Stop asinfo-gen when the as.csv download breaks off (closes #177)

parseCSV skipped a record on any read error and asked for the next one.
When the download of as.csv broke off, the response body returned the
same error on every read, so the generator logged it forever and make
asupdate never finished.

parseCSV now skips only a record that does not parse, a csv.ParseError.
Any other read error ends it with that error, so the generator exits
non-zero and script/asupdate keeps the old list.

Model: opus-5-5

TODO.md

@@ -33,6 +33,9 @@ tested.
 
 # Completed Steps
 
+- 2026-10-27: `make asupdate` stops with an error when the download of `as.csv`
+  breaks off. The generator used to retry the failed read forever. A record that
+  does not parse is still logged and skipped (closes #177)
 - 2026-10-27: `make asupdate` runs the new `script/asupdate`. It replaces the AS
   list built into RouteWatch only when the generator succeeds and finds at least
   one AS. A failed download used to replace the list with an empty file. The

cmd/asinfo-gen/main.go

@@ -85,7 +85,8 @@ func main() {
 // parseCSV reads as.csv from r and returns its ASes, with the handle and the
 // description as the file spells them. A header whose first columns are not
 // the AS number, the handle and the description is an error. A record that
-// does not parse is logged and skipped. A file left with no AS is an error.
+// does not parse is logged and skipped. A read that fails, such as on a broken
+// download, is an error. A file left with no AS is an error.
 func parseCSV(r io.Reader) ([]ASInfo, error) {
 	reader := csv.NewReader(r)
 
@@ -109,12 +110,18 @@ func parseCSV(r io.Reader) ([]ASInfo, error) {
 			break
 		}
 
-		if err != nil {
+		if _, ok := errors.AsType[*csv.ParseError](err); ok {
 			log.Printf("Error reading CSV record: %v", err)
 
 			continue
 		}
 
+		// Any other error comes from the download, such as a broken
+		// connection. Every later read returns it again.
+		if err != nil {
+			return nil, fmt.Errorf("read CSV record: %w", err)
+		}
+
 		asn, err := strconv.Atoi(record[0])
 		if err != nil {
 			log.Printf("Invalid ASN %q: %v", record[0], err)

cmd/asinfo-gen/main_test.go

@@ -2,11 +2,18 @@ package main
 
 import (
 	"errors"
+	"io"
 	"slices"
 	"strings"
 	"testing"
+	"testing/iotest"
+	"time"
 )
 
+// parseTimeLimit is how long a test waits for parseCSV to return before it
+// fails.
+const parseTimeLimit = 2 * time.Second
+
 // TestParseCSVReadsTodaysLayout parses an as.csv with the country code column
 // that the source added after the description. It expects each AS with its
 // handle and description spelled exactly as in the file, accents included.
@@ -71,3 +78,35 @@ func TestParseCSVRejectsAFileWithNoASes(t *testing.T) {
 		}
 	}
 }
+
+// TestParseCSVStopsWhenTheDownloadBreaksOff parses an as.csv whose download
+// breaks off after the first record. From then on every read fails, as on a
+// broken connection. It expects parseCSV to return that error, so that make
+// asupdate keeps the old list.
+func TestParseCSVStopsWhenTheDownloadBreaksOff(t *testing.T) {
+	t.Parallel()
+
+	const asCSV = "asn,handle,description,country-code\n" +
+		"1,LVLT-1,Level 3 Parent LLC,US\n"
+
+	download := io.MultiReader(
+		strings.NewReader(asCSV), iotest.ErrReader(io.ErrUnexpectedEOF))
+
+	// parseCSV runs on its own goroutine, so that a parseCSV that never
+	// returns fails the test instead of hanging it.
+	parsed := make(chan error, 1)
+
+	go func() {
+		_, err := parseCSV(download)
+		parsed <- err
+	}()
+
+	select {
+	case err := <-parsed:
+		if !errors.Is(err, io.ErrUnexpectedEOF) {
+			t.Errorf("parseCSV = %v, want %v", err, io.ErrUnexpectedEOF)
+		}
+	case <-time.After(parseTimeLimit):
+		t.Fatalf("parseCSV did not return within %v", parseTimeLimit)
+	}
+}