r->dsize/VtEntrySize;
+
+	size = (uvlong)r->dsize*(ds/epb);
+	size += VtEntrySize*(ds%epb);
+	return sourceSetSize(r, size);
+}
+
+ulong
+sourceGetDirSize(Source *r)
+{
+	ulong ds;
+	uvlong size;
+	int epb;
+
+	assert(sourceIsLocked(r));
+	epb = r->dsize/VtEntrySize;
+
+	size = sourceGetSize(r);
+	ds = epb*(size/r->dsize);
+	ds += (size%r->dsize)/VtEntrySize;
+	return ds;
+}
+
+int
+sourceGetEntry(Source *r, Entry *e)
+{
+	Block *b;
+
+	assert(sourceIsLocked(r));
+	b = sourceLoad(r, e);
+	if(b == nil)
+		return 0;
+	blockPut(b);
+
+	return 1;
+}
+
+/*
+ * Must be careful with this.  Doesn't record
+ * dependencies, so don't introduce any!
+ */
+int
+sourceSetEntry(Source *r, Entry *e)
+{
+	Block *b;
+	Entry oe;
+
+	assert(sourceIsLocked(r));
+	b = sourceLoad(r, &oe);
+	if(b == nil)
+		return 0;
+	entryPack(e, b->data, r->offset%r->epb);
+	blockDirty(b);
+	blockPut(b);
+
+	return 1;
+}
+
+static Block *
+blockWalk(Block *p, int index, int mode, Fs *fs, Entry *e)
+{
+	Block *b;
+	Cache *c;
+	u32int addr;
+	int type;
+	uchar oscore[VtScoreSize], score[VtScoreSize];
+	Entry oe;
+
+	c = fs->cache;
+
+	if((p->l.type & BtLevelMask) == 0){
+		assert(p->l.type == BtDir);
+		type = entryType(e);
+		b = cacheGlobal(c, e->score, type, e->tag, mode);
+	}else{
+		type = p->l.type - 1;
+		b = cacheGlobal(c, p->data + index*VtScoreSize, type, e->tag, mode);
+	}
+
+	if(b)
+		b->pc = getcallerpc(&p);
+
+	if(b == nil || mode == OReadOnly)
+		return b;
+
+	if(p->l.epoch != fs->ehi){
+		fprint(2, "blockWalk: parent not writable\n");
+		abort();
+	}
+	if(b->l.epoch == fs->ehi)
+		return b;
+
+	oe = *e;
+
+	/*
+	 * Copy on write.
+	 */
+	if(e->tag == 0){
+		assert(p->l.type == BtDir);
+		e->tag = tagGen();
+		e->flags |= VtEntryLocal;
+	}
+
+	addr = b->addr;
+	b = blockCopy(b, e->tag, fs->ehi, fs->elo);
+	if(b == nil)
+		return nil;
+
+	b->pc = getcallerpc(&p);
+	assert(b->l.epoch == fs->ehi);
+
+	blockDirty(b);
+	memmove(score, b->score, VtScoreSize);
+	if(p->l.type == BtDir){
+		memmove(e->score, b->score, VtScoreSize);
+		entryPack(e, p->data, index);
+		blockDependency(p, b, index, nil, &oe);
+	}else{
+		memmove(oscore, p->data+index*VtScoreSize, VtScoreSize);
+		memmove(p->data+index*VtScoreSize, b->score, VtScoreSize);
+		blockDependency(p, b, index, oscore, nil);
+	}
+	blockDirty(p);
+
+	if(addr != NilBlock)
+		blockRemoveLink(p, addr, type, e->tag, 0);
+
+	return b;
+}
+
+/*
+ * Change the depth of the source r.
+ * The entry e for r is contained in block p.
+ */
+static int
+sourceGrowDepth(Source *r, Block *p, Entry *e, int depth)
+{
+	Block *b, *bb;
+	u32int tag;
+	int type;
+	Entry oe;
+
+	assert(sourceIsLocked(r));
+	assert(depth <= VtPointerDepth);
+
+	type = entryType(e);
+	b = cacheGlobal(r->fs->cache, e->score, type, e->tag, OReadWrite);
+	if(b == nil)
+		return 0;
+
+	tag = e->tag;
+	if(tag == 0)
+		tag = tagGen();
+
+	oe = *e;
+
+	/*
+	 * Keep adding layers until we get to the right depth
+	 * or an error occurs.
