Dropped alt-always as an rbyd trunk terminator

Now that tree rebalancing is implemented and needed a null terminator
anyways, I think it's clear that the benefit of the alt-always pointers
as trunk terminator has pretty limited value.

Now a null or other tag is needed for every trunk, which simplifies
checks for end-of-trunk.

Alt-always tags are still emitted for deletes, etc, but there their
behavior is implicit, so no special checks are needed. Alt-always tags
are naturally cleaned up as a part of rbyd pruning.

lfs.c

@@ -625,7 +625,6 @@ enum lfsr_tag_type {
     LFSR_TAG_ALTGT          = 0x6000,
     LFSR_TAG_ALTBGT         = 0x6000,
     LFSR_TAG_ALTRGT         = 0x7000,
-    LFSR_TAG_ALTA           = 0x6000,
 
     LFSR_TAG_CRC            = 0x2000,
     LFSR_TAG_FCRC           = 0x2100,
@@ -725,14 +724,10 @@ static inline bool lfsr_tag_isalt(lfsr_tag_t tag) {
     return tag & 0x4000;
 }
 
-static inline bool lfsr_tag_istrunkstart(lfsr_tag_t tag) {
+static inline bool lfsr_tag_istrunk(lfsr_tag_t tag) {
     return (tag & 0x6000) != 0x2000;
 }
 
-static inline bool lfsr_tag_istrunkend(lfsr_tag_t tag) {
-    return !lfsr_tag_isalt(tag) || tag == LFSR_TAG_ALTA;
-}
-
 static inline lfsr_tag_t lfsr_tag_next(lfsr_tag_t tag) {
     return tag + 0x1;
 }
@@ -1718,7 +1713,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
         }
 
         // found a trunk of a tree?
-        if (lfsr_tag_istrunkstart(tag)
+        if (lfsr_tag_istrunk(tag)
                 && (!trunk || trunk >= off-d || wastrunk)) {
             // start of trunk?
             if (!wastrunk) {
@@ -1738,7 +1733,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
             weight_ += w;
 
             // end of trunk?
-            if (lfsr_tag_istrunkend(tag)) {
+            if (!lfsr_tag_isalt(tag)) {
                 wastrunk = false;
                 // update current weight
                 weight = weight_;
@@ -2448,7 +2443,7 @@ static int lfsr_rbyd_append(lfs_t *lfs, lfsr_rbyd_t *rbyd,
                                 < lfsr_tag_key(tag)))))) {
         if (lfsr_tag_isrm(tag)) {
             // if removed, make our tag unreachable
-            alt = LFSR_TAG_ALTA;
+            alt = LFSR_TAG_ALTGT(false, 0);
             weight = upper_id - lower_id - 1 + delta;
             upper_id -= weight;
         } else {
@@ -2471,7 +2466,7 @@ static int lfsr_rbyd_append(lfs_t *lfs, lfsr_rbyd_t *rbyd,
                                 > lfsr_tag_key(tag)))))) {
         if (lfsr_tag_isrm(tag)) {
             // if removed, make our tag unreachable
-            alt = LFSR_TAG_ALTA;
+            alt = LFSR_TAG_ALTGT(false, 0);
             weight = upper_id - lower_id - 1 + delta;
             upper_id -= weight;
         } else {
@@ -2505,30 +2500,30 @@ static int lfsr_rbyd_append(lfs_t *lfs, lfsr_rbyd_t *rbyd,
         goto failed;
     }
 
-    // we don't need to write the tag if we ended with an unreachable trunk
-    if (alt != LFSR_TAG_ALTA) {
 leaf:;
-        // write the actual tag
+    // write the actual tag
-        lfs_ssize_t d = lfsr_bd_progtag(lfs, rbyd->block, rbyd->off,
+    //
-                // rm => null, otherwise strip off control bits
+    // note we always need a non-alt to terminate the trunk, otherwise we
-                (lfsr_tag_isrm(tag) ? LFSR_TAG_NULL : lfsr_tag_key(tag)),
+    // can't find trunks during fetch
-                upper_id - lower_id - 1 + delta,
+    lfs_ssize_t d = lfsr_bd_progtag(lfs, rbyd->block, rbyd->off,
-                lfsr_data_size(data),
+            // rm => null, otherwise strip off control bits
-                &rbyd->crc);
+            (lfsr_tag_isrm(tag) ? LFSR_TAG_NULL : lfsr_tag_key(tag)),
-        if (d < 0) {
+            upper_id - lower_id - 1 + delta,
-            err = d;
+            lfsr_data_size(data),
-            goto failed;
+            &rbyd->crc);
-        }
+    if (d < 0) {
-        rbyd->off += d;
+        err = d;
-
+        goto failed;
-        // don't forget the data!
-        err = lfsr_bd_progdata(lfs, rbyd->block, rbyd->off, data,
-                &rbyd->crc);
-        if (err) {
-            goto failed;
-        }
-        rbyd->off += lfsr_data_size(data);
     }
+    rbyd->off += d;
+
+    // don't forget the data!
+    err = lfsr_bd_progdata(lfs, rbyd->block, rbyd->off, data,
+            &rbyd->crc);
+    if (err) {
+        goto failed;
+    }
+    rbyd->off += lfsr_data_size(data);
 
     return 0;
 

scripts/dbgbtree.py

@@ -25,7 +25,6 @@ TAG_MTREE       = 0x0306
 TAG_UATTR       = 0x0400
 TAG_SATTR       = 0x0500
 TAG_ALT         = 0x4000
-TAG_ALTA        = 0x6000
 TAG_CRC         = 0x2000
 TAG_FCRC        = 0x2100
 
