expand: reimplement globstar globbing for correctness

The previous globstar implementation was breadth-first search,
whereas Bash implements depth-first search.

Moreover, when recursing into directories, we could stop early and give
an error to the user when encountering a non-directory.
Instead, we want to simply not recurse into that path.

Add tests for both cases, too.

Fixes #829.

expand/expand.go

@@ -773,6 +773,9 @@ func (cfg *Config) glob(base, pat string) ([]string, error) {
 	//    ReadDir("/foo") glob "*"
 
 	for i, part := range parts {
+		// Keep around for debugging.
+		// log.Printf("matches %q part %d %q", matches, i, part)
+
 		wantDir := i < len(parts)-1
 		switch {
 		case part == "", part == ".", part == "..":
@@ -816,30 +819,35 @@ func (cfg *Config) glob(base, pat string) ([]string, error) {
 			matches = newMatches
 			continue
 		case part == "**" && cfg.GlobStar:
-			for i, match := range matches {
-				// "a/**" should match "a/ a/b a/b/cfg ..."; note
-				// how the zero-match case has a trailing
-				// separator.
-				matches[i] = pathJoin2(match, "")
-			}
-			// expand all the possible levels of **
-			latest := matches
-			for {
-				var newMatches []string
-				for _, dir := range latest {
-					var err error
-					newMatches, err = cfg.globDir(base, dir, rxGlobStar, wantDir, newMatches)
-					if err != nil {
-						return nil, err
-					}
+			// Find all recursive matches for "**".
+			// Note that we need the results to be in depth-first order,
+			// and to avoid recursion, we use a slice as a stack.
+			// Since we pop from the back, we populate the stack backwards.
+			stack := make([]string, 0, len(matches))
+			for i := len(matches) - 1; i >= 0; i-- {
+				match := matches[i]
+				// "a/**" should match "a/ a/b a/b/cfg ...";
+				// note how the zero-match case has a trailing separator.
+				match = pathJoin2(match, "")
+				stack = append(stack, match)
+			}
+			matches = matches[:0]
+			var newMatches []string // to reuse its capacity
+			for len(stack) > 0 {
+				dir := stack[len(stack)-1]
+				stack = stack[:len(stack)-1]
+
+				// Don't include the original "" match as it's not a valid path.
+				if dir != "" {
+					matches = append(matches, dir)
 				}
-				if len(newMatches) == 0 {
-					// not another level of directories to
-					// try; stop
-					break
+
+				// If dir is not a directory, we keep the stack as-is and continue.
+				newMatches = newMatches[:0]
+				newMatches, _ = cfg.globDir(base, dir, rxGlobStar, wantDir, newMatches)
+				for i := len(newMatches) - 1; i >= 0; i-- {
+					stack = append(stack, newMatches[i])
 				}
-				matches = append(matches, newMatches...)
-				latest = newMatches
 			}
 			continue
 		}
@@ -868,7 +876,8 @@ func (cfg *Config) globDir(base, dir string, rx *regexp.Regexp, wantDir bool, ma
 	}
 	infos, err := cfg.ReadDir(fullDir)
 	if err != nil {
-		return nil, err
+		// We still want to return matches, for the sake of reusing slices.
+		return matches, err
 	}
 	for _, info := range infos {
 		name := info.Name()

interp/interp_test.go

@@ -2557,6 +2557,18 @@ set +o pipefail
 		"shopt -s globstar; mkdir -p a/b/c; echo **/c | sed 's@\\\\@/@g'",
 		"a/b/c\n",
 	},
+	{
+		"shopt -s globstar; mkdir -p a/b; touch c; echo ** | sed 's@\\\\@/@g'",
+		"a a/b c\n",
+	},
+	{
+		"shopt -s globstar; mkdir -p a/b; touch c; echo **/ | sed 's@\\\\@/@g'",
+		"a/ a/b/\n",
+	},
+	{
+		"shopt -s globstar; mkdir -p a/b/c a/d; echo ** | sed 's@\\\\@/@g'",
+		"a a/b a/b/c a/d\n",
+	},
 	{
 		"mkdir foo; touch foo/bar; echo */bar */bar/ | sed 's@\\\\@/@g'",
 		"foo/bar */bar/\n",