scan: show each phase as N of M, with progress, elapsed time and ETA, also while it waits (closes #112)

Every scan display now names its phase and its place among the five,
as "[2/5] walk", with its progress, rate, elapsed time and ETA. load
counts the records first; the walk estimates its total from the
records under the operands; the content phase adds up the sizes of the
matching groups and says it is searching until the first record
returns. Each display is drawn from its own goroutine: several times a
second on a terminal, every 5 seconds in plain mode.

On a terminal a phase with an exact total draws its line before the
library's bar when there is room, and any other a spinner before the
line. A line too wide drops its rate, then its bar; it is cut only when
the phase, progress, elapsed time and ETA alone do not fit.

Judgement call: load and the content checks draw the bar too, once their totals are known.
Judgement call: the bar is 10 columns wide, not the width the line leaves.

Model: opus-5-5

db_test.go

@@ -309,8 +309,10 @@ func TestApplyChangesRoundTrip(t *testing.T) {
 		},
 	}
 
-	err := applyChanges(t.Context(), db, recs, nil,
-		newProgress("update", 2))
+	prog := newProgress(progressParams{label: "update", total: 4})
+	defer prog.finish()
+
+	err := applyChanges(t.Context(), db, recs, nil, prog)
 	if err != nil {
 		t.Fatalf("applyChanges: %v", err)
 	}
@@ -329,7 +331,7 @@ func TestApplyChangesRoundTrip(t *testing.T) {
 	}
 
 	err = applyChanges(t.Context(), db, []scanRec{upd},
-		[]string{"/a/tab\tnew\nline"}, newProgress("update", 2))
+		[]string{"/a/tab\tnew\nline"}, prog)
 	if err != nil {
 		t.Fatalf("applyChanges: %v", err)
 	}
@@ -357,8 +359,10 @@ func TestApplyChangesBatching(t *testing.T) {
 		})
 	}
 
-	err := applyChanges(t.Context(), db, recs, nil,
-		newProgress("update", int64(n)))
+	prog := newProgress(progressParams{label: "update", total: int64(2 * n)})
+	defer prog.finish()
+
+	err := applyChanges(t.Context(), db, recs, nil, prog)
 	if err != nil {
 		t.Fatalf("applyChanges: %v", err)
 	}
@@ -372,8 +376,7 @@ func TestApplyChangesBatching(t *testing.T) {
 		deletes = append(deletes, r.path)
 	}
 
-	err = applyChanges(t.Context(), db, nil, deletes,
-		newProgress("update", int64(n)))
+	err = applyChanges(t.Context(), db, nil, deletes, prog)
 	if err != nil {
 		t.Fatalf("applyChanges deletes: %v", err)
 	}
@@ -382,3 +385,62 @@ func TestApplyChangesBatching(t *testing.T) {
 		t.Fatalf("records = %d, want 0", len(got))
 	}
 }
+
+// TestCountContentCandidates checks that countContentCandidates counts
+// the records the content phase's search returns: those of each group
+// of two or more records of headTailMin or more with a head, where at
+// least one has no content hash.
+func TestCountContentCandidates(t *testing.T) {
+	t.Parallel()
+
+	db := openTestDB(t)
+
+	// Each group's records: size, head, and content, one per record.
+	groups := []struct {
+		size     int64
+		head     string
+		contents []string
+	}{
+		{headTailMin, "h", []string{"", "c", "c"}}, // returned
+		{headTailMin + 1, "h", []string{"", ""}},   // returned
+		{headTailMin + 2, "h", []string{"c", "c"}}, // all hashed
+		{headTailMin + 3, "h", []string{""}},       // one record
+		{headTailMin - 1, "h", []string{"", ""}},   // too small
+		{headTailMin + 4, "", []string{"", ""}},    // no head
+	}
+
+	var recs []scanRec
+
+	for _, g := range groups {
+		for _, content := range g.contents {
+			recs = append(recs, scanRec{
+				size: g.size, mtime: time.Unix(1, 0), head: g.head,
+				tail: "t", content: content,
+				path: fmt.Sprintf("/r/%d", len(recs)),
+			})
+		}
+	}
+
+	err := applyChanges(t.Context(), db, recs, nil, nil)
+	if err != nil {
+		t.Fatalf("applyChanges: %v", err)
+	}
+
+	got, err := countContentCandidates(t.Context(), db)
+	if err != nil {
+		t.Fatalf("countContentCandidates: %v", err)
+	}
+
+	var returned int64
+
+	err = loadContentCandidates(t.Context(), db,
+		func(scanRec, bool) { returned++ })
+	if err != nil {
+		t.Fatalf("loadContentCandidates: %v", err)
+	}
+
+	if got != 5 || returned != 5 {
+		t.Errorf("count = %d, records returned = %d, want 5 and 5",
+			got, returned)
+	}
+}