+	 */
+	while(e->depth < depth){
+		bb = cacheAllocBlock(r->fs->cache, type+1, tag, r->fs->ehi, r->fs->elo);
+		if(bb == nil)
+			break;
+//fprint(2, "alloc %lux grow %V\n", bb->addr, b->score);
+		memmove(bb->data, b->score, VtScoreSize);
+		memmove(e->score, bb->score, VtScoreSize);
+		e->depth++;
+		type++;
+		e->tag = tag;
+		e->flags |= VtEntryLocal;
+		blockDependency(bb, b, 0, vtZeroScore, nil);
+		blockPut(b);
+		b = bb;
+		blockDirty(b);
+	}
+
+	entryPack(e, p->data, r->offset % r->epb);
+	blockDependency(p, b, r->offset % r->epb, nil, &oe);
+	blockPut(b);
+	blockDirty(p);
+
+	return e->depth == depth;
+}
+
+static int
+sourceShrinkDepth(Source *r, Block *p, Entry *e, int depth)
+{
+	Block *b, *nb, *ob, *rb;
+	u32int tag;
+	int type, d;
+	Entry oe;
+
+	assert(sourceIsLocked(r));
+	assert(depth <= VtPointerDepth);
+
+	type = entryType(e);
+	rb = cacheGlobal(r->fs->cache, e->score, type, e->tag, OReadWrite);
+	if(rb == nil)
+		return 0;
+
+	tag = e->tag;
+	if(tag == 0)
+		tag = tagGen();
+
+	/*
+	 * Walk down to the new root block.
+	 * We may stop early, but something is better than nothing.
+	 */
+	oe = *e;
+
+	ob = nil;
+	b = rb;
+/* BUG: explain type++.  i think it is a real bug */
+	for(d=e->depth; d > depth; d--, type++){
+		nb = cacheGlobal(r->fs->cache, b->data, type-1, tag, OReadWrite);
+		if(nb == nil)
+			break;
+		if(ob!=nil && ob!=rb)
+			blockPut(ob);
+		ob = b;
+		b = nb;
+	}
+
+	if(b == rb){
+		blockPut(rb);
+		return 0;
+	}
+
+	/*
+	 * Right now, e points at the root block rb, b is the new root block,
+	 * and ob points at b.  To update:
+	 *
+	 *	(i) change e to point at b
+	 *	(ii) zero the pointer ob -> b
+	 *	(iii) free the root block
+	 *
+	 * p (the block containing e) must be written before
+	 * anything else.
+ 	 */
+
+	/* (i) */
+	e->depth = d;
+	/* might have been local and now global; reverse cannot happen */
+	if(globalToLocal(b->score) == NilBlock)
+		e->flags &= ~VtEntryLocal;
+	memmove(e->score, b->score, VtScoreSize);
+	entryPack(e, p->data, r->offset % r->epb);
+	blockDependency(p, b, r->offset % r->epb, nil, &oe);
+	blockDirty(p);
+
+	/* (ii) */
+	memmove(ob->data, vtZeroScore, VtScoreSize);
+	blockDependency(ob, p, 0, b->score, nil);
+	blockDirty(ob);
+
+	/* (iii) */
+	if(rb->addr != NilBlock)
+		blockRemoveLink(p, rb->addr, rb->l.type, rb->l.tag, 1);
+
+	blockPut(rb);
+	if(ob!=nil && ob!=rb)
+		blockPut(ob);
+	blockPut(b);
+
+	return d == depth;
+}
+
+/*
+ * Normally we return the block at the given number.
+ * If early is set, we stop earlier in the tree.  Setting early
+ * to 1 gives us the block that contains the pointer to bn.
