[frankmcsherry]

Replaces #982. Uses #1023's BackendFrontier.

A program installed after a published trace had compacted read the trace's history as stored. Before the compaction frontier, a batch can hold a retraction advanced to the frontier beside the insertion it retracts, which is a count no time had. An iterative scope seeded from such an import need not converge, and tick hung: 3 of 620 differential test seeds with late installs.

Server::import_as_of() is the epoch before the current one, which is also the frontier tick compacts every published trace to. Every import reads as of it, the way Materialize installs a computation "as of" a time:

• Arrangements: the corgi arrangement Arr carries since. A join or reduce of an import reads through int_proxy: BackendFrontier reads a proxy backend's inputs as of a frontier #1023's BackendFrontier, and a join of two imports reads as of the join of their times.
• Collections: a collection read from an import has its copied time lanes advanced.
• Every import path: programs, bindings, snapshots and tails all import through the same path. A tail started late reports the history as one epoch.
• When there's nothing to do: an arrangement the program built has since at the minimum. Every join and reduce is still wrapped, so each compiles to one operator; at the minimum the wrapper returns after one comparison per unit.

The half join still reads the arrangement as stored. That is correct while the stream is at or beyond the as-of time and published times are at the root. Once #1021 and #1022 land, a follow-up wraps its backend too.

A re-exported import (export "x" = import "y") publishes the producer's arrangement as it is; a reader of x imports it as of its own start.

Tests

• Regression test: late_install_regressions runs the three seeds that hung (40000075, 40000160, 40000519) under the split, bind and reinstall oracles in every cargo test, in 1.6 s in debug. Without the as-of reads it does not finish.
• CI sweep: a new step in the release job runs 500 late-install seeds, in about 5 s once built.
• server_late_import.rs: installs programs late and reads imports as containers, as rows, and as arrangements read by a join, a reduce and a loop.
• Timing: a probe kept out of the tree times a late install against a million-row history. Install plus first tick takes 83–85 ms, against about 150 ms on master-next, because the reduce of the import now evaluates at one time instead of at every stored epoch.

Checks

• cargo test -p differential-dataflow -p interactive -p ddir-server: 358 passed.
• Difftest with all nine oracles: 2,000 passed. Late installs only: 2,500 seeds, including the three that hung, all passed.
• Release explain suite: 29 passed.
• Every step of the LDBC CI job: exit 0, with all 41 authored answers matching.

🤖 Generated with Claude Code

https://claude.ai/code/session_019XsakwFGwtESwZ1gSYwG4o