cmd/vulnreport: only write files if they would be modified

Before writing YAML/OSV/CVE files, check if their contents would
be modified. The main benefit of this is that we now only print
out the written filename if its contents change, which is helpful
for determining the impact of batch operations.

Change-Id: Ieae133a1697d98b99cb40dda56826a5c46f40487
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/599179
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: tatianab

cmd/vulnreport/run_test.go

@@ -45,9 +45,14 @@ func newInMemoryWFS() *memWFS {
 	return &memWFS{written: make(map[string][]byte)}
 }
 
-func (m *memWFS) WriteFile(fname string, b []byte) error {
+var _ wfs = &memWFS{}
+
+func (m *memWFS) WriteFile(fname string, b []byte) (bool, error) {
+	if bytes.Equal(m.written[fname], b) {
+		return false, nil
+	}
 	m.written[fname] = b
-	return nil
+	return true, nil
 }
 func testFilename(t *testing.T) string {
 	return filepath.Join("testdata", t.Name()+".txtar")

cmd/vulnreport/writer.go

@@ -28,10 +28,11 @@ func (f *fileWriter) write(r *yamlReport) error {
 	if err := r.Encode(w); err != nil {
 		return err
 	}
-	if err := f.WriteFile(r.Filename, w.Bytes()); err != nil {
+	modified, err := f.WriteFile(r.Filename, w.Bytes())
+	if err != nil {
 		return err
 	}
-	return ok(r.Filename)
+	return ok(r.Filename, modified)
 }
 
 func (f *fileWriter) writeOSV(r *yamlReport) error {
@@ -71,24 +72,33 @@ func writeJSON(wfs wfs, fname string, v any) error {
 	if err != nil {
 		return err
 	}
-	if err := wfs.WriteFile(fname, j); err != nil {
+	modified, err := wfs.WriteFile(fname, j)
+	if err != nil {
 		return err
 	}
-	return ok(fname)
+	return ok(fname, modified)
 }
 
-func ok(fname string) error {
-	log.Out(filepath.ToSlash(fname))
+func ok(fname string, modified bool) error {
+	if modified {
+		log.Out(filepath.ToSlash(fname))
+	}
 	return nil
 }
 
 // a simple representation of a writeable file system
 type wfs interface {
-	WriteFile(string, []byte) error
+	WriteFile(string, []byte) (bool, error)
 }
 
 type defaultWFS struct{}
 
-func (defaultWFS) WriteFile(filename string, b []byte) error {
-	return os.WriteFile(filename, b, 0644)
+var _ wfs = &defaultWFS{}
+
+func (defaultWFS) WriteFile(filename string, b []byte) (bool, error) {
+	// writing the file would not change its contents
+	if existing, err := os.ReadFile(filename); err == nil && bytes.Equal(existing, b) {
+		return false, nil
+	}
+	return true, os.WriteFile(filename, b, 0644)
 }