+ */
+Block *
+_sourceBlock(Source *r, ulong bn, int mode, int early, ulong tag)
+{
+	Block *b, *bb;
+	int index[VtPointerDepth+1];
+	Entry e;
+	int i, np;
+	int m;
+
+	assert(sourceIsLocked(r));
+	assert(bn != NilBlock);
+
+	/* mode for intermediate block */
+	m = mode;
+	if(m == OOverWrite)
+		m = OReadWrite;
+
+	b = sourceLoad(r, &e);
+	if(b == nil)
+		return nil;
+	if(r->issnapshot && (e.flags & VtEntryNoArchive)){
+		blockPut(b);
+		vtSetError(ENotArchived);
+		return nil;
+	}
+
+	if(tag){
+		if(e.tag == 0)
+			e.tag = tag;
+		else if(e.tag != tag){
+			fprint(2, "tag mismatch\n");
+			vtSetError("tag mismatch");
+			goto Err;
+		}
+	}
+
+	np = e.psize/VtScoreSize;
+	memset(index, 0, sizeof(index));
+	for(i=0; bn > 0; i++){
+		if(i >= VtPointerDepth){
+			vtSetError(EBadAddr);
+			goto Err;
+		}
+		index[i] = bn % np;
+		bn /= np;
+	}
+
+	if(i > e.depth){
+		if(mode == OReadOnly){
+			vtSetError(EBadAddr);
+			goto Err;
+		}
+		if(!sourceGrowDepth(r, b, &e, i))
+			goto Err;
+	}
+
+	index[e.depth] = r->offset % r->epb;
+
+	for(i=e.depth; i>=early; i--){
+		bb = blockWalk(b, index[i], m, r->fs, &e);
+		if(bb == nil)
+			goto Err;
+		blockPut(b);
+		b = bb;
+	}
+	b->pc = getcallerpc(&r);
+	return b;
+Err:
+	blockPut(b);
+	return nil;
+}
+
+Block*
+sourceBlock(Source *r, ulong bn, int mode)
+{
+	Block *b;
+
+	b = _sourceBlock(r, bn, mode, 0, 0);
+	if(b)
+		b->pc = getcallerpc(&r);
+	return b;
+}
+
+void
+sourceClose(Source *r)
+{
+	if(r == nil)
+		return;
+	vtLock(r->lk);
+	r->ref--;
+	if(r->ref){
+		vtUnlock(r->lk);
+		return;
+	}
+	assert(r->ref == 0);
+	vtUnlock(r->lk);
+	if(r->parent)
+		sourceClose(r->parent);
+	vtLockFree(r->lk);
+	memset(r, ~0, sizeof(*r));
+	vtMemFree(r);
+}
+
+/*
+ * Retrieve the block containing the entry for r.
+ * If a snapshot has happened, we might need
+ * to get a new copy of the block.  We avoid this
+ * in the common case by caching the score for
+ * the block and the last epoch in which it was valid.
+ *
+ * We use r->mode to tell the difference between active
+ * file system sources (OReadWrite) and sources for the
+ * snapshot file system (OReadOnly).
+ */
+static Block*
+sourceLoadBlock(Source *r, int mode)
+{
+	u32int addr;
+	Block *b;
+
+	switch(r->mode){
+	default:
+		assert(0);
+	case OReadWrite:
+		assert(r->mode == OReadWrite);
+		/*
+		 * This needn't be true -- we might bump the low epoch
+		 * to reclaim some old blocks, but since this score is
+		 * OReadWrite, the blocks must all still be open, so none
+		 * are reclaimed.  Thus it's okay that the epoch is so low.
+		 * Proceed.
+		assert(r->epoch >= r->fs->elo);
+		 */
+		if(r->epoch == r->fs->ehi){
+			b = cacheGlobal(r->fs->cache, r->score, BtDir, r->tag, OReadWrite);
+			if(b == nil)
+				return nil;
+			assert(r->epoch == b->l.epoch);
+			return b;
+		}
+		assert(r->parent != nil);
+		if(!sourceLock(r->parent, OReadWrite))
+			return nil;
+		b = sourceBlock(r->parent, r->offset/r->epb, OReadWrite);
+		sourceUnlock(r->parent);
+		if(b == nil)
+			return nil;
+		assert(b->l.epoch == r->fs->ehi);
+	//	fprint(2, "sourceLoadBlock %p %V => %V\n", r, r->score, b->score);
+		memmove(r->score, b->score, VtScoreSize);
+		r->scoreEpoch = b->l.epoch;
+		r->tag = b->l.tag;
+		r->epoch = r->fs->ehi;
+		return b;
+
+	case OReadOnly:
+		addr = globalToLocal(r->score);
+		if(addr == NilBlock)
+			return cacheGlobal(r->fs->cache, r->score, BtDir, r->tag, mode);
+
+		b = cacheLocalData(r->fs->cache, addr, BtDir, r->tag, mode, r->scoreEpoch);
+		if(b)
+			return b;
+
+		/*
+		 * If it failed because the epochs don't match, the block has been
+		 * archived and reclaimed.  Rewalk from the parent and get the
+		 * new pointer.  This can't happen in the OReadWrite case
+		 * above because blocks in the current epoch don't get
+		 * reclaimed.  The fact that we're OReadOnly means we're
+		 * a snapshot.  (Or else the file system is read-only, but then
+		 * the archiver isn't going around deleting blocks.)
