[helmignore] Support for '!' as a special leading sequence

.helmignore has "supported" negative matches uselessly for a long time, reported as an open ticket (https://github.com/helm/helm/issues/3622) as early as 2018. This revision implements negative matching correctly and usefully for the first time.

Signed-off-by: Talia Stocks <tstocks@hioscar.com>

internal/ignore/doc.go

@@ -62,6 +62,5 @@ Notable differences from .gitignore:
 	- The globbing library is Go's 'filepath.Match', not fnmatch(3)
 	- Trailing spaces are always ignored (there is no supported escape sequence)
 	- The evaluation of escape sequences has not been tested for compatibility
-	- There is no support for '\!' as a special leading sequence.
 */
 package ignore // import "helm.sh/helm/v3/internal/ignore"

internal/ignore/rules.go

@@ -100,22 +100,30 @@ func (r *Rules) Ignore(path string, fi os.FileInfo) bool {
 	if path == "." || path == "./" {
 		return false
 	}
+
+	// Negative rules should only override positive ignore rules, rather
+	// than exclude everything aside from the pattern (e.g. Chart.yaml).
+	// See issue:
+	// 3622 Whitelisting in .helmignore with '/*' returns "chart metadata (Chart.yaml) missing"
+	positiveMatch := false
+	negativeMatch := false
+
 	for _, p := range r.patterns {
 		if p.match == nil {
 			log.Printf("ignore: no matcher supplied for %q", p.raw)
 			return false
 		}
 
-		// For negative rules, we need to capture and return non-matches,
+		// For negative rules, we need to continue for non-matches,
-		// and continue for matches.
+		// and record matches.
 		if p.negate {
 			if p.mustDir && !fi.IsDir() {
-				return true
+				continue
 			}
 			if !p.match(path, fi) {
-				return true
+				continue
 			}
-			continue
+			negativeMatch = true
 		}
 
 		// If the rule is looking for directories, and this is not a directory,
@@ -124,10 +132,11 @@ func (r *Rules) Ignore(path string, fi os.FileInfo) bool {
 			continue
 		}
 		if p.match(path, fi) {
-			return true
+			positiveMatch = true
 		}
 	}
-	return false
+
+	return positiveMatch && !negativeMatch
 }
 
 // parseRule parses a rule string and creates a pattern, which is then stored in the Rules object.