Stop scan cleanly on SIGINT or SIGTERM (closes #5)

A first SIGINT or SIGTERM cancels the scan. It commits the hashed
records still in its batch, with a context that is not cancelled for
that one write, and starts no other write or deletion; deletions need
a complete walk, so records under paths an interrupted walk never
reached are kept. A batch whose commit failed is now kept for that
final commit instead of dropped. The progress display is finished (a
bar stopped short is no longer filled up), `scan: interrupted after N
files` goes to stderr, and the exit code is 1. A second signal ends the
process at once. A SIGINT inherited as ignored stays ignored.

Model: opus-5-5

scan_test.go

@@ -9,11 +9,13 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"os/signal"
 	"path/filepath"
 	"runtime"
 	"slices"
 	"strconv"
 	"strings"
+	"syscall"
 	"testing"
 	"time"
 )
@@ -456,7 +458,7 @@ func TestScanContentWithinOperand(t *testing.T) {
 	added := sparseFile(t, dir, "d2", headTailMin)
 
 	st := syncTree(t, db, dir)
-	if st != (scanStats{added: 1, unchanged: 2}) {
+	if st != (scanStats{walked: 3, added: 1, unchanged: 2}) {
 		t.Fatalf("rescan stats = %+v, want 1 added 2 unchanged", st)
 	}
 
@@ -502,7 +504,7 @@ func TestScanContentStalePartners(t *testing.T) {
 	sparseFile(t, dirB, "changed-copy", headTailMin+1)
 
 	st := syncTree(t, db, dirB)
-	if st != (scanStats{added: 2}) {
+	if st != (scanStats{walked: 2, added: 2}) {
 		t.Errorf("stats = %+v, want 2 added and nothing skipped", st)
 	}
 
@@ -559,7 +561,7 @@ func TestScanContentHashedStalePartners(t *testing.T) {
 	b := sparseFile(t, t.TempDir(), "copy", headTailMin)
 
 	st := syncTree(t, db, filepath.Dir(b))
-	if st != (scanStats{added: 1}) {
+	if st != (scanStats{walked: 1, added: 1}) {
 		t.Errorf("stats = %+v, want 1 added and nothing skipped", st)
 	}
 
@@ -599,7 +601,7 @@ func TestScanContentReadFailure(t *testing.T) {
 	b := sparseFile(t, dirB, "b", headTailMin)
 
 	st := syncTree(t, db, dirB)
-	if st != (scanStats{added: 1, skipped: 1}) {
+	if st != (scanStats{walked: 1, added: 1, skipped: 1}) {
 		t.Fatalf("stats = %+v, want 1 added 1 skipped", st)
 	}
 
@@ -613,7 +615,7 @@ func TestScanContentReadFailure(t *testing.T) {
 	}
 
 	st = syncTree(t, db, dirB)
-	if st != (scanStats{unchanged: 1}) {
+	if st != (scanStats{walked: 1, unchanged: 1}) {
 		t.Fatalf("rescan stats = %+v, want 1 unchanged", st)
 	}
 
@@ -659,7 +661,7 @@ func TestScanContentCheckError(t *testing.T) {
 	b := sparseFile(t, t.TempDir(), "b", headTailMin)
 
 	st := syncTree(t, db, filepath.Dir(b))
-	if st != (scanStats{added: 1, skipped: 1}) {
+	if st != (scanStats{walked: 1, added: 1, skipped: 1}) {
 		t.Fatalf("stats = %+v, want 1 added 1 skipped", st)
 	}
 
@@ -687,7 +689,7 @@ func TestScanContentHardlinks(t *testing.T) {
 	c := sparseFile(t, dir, "copy", headTailMin)
 
 	st := syncTree(t, db, dir)
-	if st != (scanStats{added: 3}) {
+	if st != (scanStats{walked: 3, added: 3}) {
 		t.Fatalf("stats = %+v, want 3 added", st)
 	}
 
@@ -1058,7 +1060,7 @@ func TestScanPipeline(t *testing.T) {
 	db := openTestDB(t)
 
 	st := syncTree(t, db, dir)
-	if st != (scanStats{added: smokeTreeFiles}) {
+	if st != (scanStats{walked: smokeTreeFiles, added: smokeTreeFiles}) {
 		t.Fatalf("stats = %+v, want %d added only", st, smokeTreeFiles)
 	}
 
