Keep the AS list when the asupdate generator fails (closes #173)

make asupdate piped the generator into gzip, so the target saw gzip's
exit status. A failed download replaced pkg/asinfo/asdata.json.gz with
an empty file and the target still exited 0.

The target now runs the new script/asupdate. It writes the output of
cmd/asinfo-gen to a temporary file. Only when the generator succeeds
does it compress the list and move it into place. Otherwise it leaves
the file alone and exits non-zero. The generator now fails when as.csv
holds no AS.

The generator reads as.csv from ipverse/as-metadata, the source's new
name. The list it builds there is unchanged, so no new list is
committed.

Model: opus-5-5

README.md

@@ -489,8 +489,8 @@ provide:
 - `script/asupdate` — rebuild the AS list built into RouteWatch,
   `pkg/asinfo/asdata.json.gz`, from the `as.csv` file of
   [ipverse/as-metadata](https://github.com/ipverse/as-metadata). It needs the
-  network. When the generator fails or finds no ASes, the old list stays and the
-  script exits non-zero.
+  network. It runs `cmd/asinfo-gen`, which fails when it finds no AS. When
+  `cmd/asinfo-gen` fails, the old list stays and the script exits non-zero.
 
 Tests and the linter run only in Docker, as the `test` and `lint` phases of the
 `Dockerfile`; the image cannot be built unless both pass. Every `docker build`

cmd/asinfo-gen/main.go

@@ -26,7 +26,10 @@ const (
 	columnsRead       = 3
 )
 
-var errUnexpectedHeader = errors.New("unexpected CSV header")
+var (
+	errUnexpectedHeader = errors.New("unexpected CSV header")
+	errNoASes           = errors.New("no AS in the CSV")
+)
 
 // ASInfo represents information about an Autonomous System
 type ASInfo struct {
@@ -82,7 +85,7 @@ 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.
+// does not parse is logged and skipped. A file left with no AS is an error.
 func parseCSV(r io.Reader) ([]ASInfo, error) {
 	reader := csv.NewReader(r)
 
@@ -126,5 +129,9 @@ func parseCSV(r io.Reader) ([]ASInfo, error) {
 		})
 	}
 
+	if len(asInfos) == 0 {
+		return nil, errNoASes
+	}
+
 	return asInfos, nil
 }

cmd/asinfo-gen/main_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"slices"
 	"strings"
 	"testing"
@@ -53,3 +54,20 @@ func TestParseCSVRejectsAnotherHeader(t *testing.T) {
 		}
 	}
 }
+
+// TestParseCSVRejectsAFileWithNoASes parses files that hold no AS, one with
+// only the header and one whose only record has no valid AS number. It
+// expects errNoASes, so that make asupdate keeps the old list.
+func TestParseCSVRejectsAFileWithNoASes(t *testing.T) {
+	t.Parallel()
+
+	for _, asCSV := range []string{
+		"asn,handle,description,country-code\n",
+		"asn,handle,description,country-code\nAS1,LVLT-1,Level 3,US\n",
+	} {
+		_, err := parseCSV(strings.NewReader(asCSV))
+		if !errors.Is(err, errNoASes) {
+			t.Errorf("parseCSV(%q) = %v, want %v", asCSV, err, errNoASes)
+		}
+	}
+}

script/asupdate

@@ -1,7 +1,7 @@
 #!/bin/sh
 # script/asupdate: rebuild the AS list built into RouteWatch from
-# as.csv with cmd/asinfo-gen. The new list replaces the old one only
-# when the generator succeeds and writes at least one AS. Otherwise
+# as.csv with cmd/asinfo-gen, which fails when it finds no AS. The new
+# list replaces the old one only when the generator succeeds. Otherwise
 # the old list stays as it was and the script exits non-zero.
 set -eu
 
@@ -25,15 +25,6 @@ main() {
         exit 1
     fi
 
-    # The generator writes a list of ASes with "[" alone on the first
-    # line. An empty list is "[]", and no list at all is "null".
-    first_line=""
-    read -r first_line <"$json" || true
-    if [ "$first_line" != "[" ]; then
-        echo "asupdate: the generator wrote no ASes; $AS_DATA is unchanged" >&2
-        exit 1
-    fi
-
     # -n leaves the time out of the file, so an unchanged list gives the
     # same bytes.
     gzip -n -c <"$json" >"$AS_DATA_TMP"