Add a test flag for alloy fmt (#1632)

Author: kavfixnel

CHANGELOG.md

@@ -280,6 +280,8 @@ v1.2.0
 
 - Add a `prometheus.exporter.catchpoint` component to collect metrics from Catchpoint. (@bominrahmani)
 
+- Add the `-t/--test` flag to `alloy fmt` to check if a alloy config file is formatted correctly. (@kavfixnel)
+
 ### Enhancements
 
 - (_Public preview_) Add native histogram support to `otelcol.receiver.prometheus`. (@wildum)

docs/sources/reference/cli/fmt.md

@@ -29,8 +29,13 @@ Otherwise, `fmt` reads and formats the file from disk specified by the argument.
 The `--write` flag can be specified to replace the contents of the original file on disk with the formatted results.
 `--write` can only be provided when `fmt` isn't reading from standard input.
 
+The `--test` flag can be specified to test if the contents of the file are formatted correctly.
+
+The `--write` and `--test` flags are mutually exclusive.
+
 The command fails if the file being formatted has syntactically incorrect {{< param "PRODUCT_NAME" >}} configuration, but doesn't validate whether {{< param "PRODUCT_NAME" >}} components are configured properly.
 
 The following flags are supported:
 
 * `--write`, `-w`: Write the formatted file back to disk when not reading from standard input.
+* `--test`, `-t`: Only test the input and return a non-zero exit code if changes would have been made.

internal/alloycli/cmd_fmt.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"reflect"
 
 	"github.com/spf13/cobra"
 
@@ -17,6 +18,7 @@ import (
 func fmtCommand() *cobra.Command {
 	f := &alloyFmt{
 		write: false,
+		test:  false,
 	}
 
 	cmd := &cobra.Command{
@@ -55,20 +57,26 @@ The -w flag can be used to write the formatted file back to disk. -w can not be
 	}
 
 	cmd.Flags().BoolVarP(&f.write, "write", "w", f.write, "write result to (source) file instead of stdout")
+	cmd.Flags().BoolVarP(&f.test, "test", "t", f.test, "exit with non-zero when changes would be made. Cannot be used with -w/--write")
 	return cmd
 }
 
 type alloyFmt struct {
 	write bool
+	test  bool
 }
 
 func (ff *alloyFmt) Run(configFile string) error {
+	if ff.write && ff.test {
+		return fmt.Errorf("Cannot use -w/--write and -t/--test at the same time")
+	}
+
 	switch configFile {
 	case "-":
 		if ff.write {
 			return fmt.Errorf("cannot use -w with standard input")
 		}
-		return format("<stdin>", nil, os.Stdin, false)
+		return format("<stdin>", nil, os.Stdin, false, ff.test)
 
 	default:
 		fi, err := os.Stat(configFile)
@@ -84,11 +92,11 @@ func (ff *alloyFmt) Run(configFile string) error {
 			return err
 		}
 		defer f.Close()
-		return format(configFile, fi, f, ff.write)
+		return format(configFile, fi, f, ff.write, ff.test)
 	}
 }
 
-func format(filename string, fi os.FileInfo, r io.Reader, write bool) error {
+func format(filename string, fi os.FileInfo, r io.Reader, write bool, test bool) error {
 	bb, err := io.ReadAll(r)
 	if err != nil {
 		return err
@@ -107,6 +115,14 @@ func format(filename string, fi os.FileInfo, r io.Reader, write bool) error {
 	// Add a newline at the end of the file.
 	_, _ = buf.Write([]byte{'\n'})
 
+	// If -t/--test flag is check, only check if file is formatted correctly
+	if test {
+		if !reflect.DeepEqual(bb, buf.Bytes()) {
+			return fmt.Errorf("File %s is not formatted correctly", filename)
+		}
+		return nil
+	}
+
 	if !write {
 		_, err := io.Copy(os.Stdout, &buf)
 		return err