+		 */
+		if(strcmp(vtGetError(), ELabelMismatch) == 0){
+			if(!sourceLock(r->parent, OReadOnly))
+				return nil;
+			b = sourceBlock(r->parent, r->offset/r->epb, OReadOnly);
+			sourceUnlock(r->parent);
+			if(b){
+				fprint(2, "sourceAlloc: lost %V found %V\n",
+					r->score, b->score);
+				memmove(r->score, b->score, VtScoreSize);
+				r->scoreEpoch = b->l.epoch;
+				return b;
+			}
+		}
+		return nil;
+	}
+}
+
+int
+sourceLock(Source *r, int mode)
+{
+	Block *b;
+
+	if(mode == -1)
+		mode = r->mode;
+
+	b = sourceLoadBlock(r, mode);
+	if(b == nil)
+		return 0;
+	/*
+	 * The fact that we are holding b serves as the
+	 * lock entitling us to write to r->b.
+	 */
+	assert(r->b == nil);
+	r->b = b;
+	if(r->mode == OReadWrite)
+		assert(r->epoch == r->b->l.epoch);
+	return 1;
+}
+
+/*
+ * Lock two (usually sibling) sources.  This needs special care
+ * because the Entries for both sources might be in the same block.
+ * We also try to lock blocks in left-to-right order within the tree.
+ */
+int
+sourceLock2(Source *r, Source *rr, int mode)
+{
+	Block *b, *bb;
+
+	if(rr == nil)
+		return sourceLock(r, mode);
+
+	if(mode == -1)
+		mode = r->mode;
+
+	if(r->parent==rr->parent && r->offset/r->epb == rr->offset/rr->epb){
+		b = sourceLoadBlock(r, mode);
+		if(b == nil)
+			return 0;
+		if(memcmp(r->score, rr->score, VtScoreSize) != 0){
+			memmove(rr->score, b->score, VtScoreSize);
+			rr->scoreEpoch = b->l.epoch;
+			rr->tag = b->l.tag;
+			rr->epoch = rr->fs->ehi;
+		}
+		blockDupLock(b);
+		bb = b;
+	}else if(r->parent==rr->parent || r->offset > rr->offset){
+		bb = sourceLoadBlock(rr, mode);
+		b = sourceLoadBlock(r, mode);
+	}else{
+		b = sourceLoadBlock(r, mode);
+		bb = sourceLoadBlock(rr, mode);
+	}
+	if(b == nil || bb == nil){
+		if(b)
+			blockPut(b);
+		if(bb)
+			blockPut(bb);
+		return 0;
+	}
+
+	/*
+	 * The fact that we are holding b and bb serves
+	 * as the lock entitling us to write to r->b and rr->b.
+	 */
+	r->b = b;
+	rr->b = bb;
+	return 1;
+}
+
+void
+sourceUnlock(Source *r)
+{
+	Block *b;
+
+	if(r->b == nil){
+		fprint(2, "sourceUnlock: already unlocked\n");
+		abort();
+	}
+	b = r->b;
+	r->b = nil;
+	blockPut(b);
+}
+
+static Block*
+sourceLoad(Source *r, Entry *e)
+{
+	Block *b;
+
+	assert(sourceIsLocked(r));
+	b = r->b;
+	if(!entryUnpack(e, b->data, r->offset % r->epb))
+		return nil;
+	if(e->gen != r->gen){
+		vtSetError(ERemoved);
+		return nil;
+	}
+	blockDupLock(b);
+	return b;
+}
+
+static int
+sizeToDepth(uvlong s, int psize, int dsize)
+{
+	int np;
+	int d;
+
+	/* determine pointer depth */
+	np = psize/VtScoreSize;
+	s = (s + dsize - 1)/dsize;
+	for(d = 0; s > 1; d++)
+		s = (s + np - 1)/np;
+	return d;
+}
+
+static u32int
+tagGen(void)
+{
+	u32int tag;
+
+	for(;;){
+		tag = lrand();
+		if(tag >= UserTag)
+			break;
+	}
+	return tag;
+}
+
+char *
+sourceName(Source *s)
+{
+	return fileName(s->file);
+}