language/go: add //go:embed all:<pattern> support (bazelbuild#1432)

Co-authored-by: Michal Bicz <m@bicz.net>

language/go/embed.go

@@ -156,24 +156,30 @@ func (er *embedResolver) resolve(embed fileEmbed) (list []string, err error) {
 		}
 	}()
 
+	glob := embed.path
+	all := strings.HasPrefix(embed.path, "all:")
+	if all {
+		glob = strings.TrimPrefix(embed.path, "all:")
+	}
+
 	// Check whether the pattern is valid at all.
-	if _, err := path.Match(embed.path, ""); err != nil || !validEmbedPattern(embed.path) {
+	if _, err := path.Match(glob, ""); err != nil || !validEmbedPattern(glob) {
 		return nil, fmt.Errorf("invalid pattern syntax")
 	}
 
 	// Match the pattern against each path in the tree. If the pattern matches a
 	// directory, we need to include each file in that directory, even if the file
-	// doesn't match the pattern separate, unless it is a hidden file (starting
-	// with . or _).
+	// doesn't match the pattern separate. By default, hidden files (starting
+	// with . or _) are excluded but all: prefix forces them to be included.
 	//
 	// For example, the pattern "*" matches "a", ".b", and "_c". If "a" is a
 	// directory, we would include "a/d", even though it doesn't match "*". We
 	// would not include "a/.e".
 	var visit func(*embeddableNode, bool)
 	visit = func(f *embeddableNode, add bool) {
 		convertedPath := filepath.ToSlash(f.path)
-		match, _ := path.Match(embed.path, convertedPath)
-		add = match || (add && !f.isHidden())
+		match, _ := path.Match(glob, convertedPath)
+		add = match || (add && (!f.isHidden() || all))
 		if !f.isDir() {
 			if add {
 				list = append(list, convertedPath)

language/go/testdata/embedsrcs/BUILD.want

@@ -10,6 +10,7 @@ go_library(
         "m_gen.txt",
         "m_static.txt",
         "n_/static.txt",
+        "o_dir/_hidden.txt",
     ],
     importpath = "example.com/repo/embedsrcs",
     visibility = ["//visibility:public"],

language/go/testdata/embedsrcs/embedsrcs.go

@@ -2,5 +2,5 @@ package embedsrcs
 
 import "embed"
 
-//go:embed *m_* n_/*
+//go:embed *m_* n_/* all:o*
 var fs embed.FS

language/go/testdata/embedsrcs/o_dir/_hidden.txt

@@ -0,0 +1,2 @@
+# o_dir/_hidden.txt should be embeded because o_dir overrides exclusion rule
+# for hidden files with :all.