internal/driver: return an error on failed source fetching

This change inverts checks to ensure that we return
a wrapped fs.ErrNotExist error if we completely fail
to find a source fetcher, instead of panicking.

Fixes google#863

internal/driver/fetch.go

@@ -18,6 +18,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"io/fs"
 	"net/http"
 	"net/url"
 	"os"
@@ -510,10 +511,19 @@ func fetch(source string, duration, timeout time.Duration, ui plugin.UI, tr http
 			src = sourceURL
 		}
 	}
-	if err == nil {
-		defer f.Close()
-		p, err = profile.Parse(f)
+
+	if err != nil {
+		return
+	}
+
+	if f == nil {
+		err = fmt.Errorf("could not fetch source: %q: %w", source, fs.ErrNotExist)
+		return
 	}
+
+	defer f.Close()
+	p, err = profile.Parse(f)
+
 	return
 }
 

internal/driver/fetch_test.go

@@ -21,7 +21,9 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/pem"
+	"errors"
 	"fmt"
+	"io/fs"
 	"math/big"
 	"net"
 	"net/http"
@@ -824,3 +826,16 @@ func selfSignedCert(t *testing.T, host string) (tls.Certificate, []byte, []byte)
 	}
 	return cert, bc, bk
 }
+
+func TestFetchDoesNotPanicOnMissingSource(t *testing.T) {
+	_, _, err := fetch("non-existent-source", 10*time.Millisecond, 10*time.Millisecond, nil, http.DefaultTransport)
+	if err == nil {
+		t.Fatal("expected a non-nil error")
+	}
+	if g, w := err, fs.ErrNotExist; !errors.Is(g, w) {
+		t.Fatalf("mismatched type: got %T, want %T", g, w)
+	}
+	if g, w := err.Error(), "could not fetch"; !strings.Contains(g, w) {
+		t.Fatalf("error substring mismatch\n\tGot  %q\n\tWant %q", g, w)
+	}
+}