usage: the parse is the log fingerprint, dropping the build's full re-read before it (#1357)

What changed. A usage-log snapshot's fingerprint is now produced by the
parse itself: each complete line read is fed into a SHA-256. When a build
continues the old snapshot's blocks, it continues the old snapshot's digest
too (crypto/sha256's state, kept on the snapshot as `digest`); otherwise the
digest starts fresh from byte 0 with the parse. The build's before-read
logRecordHash over the whole file is gone. The fingerprint covers the bytes
actually parsed (`off`), not info.Size(), so a half line another writer is
in the middle of appending is no longer counted as read; the unchanged
check, the prefix check and the after-read check all hash `off` bytes.

What stays, as #1328's tests require (ggbdpq's analysis on #1357): the
unsettled unchanged check, the prefix re-verification on a non-trusted
continuation, and the after-read check, which now compares the parse's own
digest against the file as it is after the read. A same-tick rewrite is
still caught at the build, not at a later read. On the trusted, settled
path nothing is re-read, as before; that snapshot now also carries a
fingerprint (the continued digest), so a later non-trusted append can
verify its prefix instead of rebuilding from byte 0.

Test changed: TestUsageRewriteBeforeFingerprintIsNotCached counted
logRecordHash calls and rewrote the file at call 1 (after the before-read
hash, before the parse) or call 2 (after the read). With the parse being
the fingerprint there is no call 1 any more, so the hash=2 case could no
longer fire. Its guarantee still holds and is still tested at both
boundaries: "before-read" rewrites at the build's first change-time look,
after its stat and before it opens and parses; "after-read" rewrites at the
after-read fingerprint check. Removing the after-read check makes
after-read fail. No other test changed.

New tests:
- TestUsageAppendReadHashesNoPreReadScan: 20,000-row settled log
  (2,760,615 bytes), 5 x Append + Summarize(Today), counts the bytes
  logRecordHash re-reads. origin/main: 7,728,984 bytes per cycle (2.80x the
  log), FAIL; this commit: 4,968,615 (1.80x), PASS. Wall time per cycle on
  an M-series Mac, 3 runs each: 3.43-3.52 ms before, 2.28-2.52 ms after
  (~32% less). The remaining two passes are the prefix check and the
  after-read check, which the same-tick guarantee needs.
- TestUsagePartialLineThenCompletion (settled and unsettled): another
  writer's half line, a read, then the line completed; the fingerprint
  covers the parsed bytes only and the completed record is counted. On
  origin/main it fails on the fingerprint check (it covered the half line).

Break checks: no after-read check -> RewriteBeforeFingerprint/after-read
fails; no prefix verification on a continuation -> SameSizeAndMtimeRewrite
append=true, SameTickRewrite append=true and LogGrowingRewriteThenAppend
fail; lines not fed into the digest -> SameTickRewrite fails.

Run: go test -tags nogui ./internal/usage/... ./internal/gateway/... (ok,
temp HOME, pinned caches, removed after); go test -race ./internal/usage/...
(ok); -race -count=10 on the rewrite/append tests (ok); gofmt -l (empty);
go vet, GOOS=linux and GOOS=windows go vet -tags nogui ./internal/usage/
(ok). Not tried on Windows or Linux hardware; the change is
platform-independent hashing over the same read.

internal/usage/cache_bounds_test.go

@@ -125,9 +125,12 @@ func holdLogTick(t *testing.T, tick time.Time) {
 	t.Cleanup(func() { logChangeTime, logIndexNow = changeTime, now })
 }
 
+// The parse is the fingerprint (#1357), so a rewrite can land before the read
+// (after the stat that decided to read) or between the read and the
+// after-read fingerprint check. Neither may leave the old bytes cached.
 func TestUsageRewriteBeforeFingerprintIsNotCached(t *testing.T) {
-	for _, rewriteAt := range []int{1, 2} {
-		t.Run(fmt.Sprint("hash=", rewriteAt), func(t *testing.T) {
+	for _, rewriteAt := range []string{"before-read", "after-read"} {
+		t.Run(rewriteAt, func(t *testing.T) {
 			pageHome(t)
 			now := holdClock(t, time.Now())
 			tick := time.Now().Truncate(time.Second)
@@ -146,23 +149,34 @@ func TestUsageRewriteBeforeFingerprintIsNotCached(t *testing.T) {
 			if bytes.Equal(edited, data) || len(edited) != len(data) {
 				t.Fatal("fixture must change at the same size")
 			}
-			hash := logRecordHash
-			hashCalls := 0
 			rewritten := false
-			logRecordHash = func(path string, n int64) string {
-				hashCalls++
-				if hashCalls == rewriteAt {
-					rewritten = true
-					if err := os.WriteFile(path, edited, 0600); err != nil {
-						t.Fatal(err)
-					}
-					if err := os.Chtimes(path, info.ModTime(), info.ModTime()); err != nil {
-						t.Fatal(err)
-					}
+			rewrite := func(path string) {
+				if rewritten {
+					return
+				}
+				rewritten = true
+				if err := os.WriteFile(path, edited, 0600); err != nil {
+					t.Fatal(err)
+				}
+				if err := os.Chtimes(path, info.ModTime(), info.ModTime()); err != nil {
+					t.Fatal(err)
 				}
-				return hash(path, n)
 			}
-			t.Cleanup(func() { logRecordHash = hash })
+			hash, changeTime := logRecordHash, logChangeTime
+			if rewriteAt == "before-read" {
+				// The build's first look at the change time follows its stat
+				// and precedes the open that reads and hashes the lines.
+				logChangeTime = func(i os.FileInfo) time.Time {
+					rewrite(Path())
+					return changeTime(i)
+				}
+			} else {
+				logRecordHash = func(path string, n int64) string {
+					rewrite(path)
+					return hash(path, n)
+				}
+			}
+			t.Cleanup(func() { logRecordHash, logChangeTime = hash, changeTime })
 			Summarize(All)
 			if !rewritten {
 				t.Fatal("fixture did not rewrite at the read boundary")

