chore(internal/actions): add touch flag to changefinder

internal/actions/cmd/changefinder/README.md

@@ -20,6 +20,8 @@ The available flags are as follows:
  * `-content-pattern=[regex]`: A regex to match on diff contents.
  * `-commit-message=[commit message]`: Message to use in the nested commit block
  * `-commit-scope=[conventional commit scope]`: Scope to use for the commit e.g. `fix`
+ * `-touch`: touches the `CHANGES.md` file to elicit a submodule change - only
+ works when used with `-format=commit`
 
 Example usages from this repo root:
 
@@ -33,3 +35,34 @@ go run ./internal/actions/cmd/changefinder -q -format=github
 # quiet mode, github format, github var name "foo"
 go run ./internal/actions/cmd/changefinder -q -format=github -gh-var=foo
 ```
+
+## Example  using `-touch`
+
+The `-touch` flag is best used when attempting to generate nested commits for an
+already submitted change. For example, a GAPIC generator change touches every
+gapic module, but the PR doesn't have nested commits for each changed module.
+The PR is submitted as is, perhaps because it was included with a bunch of other
+changes or for timing purposes, and the nested commits were not added with
+`changefinder` while the PR was open. In this case, we can do the following
+using `changefinder`:
+
+1. checkout the merged commit with the target changes: `git checkout <some commit has>`
+2. "save" this to a branch: `git checkout -b changed`
+3. checkout the commit just before the target commit: `git checkout <the commit just before>`
+4. "save" this to a branch : `git checkout -b base`
+5. checkout the changes: `git checkout changed`
+6. generate the nested commits + touch each module:
+```
+go run ./internal/actions/cmd/changefinder -q \
+  -format=commit \
+  -commit-scope=fix \
+  -commit-message="describe the change" \
+  -touch
+```
+7. checkout main, new branch, commit:
+```
+git checkout main && \
+  git checkout -b bump-modules && \
+  git commit -a -m 'chore: bump changed modules' && \
+  git push
+```

internal/actions/cmd/changefinder/main.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"path"
 	"strings"
 
 	"cloud.google.com/go/internal/actions/logg"
@@ -37,6 +38,7 @@ var (
 	contentPattern = flag.String("content-regex", "", "regular expression to execute against contents of diff")
 	commitMessage  = flag.String("commit-message", "", "message to use with the module in nested commit format")
 	commitScope    = flag.String("commit-scope", "", "scope to use in commit message - only for format=commit")
+	touch          = flag.Bool("touch", false, "touches the CHANGES.md file to elicit a submodule change - only works when used with -format=commit")
 )
 
 func main() {
@@ -45,6 +47,9 @@ func main() {
 	if *format == "commit" && (*commitMessage == "" || *commitScope == "") {
 		logg.Fatalf("requested format=commit and missing commit-message or commit-scope")
 	}
+	if *touch && *format != "commit" {
+		logg.Fatalf("requested modules be touched without using format=commit")
+	}
 	rootDir, err := os.Getwd()
 	if err != nil {
 		logg.Fatal(err)
@@ -79,6 +84,11 @@ func main() {
 			logg.Printf("changes in submodule: %s", submodDir)
 			updatedSubmoduleDirs = append(updatedSubmoduleDirs, submodDir)
 			modulesSeen[submodDir] = true
+			if *touch {
+				if err := touchModule(rootDir, submodDir); err != nil {
+					logg.Printf("error touching module %q: %v", submodDir, err)
+				}
+			}
 		}
 	}
 
@@ -168,3 +178,23 @@ func gitFilesChanges(dir string) ([]string, error) {
 	logg.Printf("Files changed:\n%s", b)
 	return strings.Split(string(b), "\n"), nil
 }
+
+func touchModule(root, mod string) error {
+	c := exec.Command("echo")
+	logg.Printf(c.String())
+
+	f, err := os.OpenFile(path.Join(root, mod, "CHANGES.md"), os.O_APPEND|os.O_WRONLY, 0644)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	c.Stdout = f
+
+	err = c.Run()
+	if err != nil {
+		return err
+	}
+
+	logg.Printf("Module touched: %s", mod)
+	return nil
+}