fix(gazelle): generate empty py_library only when need (bazel-contrib#1905)

bazel-contrib#1887 incorrectly generated an empty py_library for all dirs, although
it will eventually be removed because it is empty, but it will easily
conflict with other existing rules.

Fix this error, and only generate an empty py_library for bazel packages
with the same name py_library rule.

Author: hunshcn

gazelle/python/generate.go

@@ -27,11 +27,12 @@ import (
 	"github.com/bazelbuild/bazel-gazelle/label"
 	"github.com/bazelbuild/bazel-gazelle/language"
 	"github.com/bazelbuild/bazel-gazelle/rule"
-	"github.com/bazelbuild/rules_python/gazelle/pythonconfig"
 	"github.com/bmatcuk/doublestar/v4"
 	"github.com/emirpasic/gods/lists/singlylinkedlist"
 	"github.com/emirpasic/gods/sets/treeset"
 	godsutils "github.com/emirpasic/gods/utils"
+
+	"github.com/bazelbuild/rules_python/gazelle/pythonconfig"
 )
 
 const (
@@ -270,6 +271,23 @@ func (py *Python) GenerateRules(args language.GenerateArgs) language.GenerateRes
 			}
 		}
 
+		// If we're doing per-file generation, srcs could be empty at this point, meaning we shouldn't make a py_library.
+		// If there is already a package named py_library target before, we should generate an empty py_library.
+		if srcs.Empty() {
+			if args.File == nil {
+				return
+			}
+			generateEmptyLibrary := false
+			for _, r := range args.File.Rules {
+				if r.Kind() == actualPyLibraryKind && r.Name() == pyLibraryTargetName {
+					generateEmptyLibrary = true
+				}
+			}
+			if !generateEmptyLibrary {
+				return
+			}
+		}
+
 		// Check if a target with the same name we are generating already
 		// exists, and if it is of a different kind from the one we are
 		// generating. If so, we have to throw an error since Gazelle won't

gazelle/python/testdata/remove_invalid_library/others/BUILD.in

@@ -0,0 +1,5 @@
+genrule(
+    name = "others",  # same to directory name
+    outs = ["data.txt"],
+    cmd = "echo foo bar baz > $@",
+)

gazelle/python/testdata/remove_invalid_library/others/BUILD.out

@@ -0,0 +1,5 @@
+genrule(
+    name = "others",  # same to directory name
+    outs = ["data.txt"],
+    cmd = "echo foo bar baz > $@",
+)