@@ -164,12 +163,6 @@ def tagrepr(tag, w, size, off=None):
         return 'fcrc%s %d' % (
             ' 0x%x' % w if w > 0 else '',
             size)
-    elif tag == TAG_ALTA:
-        return 'alta w%d %s' % (
-            w,
-            '0x%x' % (0xffffffff & (off-size))
-                if off is not None
-                else '-%d' % off)
     elif tag & 0x4000:
         return 'alt%s%s 0x%x w%d %s' % (
             'r' if tag & 0x1000 else 'b',
@@ -290,7 +283,7 @@ class Rbyd:
                 weight__ += w
 
                 # end of trunk?
-                if not tag & 0x4000 or tag == TAG_ALTA:
+                if not tag & 0x4000:
                     wastrunk = False
                     # update weight
                     weight_ = weight__

scripts/dbgmtree.py

@@ -25,7 +25,6 @@ TAG_MTREE       = 0x0306
 TAG_UATTR       = 0x0400
 TAG_SATTR       = 0x0500
 TAG_ALT         = 0x4000
-TAG_ALTA        = 0x6000
 TAG_CRC         = 0x2000
 TAG_FCRC        = 0x2100
 
@@ -173,12 +172,6 @@ def tagrepr(tag, w, size, off=None):
         return 'fcrc%s %d' % (
             ' 0x%x' % w if w > 0 else '',
             size)
-    elif tag == TAG_ALTA:
-        return 'alta w%d %s' % (
-            w,
-            '0x%x' % (0xffffffff & (off-size))
-                if off is not None
-                else '-%d' % off)
     elif tag & 0x4000:
         return 'alt%s%s 0x%x w%d %s' % (
             'r' if tag & 0x1000 else 'b',
@@ -299,7 +292,7 @@ class Rbyd:
                 weight__ += w
 
                 # end of trunk?
-                if not tag & 0x4000 or tag == TAG_ALTA:
+                if not tag & 0x4000:
                     wastrunk = False
                     # update weight
                     weight_ = weight__

scripts/dbgrbyd.py

@@ -34,7 +34,6 @@ TAG_MTREE       = 0x0306
 TAG_UATTR       = 0x0400
 TAG_SATTR       = 0x0500
 TAG_ALT         = 0x4000
-TAG_ALTA        = 0x6000
 TAG_CRC         = 0x2000
 TAG_FCRC        = 0x2100
 
@@ -166,12 +165,6 @@ def tagrepr(tag, w, size, off=None):
         return 'fcrc%s %d' % (
             ' 0x%x' % w if w > 0 else '',
             size)
-    elif tag == TAG_ALTA:
-        return 'alta w%d %s' % (
-            w,
-            '0x%x' % (0xffffffff & (off-size))
-                if off is not None
-                else '-%d' % off)
     elif tag & 0x4000:
         return 'alt%s%s 0x%x w%d %s' % (
             'r' if tag & 0x1000 else 'b',
@@ -309,15 +302,14 @@ def dbg_log(data, block_size, rev, off, weight, *,
                 else:
                     upper_ += w
 
-                if not tag & 0x4000 or tag == TAG_ALTA:
+                if not tag & 0x4000:
                     wastrunk = False
                     # derive the current tag's id from alt weights
                     delta = (lower_+upper_) - weight_
                     weight_ = lower_+upper_
                     id = lower_ + w-1
 
-            if ((tag & 0xe000) != 0x2000
+            if (tag & 0xe000) != 0x2000 and not tag & 0x4000:
-                    and (not tag & 0x4000 or tag == TAG_ALTA)):
                 # note we ignore out-of-bounds here for debugging
                 if delta > 0:
                     # grow lifetimes
@@ -466,7 +458,7 @@ def dbg_log(data, block_size, rev, off, weight, *,
             else:
                 upper_ += w
 
-            if not tag & 0x4000 or tag == TAG_ALTA:
+            if not tag & 0x4000:
                 wastrunk = False
                 # derive the current tag's id from alt weights
                 id = lower_ + w-1
@@ -478,8 +470,7 @@ def dbg_log(data, block_size, rev, off, weight, *,
             '\x1b[m' if color and j >= off else '',
             lifetime_width, lifetimerepr(j) if args.get('lifetimes') else '',
             '\x1b[90m' if color and j >= off else '',
-            w_width, '-' if tag == TAG_ALTA
+            w_width, '' if (tag & 0xe000) != 0x0000
-                else '' if (tag & 0xe000) != 0x0000
                 else '%d-%d' % (id-(w-1), id) if w > 1
                 else id,
             '%-22s%s' % (
@@ -896,7 +887,7 @@ def main(disk, blocks=None, *,
                 weight__ += w
 
                 # end of trunk?
-                if not tag & 0x4000 or tag == TAG_ALTA:
+                if not tag & 0x4000:
                     wastrunk = False
                     # update weight
                     weight_ = weight__