Check committed Go formatting

internal/validation/validation.go

@@ -2,8 +2,10 @@
 package validation
 
 import (
+	"bytes"
 	"context"
 	"fmt"
+	"go/format"
 	"io"
 	"os"
 	"strings"
@@ -16,6 +18,7 @@ type Settings struct {
 	Race           bool
 	FullVet        bool
 	CompileTargets []Target
+	GoFormat       bool
 }
 
 // Target selects one package-compilation target.
@@ -88,5 +91,55 @@ func Run(ctx context.Context, request Request) error {
 			}
 		}
 	}
+	if request.Settings.GoFormat {
+		tree, err := request.ReadCandidateTree(ctx)
+		if err != nil {
+			return &Failure{Check: GoFormat, Cause: err}
+		}
+		if err := checkGoFormat(ctx, tree); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func checkGoFormat(ctx context.Context, tree *CandidateTree) error {
+	var findings []Finding
+	for _, entry := range tree.Entries() {
+		if entry.Type != "blob" || (entry.Mode != "100644" && entry.Mode != "100755") || !strings.HasSuffix(entry.Path, ".go") {
+			continue
+		}
+		if err := ctx.Err(); err != nil {
+			return &Failure{Check: GoFormat, Cause: err}
+		}
+		contents, err := tree.ReadFile(ctx, entry.Path)
+		if err != nil {
+			return &Failure{
+				Check: GoFormat,
+				Findings: []Finding{{
+					Path:    entry.Path,
+					Message: "unable to inspect committed Go source",
+				}},
+				Cause: fmt.Errorf("inspect committed Go file %q: %w", entry.Path, err),
+			}
+		}
+		formatted, err := format.Source(contents)
+		if err != nil {
+			return &Failure{
+				Check: GoFormat,
+				Findings: []Finding{{
+					Path:    entry.Path,
+					Message: "committed Go source cannot be formatted",
+				}},
+				Cause: fmt.Errorf("format committed Go file %q: %w", entry.Path, err),
+			}
+		}
+		if !bytes.Equal(contents, formatted) {
+			findings = append(findings, Finding{Path: entry.Path, Message: "file is not gofmt-formatted"})
+		}
+	}
+	if len(findings) != 0 {
+		return &Failure{Check: GoFormat, Findings: findings}
+	}
 	return nil
 }