internal/usage/log_hash_cost_test.go

@@ -0,0 +1,100 @@
+package usage
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"testing"
+	"time"
+)
+
+// While the gateway writes, the log's change time is nearly always under a
+// second old, so each read verifies the snapshot by a fingerprint (#1357).
+// The parse itself produces that fingerprint: a rebuild re-reads the file only
+// to verify the prefix it continues and to check nothing was rewritten during
+// the read, not a third time before the read. Hashing doesn't allocate, so
+// the allocation tests don't see this; this counts the bytes re-read.
+func TestUsageAppendReadHashesNoPreReadScan(t *testing.T) {
+	pageHome(t)
+	holdClock(t, time.Now())
+	historyLog(t, 20000)
+	now := logIndexNow
+	logIndexNow = func() time.Time { return time.Now().Add(2 * logStampSettle) }
+	Summarize(Today) // settled first, as a log the gateway hasn't touched for a while
+	logIndexNow = now
+	t.Cleanup(func() { logIndexNow = now })
+	hash := logRecordHash
+	var hashed int64
+	logRecordHash = func(path string, n int64) string {
+		hashed += n
+		return hash(path, n)
+	}
+	t.Cleanup(func() { logRecordHash = hash })
+	const cycles = 5
+	start := time.Now()
+	for range cycles {
+		Append(Record{Agent: "codex", Provider: "relay", Model: "m", Input: 1, Status: 200})
+		Summarize(Today)
+	}
+	took := time.Since(start)
+	info, err := os.Stat(Path())
+	if err != nil {
+		t.Fatal(err)
+	}
+	perCycle := hashed / cycles
+	t.Logf("log %d bytes: %d bytes hashed per append + read (%.2f× the log), %v per cycle", info.Size(), perCycle, float64(perCycle)/float64(info.Size()), took/cycles)
+	if s := Summarize(Today); s.Calls != 1+cycles {
+		t.Fatalf("calls today: %d", s.Calls)
+	}
+	// An unsettled append verifies its prefix and re-hashes after the read:
+	// two passes. A third, before the read, is the scan the parse replaces.
+	if perCycle > 2*info.Size()+info.Size()/10 {
+		t.Fatalf("an append + read hashed %d bytes over a %d-byte log, more than the prefix check and the after-read check", perCycle, info.Size())
+	}
+}
+
+// Another writer can be mid-line when a read stats the log. The fingerprint
+// covers only the complete lines parsed, so the line's completion continues
+// the snapshot rather than counting the half line as read.
+func TestUsagePartialLineThenCompletion(t *testing.T) {
+	for _, settle := range []bool{false, true} {
+		t.Run(fmt.Sprint("settled=", settle), func(t *testing.T) {
+			pageHome(t)
+			now := holdClock(t, time.Now())
+			if settle {
+				settleLogClock(t)
+			}
+			historyLog(t, logBlockRows+3)
+			line, err := json.Marshal(Record{Time: now, Agent: "codex", Provider: "relay", Model: "m", Session: "late", Input: 41, Status: 200})
+			if err != nil {
+				t.Fatal(err)
+			}
+			line = append(line, '\n')
+			half := len(line) / 2
+			write := func(b []byte) {
+				t.Helper()
+				f, err := os.OpenFile(Path(), os.O_APPEND|os.O_WRONLY, 0600)
+				if err != nil {
+					t.Fatal(err)
+				}
+				if _, err := f.Write(b); err != nil {
+					t.Fatal(err)
+				}
+				if err := f.Close(); err != nil {
+					t.Fatal(err)
+				}
+			}
+			write(line[:half])
+			before := Summarize(All)
+			if s := readLogSnapshot(); s.hash != "" && s.hash != recordHash(Path(), s.off) {
+				t.Fatal("fingerprint does not cover the parsed bytes")
+			}
+			write(line[half:])
+			got, want := Summarize(All), summarize(All, now, Load(time.Time{}))
+			if got.Totals != want.Totals || got.Calls != before.Calls+1 {
+				t.Fatalf("completed line: got %+v (%d calls, %d before) want %+v", got.Totals, got.Calls, before.Calls, want.Totals)
+			}
+			equalPage(t, QueryPage(All, Filter{}, 0, 100), pageFromLedger(All, Filter{}, 0, 100, LedgerOf(All, Filter{})))
+		})
+	}
+}

