nandi/frq · bffe879

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Read the clock in the reader's zone when /etc/localtime is a file

Every timestamp came out in UTC under the nix package: eight in the
morning next to a clock reading one. The zone is found by resolving
/etc/localtime and taking the name out of the path, and in a container
that file is often the bytes themselves rather than a symlink — Arch
under distrobox has it as an overlay mount — so readlink answers with
the path it was given, no name comes out, and the chain fell through to
UTC. libc knew the zone throughout; only the name recovery failed.

So fall back to libc's colon form, TZ=:/etc/localtime, which names the
tzfile by path instead of by zone and reads it with its transitions
intact: -25200 in September and -28800 in February, not one frozen
offset. Nothing downstream minds that it is a path, the zone only ever
reaching tz-offset-seconds.

TZDIR on the jolt wrapper for the other half of the same question: a
zone *name* needs a zoneinfo tree to resolve against, and a NixOS host
has no /usr/share/zoneinfo. Set as a default, so one already set wins.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nandi committed 2026-09-01T01:18:04-07:00 Browse files

bffe879 parent: 7434fee

added CLAUDE.md +20 -0 Viewed

new file mode 100644
@@ -0,0 +1,20 @@
	1	+# Working in this repo
	2	+
	3	+## Nix
	4	+
	5	+`nix` is not installed on the host. It lives in the Arch distrobox, so run every
	6	+nix command through that:
	7	+
	8	+```bash
	9	+distrobox enter arch -- bash -lc 'cd <this directory> && nix build .#frq'
	10	+```
	11	+
	12	+The worktree path is the same inside the container as outside, so `cd "$PWD"`
	13	+works. A remote builder (`eu.nixbuild.net`) is already configured there; the
	14	+Android outputs want `--store ssh-ng://eu.nixbuild.net --eval-store auto`, for
	15	+the reason nix/android.nix gives at the top.
	16	+
	17	+One thing that container is not representative of: `/etc/localtime` is a
	18	+regular file there rather than a symlink, so anything that reads the zone out
	19	+of its path sees nothing. That is a real deployment shape, not an artefact —
	20	+frq.clock handles it.

++# Working in this repo
++
++## Nix
++
++`nix` is not installed on the host. It lives in the Arch distrobox, so run every
++nix command through that:
++
++```bash
++distrobox enter arch -- bash -lc 'cd <this directory> && nix build .#frq'
++```
++
++The worktree path is the same inside the container as outside, so `cd "$PWD"`
++works. A remote builder (`eu.nixbuild.net`) is already configured there; the
++Android outputs want `--store ssh-ng://eu.nixbuild.net --eval-store auto`, for
++the reason nix/android.nix gives at the top.
++
++One thing that container is *not* representative of: `/etc/localtime` is a
++regular file there rather than a symlink, so anything that reads the zone out
++of its path sees nothing. That is a real deployment shape, not an artefact —
++frq.clock handles it.

modified flake.nix +7 -0 Viewed

@@ -165,10 +165,17 @@
165	165
166	166	# jolt.deps shells out to git and unzip, and jolt.mvn-http dlopens
167	167	# OpenSSL through the JOLT_OPENSSL_LIBDIR seam.
	168	+ #
	169	+ # TZDIR so a zone name resolves wherever this runs: frq.clock
	170	+ # hands one to tzset, and glibc then looks for the tzfile under
	171	+ # /usr/share/zoneinfo unless told otherwise — which a NixOS host
	172	+ # does not have. The store's own tzdata is there on both kinds of
	173	+ # machine. --set-default, so a TZDIR the user set still wins.
168	174	postFixup = ''
169	175	wrapProgram "$out/bin/jolt" \
170	176	--prefix PATH : "${pkgs.lib.makeBinPath [ pkgs.git pkgs.unzip ]}" \
171	177	--set-default JOLT_OPENSSL_LIBDIR "${pkgs.lib.makeLibraryPath [ pkgs.openssl ]}" \
	178	+ --set-default TZDIR "${pkgs.tzdata}/share/zoneinfo" \
172	179	--set-default SSL_CERT_FILE "${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt"
173	180	'';
174	181	};