@@ -1081,7 +1083,7 @@ func TestSyncScanUnchangedReuse(t *testing.T) {
 	writeFile(t, dir, "b.bin", pattern(2, 600))
 
 	st := syncTree(t, db, dir)
-	if st != (scanStats{added: 2}) {
+	if st != (scanStats{walked: 2, added: 2}) {
 		t.Fatalf("first scan stats = %+v, want 2 added", st)
 	}
 
@@ -1095,7 +1097,7 @@ func TestSyncScanUnchangedReuse(t *testing.T) {
 	}
 
 	st = syncTree(t, db, dir)
-	if st != (scanStats{unchanged: 2}) {
+	if st != (scanStats{walked: 2, unchanged: 2}) {
 		t.Fatalf("rescan stats = %+v, want 2 unchanged", st)
 	}
 
@@ -1124,7 +1126,7 @@ func TestSyncScanMtimeBump(t *testing.T) {
 	}
 
 	st := syncTree(t, db, dir)
-	if st != (scanStats{updated: 1}) {
+	if st != (scanStats{walked: 1, updated: 1}) {
 		t.Fatalf("mtime-bump stats = %+v, want 1 updated", st)
 	}
 
@@ -1152,7 +1154,7 @@ func TestSyncScanAddRemove(t *testing.T) {
 	}
 
 	st := syncTree(t, db, dir)
-	if st != (scanStats{added: 1, removed: 1, unchanged: 1}) {
+	if st != (scanStats{walked: 2, added: 1, removed: 1, unchanged: 1}) {
 		t.Fatalf("add/remove stats = %+v, want 1 added 1 removed 1 unchanged",
 			st)
 	}
@@ -1269,7 +1271,7 @@ func TestSyncScanOverlappingRoots(t *testing.T) {
 	// A file reachable via two overlapping operands is deduplicated
 	// by path in the shared walk and processed once.
 	st := syncTree(t, db, dir, filepath.Join(dir, "sub"))
-	if st != (scanStats{added: 1}) {
+	if st != (scanStats{walked: 1, added: 1}) {
 		t.Fatalf("stats = %+v, want 1 added", st)
 	}
 
@@ -1290,7 +1292,7 @@ func TestScanSkipsUniqueSizes(t *testing.T) {
 	// Neither size is shared, so neither file is read: both records
 	// are written without hashes and no duplicates are reported.
 	st := syncTree(t, db, dir)
-	if st != (scanStats{added: 2}) {
+	if st != (scanStats{walked: 2, added: 2}) {
 		t.Fatalf("stats = %+v, want 2 added", st)
 	}
 
@@ -1312,7 +1314,7 @@ func TestScanSkipsUniqueSizes(t *testing.T) {
 	c := writeFile(t, dir, "c.bin", pattern(1, 500))
 
 	st = syncTree(t, db, dir)
-	if st != (scanStats{added: 1, updated: 1, unchanged: 1}) {
+	if st != (scanStats{walked: 3, added: 1, updated: 1, unchanged: 1}) {
 		t.Fatalf("rescan stats = %+v, want 1 added 1 updated 1 unchanged",
 			st)
 	}
@@ -1376,7 +1378,7 @@ func TestScanHardlinksReadOnce(t *testing.T) {
 	}
 
 	st := syncTree(t, db, dir)
-	if st != (scanStats{added: 2}) {
+	if st != (scanStats{walked: 2, added: 2}) {
 		t.Fatalf("stats = %+v, want 2 added", st)
 	}
 
@@ -1498,6 +1500,13 @@ func injectWriteFailure(t *testing.T, path string) {
 func baselineGoroutines(t *testing.T) int {
 	t.Helper()
 
+	// The first scan in a process starts os/signal's goroutine, which
+	// never exits. Start it now, so the baseline counts it instead of
+	// the scan seeming to leave it behind.
+	ch := make(chan os.Signal, 1)
+	signal.Notify(ch, syscall.SIGINT)
+	signal.Stop(ch)
+
 	deadline := time.Now().Add(goroutineSettle)
 	last := runtime.NumGoroutine()