Timeline

emitMatches walked a hash bucket's entire collision chain at every target position with no limit. rollingHash is a weak multiply-accumulate over a 16-byte window, so low-entropy content — repeated tokens, runs of whitespace, ordinary SQL and C boilerplate — funnels an enormous number of source windows into one bucket, making Create quadratic in target length. Measured on the issue's repro (repeated 8-byte token, one 10-byte edit): 32 KiB 0.133s, 64 KiB 0.496s, 128 KiB 1.950s, 256 KiB 7.765s — ~3.9x per doubling — against 0.013s for a 128 KiB high-entropy control.

Bound the walk at maxChainCandidates, matching the `int limit = 250` that fossil's delta_create (src/delta.c) puts on the identical loop. The same walk also computed each candidate's match length twice, once to test it against the nHash threshold and again to read the value; compute it once.

Extract the walk into bestMatch so the bound is testable on a candidate count rather than on wall-clock time, which cannot flake on loaded CI.

Delta output changes, as fossil's own limit makes it change: a capped search can settle for a shorter match, so the encoder emits more copy commands. This lands our output on canonical fossil's — 128 KiB: 266 bytes vs fossil's 260; 1 MiB: 2142 vs 2120 — where before the unbounded walk produced 44 and 50 bytes and paid quadratic time for them.

After: 128 KiB 1.950s -> 0.018s, and 1 MiB 125.036s -> 0.062s.

Verified the deltas still apply byte-for-byte, by our Apply and by canonical fossil 2.28's test-delta-apply.

Co-authored-by: Dan Mestas <[email protected]> check-in: b1b6c09b41 user: [email protected] tags: trunk, v0.7.0

afterDephantomize crosslinked every artifact it discovered through crosslinkSingle, which opened its own write transaction per artifact and passed an explicit nil content cache into crosslinkCheckin. Both choices suit an isolated artifact and neither suits the cascade: one phantom fill unblocks hundreds or thousands of related artifacts that share delta bases and parent manifests, so the walk paid a transaction commit per artifact and re-expanded content a sibling had just expanded. A CPU profile of round 7 of a fossil-corpus clone showed exactly that pair -- _vdbeCommit plus expandChain/decompress reached through crosslinkSingle -- as the remaining 300+ seconds after the statement-cache fix.

crosslinkSingle had no callers outside the cascade, so it is replaced rather than kept alongside a second strategy. cascadeLinker buffers the artifacts the walk discovers and links them through linkArtifact -- the same type switch the whole-repository sweep uses, so the two paths no longer carry near-duplicate switches -- on one content.Cache for the whole cascade, committing once per crosslinkBatchSize artifacts instead of once per artifact. The sweep's own batching, cache, and tuning constants are untouched.

The walk's delta-child query filters on committed mlink rows, so deferring writes past it could change which artifacts it visits. Orphans are the only discovery that query can observe -- delta.rid is a primary key, so every other buffered artifact was itself produced by that query and cannot come back -- so the buffer flushes after an orphan batch and before the query, preserving the original order: orphans crosslinked, orphan rows cleared, children queried.

Failure handling keeps what per-artifact transactions bought. An artifact that cannot be expanded, parsed, or linked is logged with the same wording and skipped while the rest of the batch commits, and each artifact's writes are wrapped in a savepoint so a link that fails partway leaves no partial event/plink/mlink/tagxref behind. Cancellation is unchanged in the walk and newly observed inside a flush on the sweep's stride; an open batch is abandoned rather than committed, so committed batches stand and the next Crosslink sweep re-selects whatever still has no event row.

Tests: a characterization test pins the cascade's derived rows and visit count over a graph covering both sides of the mlink filter -- an orphan that writes mlink rows and one that does not, and so is still visited twice -- against the dump the per-artifact shape produced. A regression guard asserts one commit and one expansion per artifact with the parent manifest served from cache for a 300-artifact burst, plus tests for whole-batch flushing, savepoint isolation, and mid-flush cancellation. check-in: bfb2ad3aa4 user: [email protected] tags: trunk