Drop exifreader's faulty-value text; test unnamed EXIF tags

A text tag whose value lies outside the file, which exifreader reads as
"<faulty value>", no longer gives a make, model, lens model or offset
field. Tests cover such a JPEG and the backup-metadata dump keying a tag
exifreader has no name for as undefined-<tag number>.

Model: opus-5-5

src/exif.ts

@@ -58,8 +58,12 @@ export interface PhotoExif {
     gpsAltitude?: number;
 }
 
+// exifreader gives "<faulty value>" for a tag whose value lies outside the
+// file; that tag is left out like one the file lacks.
 const asString = (v: unknown): string | undefined =>
-    typeof v === "string" && v.length > 0 ? v : undefined;
+    typeof v === "string" && v.length > 0 && v !== "<faulty value>"
+        ? v
+        : undefined;
 
 const asNumber = (v: unknown): number | undefined =>
     typeof v === "number" && Number.isFinite(v) ? v : undefined;

test/cli/metadata-exif.test.ts

@@ -63,6 +63,30 @@ const TIFF_ALTITUDE_WITHOUT_REF = [
     ...[0x00, 0x00, 0x00, 0x19, 0x00, 0x00, 0x00, 0x02], // 25/2, at 44
 ];
 
+// A big-endian TIFF block holding Orientation 6 and a Make whose value lies
+// past the end of the file.
+const TIFF_MAKE_PAST_END = [
+    ...[0x4d, 0x4d, 0x00, 0x2a, 0x00, 0x00, 0x00, 0x08], // the first IFD at 8
+    // The first IFD, at 8: two entries, then no next IFD.
+    ...[0x00, 0x02],
+    // Make (0x010f), 6 ASCII bytes at 4096.
+    ...[0x01, 0x0f, 0x00, 0x02, 0x00, 0x00, 0x00, 0x06, 0x00, 0x00, 0x10, 0x00],
+    // Orientation (0x0112), SHORT, 6.
+    ...[0x01, 0x12, 0x00, 0x03, 0x00, 0x00, 0x00, 0x01, 0x00, 0x06, 0x00, 0x00],
+    ...[0x00, 0x00, 0x00, 0x00],
+];
+
+// A big-endian TIFF block with one IFD entry that exifreader has no name for:
+// tag 0xc000 (49152), SHORT, 7.
+const TIFF_UNNAMED_TAG = [
+    ...[0x4d, 0x4d, 0x00, 0x2a, 0x00, 0x00, 0x00, 0x08], // the first IFD at 8
+    // The first IFD, at 8: one entry, then no next IFD.
+    ...[0x00, 0x01],
+    // Tag 0xc000, SHORT, 7.
+    ...[0xc0, 0x00, 0x00, 0x03, 0x00, 0x00, 0x00, 0x01, 0x00, 0x07, 0x00, 0x00],
+    ...[0x00, 0x00, 0x00, 0x00],
+];
+
 // An APP1 segment whose length field matches its data.
 const app1 = (data: number[]): number[] => {
     const len = data.length + 2;
@@ -94,6 +118,13 @@ describe("readPhotoExif", () => {
         });
     });
 
+    it("gives no make for a Make whose value lies past the end of the file", () => {
+        const data = [...EXIF_HEADER, ...TIFF_MAKE_PAST_END];
+        expect(readPhotoExif(bytes(SOI, app1(data), SOS))).toStrictEqual({
+            orientation: 6,
+        });
+    });
+
     it.each([
         [
             "a file that is not an image",
@@ -170,6 +201,14 @@ describe("extractImageMetadata", () => {
         });
     });
 
+    it("keys a tag exifreader has no name for by its number", () => {
+        const meta = extractImageMetadata(
+            bytes(SOI, app1([...EXIF_HEADER, ...TIFF_UNNAMED_TAG]), SOS),
+        );
+        expect(meta?.exifError).toBeUndefined();
+        expect(meta?.exif).toMatchObject({ "undefined-49152": { value: 7 } });
+    });
+
     it("returns nothing for a file that is not an image", () => {
         const text = new TextEncoder().encode("just some text, not an image");
         expect(extractImageMetadata(text)).toBeUndefined();