internal/usage/log_index.go

@@ -5,7 +5,11 @@ package usage
 // Readers decode only blocks which can overlap their time window.
 import (
 	"bufio"
+	"crypto/sha256"
+	"encoding"
 	"encoding/json"
+	"fmt"
+	"hash"
 	"io"
 	"maps"
 	"os"
@@ -29,7 +33,8 @@ type logSnapshot struct {
 	path         string
 	info         os.FileInfo
 	off          int64
-	hash         string
+	hash         string // SHA-256 of the log's first off bytes, the ones parsed
+	digest       []byte // the hash's running state, which the next parse continues
 	version      uint64
 	blocks       []*rowChunk
 	first        time.Time
@@ -67,7 +72,7 @@ func logSnapshotFor(metadataOnly bool) *logSnapshot {
 	unchanged := old != nil && old.path == path && (err != nil && old.info == nil || err == nil && sameLogInfo(old.info, info))
 	if unchanged && info != nil {
 		if logChangeStamp(info) == "" || !old.settled {
-			unchanged = old.hash != "" && logRecordHash(path, info.Size()) == old.hash
+			unchanged = old.hash != "" && logRecordHash(path, old.off) == old.hash
 			if unchanged && settled {
 				promoted := *old
 				promoted.info, promoted.settled = info, true
@@ -94,8 +99,11 @@ func logSnapshotFor(metadataOnly bool) *logSnapshot {
 	notePath, noteBase, noteLast := logAppends.path, logAppends.base, logAppends.last
 	logAppends.Unlock()
 	trusted := old != nil && old.settled && logChangeStamp(info) != "" && notePath == path && sameLogInfo(oldInfo(old), noteBase) && sameLogInfo(info, noteLast)
-	continued := old != nil && old.path == path && old.info != nil && sameLogFile(old.info, info) && info.Size() > old.info.Size() && (trusted || old.hash != "" && logRecordHash(path, old.info.Size()) == old.hash)
-	if continued && !old.uncached {
+	continued := old != nil && old.path == path && old.info != nil && sameLogFile(old.info, info) && info.Size() > old.info.Size() && (trusted || old.hash != "" && logRecordHash(path, old.off) == old.hash)
+	// The parse is the fingerprint: it hashes the lines it reads, continuing
+	// the old snapshot's hash when it continues its blocks.
+	sum := sha256.New()
+	if continued && !old.uncached && resumeLogHash(sum, old.digest) {
 		next.off, next.first = old.off, old.first
 		next.blocks = slices.Clone(old.blocks)
 		next.keyProviders = maps.Clone(old.keyProviders)
@@ -108,9 +116,6 @@ func logSnapshotFor(metadataOnly bool) *logSnapshot {
 	if _, err = f.Seek(next.off, 0); err != nil {
 		return next
 	}
-	if !trusted || !settled {
-		next.hash = logRecordHash(path, info.Size())
-	}
 	var tail *rowChunk
 	if n := len(next.blocks); n > 0 && next.blocks[n-1].Count < logBlockRows {
 		tail = next.blocks[n-1].writable()
@@ -123,6 +128,7 @@ func logSnapshotFor(metadataOnly bool) *logSnapshot {
 			break
 		} // a partial final line is read again after it completes
 		next.off += int64(len(b))
+		sum.Write(b)
 		var r Record
 		if json.Unmarshal(b, &r) != nil {
 			continue
@@ -145,9 +151,10 @@ func logSnapshotFor(metadataOnly bool) *logSnapshot {
 	if tail != nil {
 		next.blocks = append(next.blocks, tail.freeze())
 	}
+	next.hash, next.digest = fmt.Sprintf("%x", sum.Sum(nil)), marshalLogHash(sum)
 	// A rewrite during the read must not give old blocks the new content's hash.
-	if (!trusted || !settled) && next.hash != logRecordHash(path, info.Size()) {
-		next.hash = ""
+	if (!trusted || !settled) && next.hash != logRecordHash(path, next.off) {
+		next.hash, next.digest = "", nil
 		next.settled = false
 	}
 	logAppends.Lock()
@@ -212,6 +219,27 @@ func (c *rowChunk) freeze() *rowChunk {
 	return c.pack()
 }
 
+func resumeLogHash(h hash.Hash, state []byte) bool {
+	u, ok := h.(encoding.BinaryUnmarshaler)
+	if !ok || state == nil || u.UnmarshalBinary(state) != nil {
+		h.Reset()
+		return false
+	}
+	return true
+}
+
+func marshalLogHash(h hash.Hash) []byte {
+	m, ok := h.(encoding.BinaryMarshaler)
+	if !ok {
+		return nil
+	}
+	state, err := m.MarshalBinary()
+	if err != nil {
+		return nil
+	}
+	return state
+}
+
 func oldInfo(s *logSnapshot) os.FileInfo {
 	if s == nil {
 		return nil