              # jolt.deps shells out to git and unzip, and jolt.mvn-http dlopens
              # OpenSSL through the JOLT_OPENSSL_LIBDIR seam.
++            #
++            # TZDIR so a zone *name* resolves wherever this runs: frq.clock
++            # hands one to tzset, and glibc then looks for the tzfile under
++            # /usr/share/zoneinfo unless told otherwise — which a NixOS host
++            # does not have. The store's own tzdata is there on both kinds of
++            # machine. --set-default, so a TZDIR the user set still wins.
              postFixup = ''
                wrapProgram "$out/bin/jolt" \
                  --prefix PATH : "${pkgs.lib.makeBinPath [ pkgs.git pkgs.unzip ]}" \
                  --set-default JOLT_OPENSSL_LIBDIR "${pkgs.lib.makeLibraryPath [ pkgs.openssl ]}" \
++                --set-default TZDIR "${pkgs.tzdata}/share/zoneinfo" \
                  --set-default SSL_CERT_FILE "${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt"
              '';
            };

modified src/frq/clock.jolt +13 -0 Viewed

@@ -12,12 +12,25 @@
12	12	(def ^:private zone
13	13	;; TZ if it is set, otherwise whatever /etc/localtime points at, otherwise
14	14	;; UTC — which is wrong by hours but never wrong by a day's worth of parsing.
	15	+ ;;
	16	+ ;; /etc/localtime is not always a symlink. In a container it is often the
	17	+ ;; bytes themselves — distrobox's Arch has it as an overlay-mounted regular
	18	+ ;; file — and then `readlink -f` answers with the path it was given and there
	19	+ ;; is no zone name to read out of it at all. That fell through to UTC, so a
	20	+ ;; channel read seven hours ahead of the clock on the same screen.
	21	+ ;;
	22	+ ;; The colon form is what libc has for exactly this: `TZ=:/etc/localtime`
	23	+ ;; names the tzfile by path rather than by zone, and tzset reads it with the
	24	+ ;; transitions intact — the offset it gives is DST-correct per instant, not a
	25	+ ;; fixed one. Nothing downstream cares that this is a path: the zone is only
	26	+ ;; ever handed back to tz-offset-seconds, never shown.
15	27	(delay
16	28	(or (let [tz (host/getenv "TZ")] (when (seq tz) tz))
17	29	(try
18	30	(second (re-find #"/zoneinfo/(.+)$"
19	31	(str/trim (str (host/sh-out "readlink -f /etc/localtime")))))
20	32	(catch Exception _ nil))
	33	+ (when (host/file-exists? "/etc/localtime") ":/etc/localtime")
21	34	"UTC")))
22	35
23	36	(defn now-ms [] (quot (host/wall-nanos) 1000000))

  (def ^:private zone
    ;; TZ if it is set, otherwise whatever /etc/localtime points at, otherwise
    ;; UTC — which is wrong by hours but never wrong by a day's worth of parsing.
++  ;;
++  ;; /etc/localtime is not always a symlink. In a container it is often the
++  ;; bytes themselves — distrobox's Arch has it as an overlay-mounted regular
++  ;; file — and then `readlink -f` answers with the path it was given and there
++  ;; is no zone name to read out of it at all. That fell through to UTC, so a
++  ;; channel read seven hours ahead of the clock on the same screen.
++  ;;
++  ;; The colon form is what libc has for exactly this: `TZ=:/etc/localtime`
++  ;; names the tzfile by path rather than by zone, and tzset reads it with the
++  ;; transitions intact — the offset it gives is DST-correct per instant, not a
++  ;; fixed one. Nothing downstream cares that this is a path: the zone is only
++  ;; ever handed back to tz-offset-seconds, never shown.
    (delay
      (or (let [tz (host/getenv "TZ")] (when (seq tz) tz))
          (try
            (second (re-find #"/zoneinfo/(.+)$"
                             (str/trim (str (host/sh-out "readlink -f /etc/localtime")))))
            (catch Exception _ nil))
++        (when (host/file-exists? "/etc/localtime") ":/etc/localtime")
          "UTC")))
  (defn now-ms [] (quot (host/wall-nanos) 1000000))