README.md

@@ -246,7 +246,7 @@ block is the canonical description; this side owns the names.
 |---|---|---|
 | `.stage-<tag>` | `cache-lib.sh`, `hardlink_clone_into()` | stranded; only the arbiter reclaims it |
 | `.publish-new-<tag>` | `publish-snapshot.sh` | stranded; see `daniel/gitdan#30` |
-| `.publish-old-<key>-<tag>` | `publish-snapshot.sh` | swept by the next publish of that key |
+| `.publish-old-<key>-<tag>` | `publish-snapshot.sh` | swept by the next publish of that key, if there is one |
 | `.evicting-<name>-<pid>` | `prune-cache.sh`, `evict_dir()` | swept at the start of the next prune pass |
 | `.reading-<source>-<tag>` | `cache-lib.sh`, `reader_lock_acquire()` | not garbage — see below |
 
@@ -269,9 +269,11 @@ as `.stage-` does, so it is the one that needed catching. Bringing both under
 the arbiter's enumeration is tracked as `daniel/gitdan#30`.
 
 The two lists are meant to be the same length — the five names above, and the
-prefixes `ci-cache-reclaim.sh` enumerates. A mismatch means one side gained a
+prefix constants `ci-cache-reclaim.sh` *declares*. Not the subset it enumerates
-shape without telling the other, which is the drift the rule exists to catch
+as reclaim candidates: that one is smaller, because `.reading-` is read and
-and the cheapest thing to check.
+never swept. A mismatch means one side gained a shape without telling the
+other, which is the drift the rule exists to catch and the cheapest thing to
+check.
 
 **The staleness constants are part of the same contract, and that half has a
 direction.** `CACHE_READ_STALE_SECONDS` (`cache-lib.sh`) and

scripts/cache-lib.sh

@@ -32,7 +32,9 @@
 #                             its clone and the swap. Strands the same way.
 #   .publish-old-<key>-<tag>  publish-snapshot.sh: the rotated-away generation,
 #                             kept while a reader still holds it and swept by
-#                             the next publish of the same key.
+#                             the next publish of the same key — if there ever
+#                             is one. A merged or renamed branch never
+#                             publishes again, and its last one stays.
 #   .evicting-<name>-<pid>    prune-cache.sh, evict_dir(): a cache renamed
 #                             aside so the decision to unlink it can be retaken
 #                             after the rename. Swept at the start of every
@@ -63,9 +65,11 @@
 # precisely the shape only the arbiter can reach. Bringing both in is tracked
 # as daniel/gitdan#30.
 #
-# The two lists are meant to be the same length. A shape here without a prefix
+# The two lists are meant to be the same length: the five names above, and the
-# there is one side having changed without telling the other, and it is
+# prefix constants that script DECLARES — not the subset it enumerates as
-# cheapest to notice by counting.
+# reclaim candidates, which is smaller because `.reading-` is read and never
+# swept. A shape here without a constant there is one side having changed
+# without telling the other, and it is cheapest to notice by counting.
 #
 # The two staleness constants the arbiter mirrors are part of the same
 # contract, and that half has a direction to it — see CACHE_READ_STALE_SECONDS

scripts/publish-snapshot.sh

@@ -12,9 +12,11 @@
 # The swap is two renames, not one, because POSIX rename() can only replace an
 # EMPTY directory and a snapshot from a prior publish is not one:
 #
-#   1. stage the new snapshot at .stage-<tag>   (copy time is off every
+#   1. stage the new snapshot at .publish-new-<tag>, which hardlink_clone_into
-#      consumer's hot path — nothing reads a staging path);
+#      builds at its own .stage-<tag> and renames there (copy time is off
-#   2. rename the current snapshot aside to .publish-old-<tag>, if present;
+#      every consumer's hot path — nothing reads either path);
+#   2. rename the current snapshot aside to .publish-old-<key>-<tag>, if
+#      present;
 #   3. rename the staged snapshot into place.
 #
 # Step 3 is a single atomic rename onto a path now guaranteed absent, so it
@@ -60,8 +62,10 @@ DST=$(snapshot_dir_for "$ROOT" "$OWN_KEY")
 # per job per run, so a publisher killed between staging its snapshot and the
 # swap below strands a full hardlink clone that no later run of this script
 # will ever match. `.publish-old-` is milder — the sweep further down reclaims
-# it on the next publish of the same key. Bringing both under the arbiter's
+# it on the next publish of the same key, if there ever is one; a key whose
-# enumeration is tracked as daniel/gitdan#30.
+# branch is merged, deleted or renamed is never published again, and its
+# rotated generation stays until something outside this repo takes it.
+# Bringing both under the arbiter's enumeration is tracked as daniel/gitdan#30.
 OLD="${ROOT}/.publish-old-${OWN_KEY}-${TAG}"
 SNAP_NAME=$(basename "$DST")
 GRACE="${CACHE_READ_GRACE_SECONDS}"
@@ -97,6 +101,8 @@ start=$(date +%s)
 # the snapshot it just published — the same aliasing hazard the consume side
 # closes, pointing the other way. The staging path is not the final name, so
 # `hardlink_clone_into`'s rename lands on DST only after OLD is out of the way.
+# `.publish-new-` is a contract name — see the note at OLD above before
+# changing it.
 TMP_DST="${ROOT}/.publish-new-${TAG}"
 rm -rf "$TMP_DST"
 hardlink_clone_into "$SRC" "$TMP_DST" "$TAG" || {