| 1 | package builtin |
| 2 | |
| 3 | import ( |
| 4 | "bytes" |
| 5 | "fmt" |
| 6 | "io" |
| 7 | "strings" |
| 8 | "testing" |
| 9 | ) |
| 10 | |
| 11 | type countingReader struct { |
| 12 | r io.Reader |
| 13 | n int |
| 14 | } |
| 15 | |
| 16 | func (c *countingReader) Read(p []byte) (int, error) { |
| 17 | n, err := c.r.Read(p) |
| 18 | c.n += n |
| 19 | return n, err |
| 20 | } |
| 21 | |
| 22 | // TestScanWindowedReadDoesNotConsumeWholeFile guards the regression where scan() |
| 23 | // drained the entire file (a second `for scanner.Scan()` loop) just to count the |
| 24 | // remaining lines for the pagination trailer. A small windowed read of a large |
| 25 | // file must read only a small prefix, not all of it. |
| 26 | func TestScanWindowedReadDoesNotConsumeWholeFile(t *testing.T) { |
| 27 | var buf bytes.Buffer |
| 28 | for i := 1; i <= 100_000; i++ { |
| 29 | fmt.Fprintf(&buf, "line %d\n", i) |
| 30 | } |
| 31 | total := buf.Len() |
| 32 | if total < 500*1024 { |
| 33 | t.Fatalf("test fixture too small (%d bytes) to be meaningful", total) |
| 34 | } |
| 35 | |
| 36 | cr := &countingReader{r: bytes.NewReader(buf.Bytes())} |
| 37 | out, err := readFile{}.scan(cr, 0, 3) |
| 38 | if err != nil { |
| 39 | t.Fatalf("scan: %v", err) |
| 40 | } |
| 41 | |
| 42 | if !strings.Contains(out, "1→line 1") || !strings.Contains(out, "3→line 3") { |
| 43 | t.Fatalf("window content wrong:\n%s", out) |
| 44 | } |
| 45 | if strings.Contains(out, "line 4") { |
| 46 | t.Fatalf("window leaked line 4:\n%s", out) |
| 47 | } |
| 48 | if !strings.Contains(out, "more lines below") { |
| 49 | t.Fatalf("pagination trailer missing:\n%s", out) |
| 50 | } |
| 51 | if cr.n > 100*1024 { |
| 52 | t.Fatalf("read %d of %d bytes for a 3-line window; should read only a small prefix", cr.n, total) |
| 53 | } |
| 54 | t.Logf("read %d of %d bytes (%.1f%%) for a 3-line window", cr.n, total, 100*float64(cr.n)/float64(total)) |
| 55 | } |
| 56 |