graphqlbackend: Implement diff and reproducedDiff

cmd/frontend/graphqlbackend/schema.graphql

@@ -6833,10 +6833,19 @@ type SiteConfigurationChange implements Node {
     via an internal process (example: site startup or SITE_CONFIG_FILE being reloaded).
     """
     author: User
+
+    """
+    A flag to indicate if the diff of changes to the previous site configuration
+    was reproduced or not. Sometimes it may not be possible to generate a diff
+    (for example the redacted contents are not available) in which case the
+    value of the flag will be false.
+    """
+    reproducedDiff: Boolean!
+
     """
-    The diff string when diffed against the site config at previousID.
+    The diff string when diffed against the previous site config. When this is null there was no diff to the previous change.
     """
-    diff: String!
+    diff: String
     """
     The timestamp when this change in the site config was applied.
     """

cmd/frontend/graphqlbackend/site_config_change.go

@@ -2,11 +2,15 @@ package graphqlbackend
 
 import (
 	"context"
+	"fmt"
 
 	"github.com/graph-gophers/graphql-go"
 	"github.com/graph-gophers/graphql-go/relay"
 	"github.com/sourcegraph/sourcegraph/internal/database"
 	"github.com/sourcegraph/sourcegraph/internal/gqlutil"
+
+	"github.com/hexops/gotextdiff"
+	"github.com/hexops/gotextdiff/myers"
 )
 
 const siteConfigurationChangeKind = "SiteConfigurationChange"
@@ -21,11 +25,6 @@ func (r SiteConfigurationChangeResolver) ID() graphql.ID {
 	return marshalSiteConfigurationChangeID(r.siteConfig.ID)
 }
 
-// One line wrapper to be able to use in tests as well.
-func marshalSiteConfigurationChangeID(id int32) graphql.ID {
-	return relay.MarshalID(siteConfigurationChangeKind, &id)
-}
-
 func (r SiteConfigurationChangeResolver) Author(ctx context.Context) (*UserResolver, error) {
 	if r.siteConfig.AuthorUserID == 0 {
 		return nil, nil
@@ -39,17 +38,48 @@ func (r SiteConfigurationChangeResolver) Author(ctx context.Context) (*UserResol
 	return user, nil
 }
 
-// TODO: Implement this.
-func (r SiteConfigurationChangeResolver) Diff() string {
-	// TODO: We will do something like this, but for now return an empty string to not leak secrets
-	// until we have implemented redaction.
-	//
-	// if r.previousSiteConfig == nil {
-	// 	return ""
-	// }
+func (r SiteConfigurationChangeResolver) ReproducedDiff() bool {
+	// If we were able to redact contents for both siteConfig and previousSiteConfig and store it in
+	// the DB, then we can generate a diff.
+	if r.siteConfig != nil {
+		// As a special case, if previousSiteConfig is nil (first entry in the DB) and we also have
+		// redactedContents available for this siteConfig, we can generate a diff (it will be all
+		// lines added).
+		if r.previousSiteConfig == nil && r.siteConfig.RedactedContents != "" {
+			return true
+		}
+
+		if r.siteConfig.RedactedContents != "" && r.previousSiteConfig.RedactedContents != "" {
+			return true
+		}
+	}
+
+	return false
+}
+
+func (r SiteConfigurationChangeResolver) Diff() *string {
+	if !r.ReproducedDiff() {
+		return nil
+	}
+
+	var prevID int32
+	var prevRedactedContents string
+	if r.previousSiteConfig != nil {
+		prevID = r.previousSiteConfig.ID
+
+		// 🚨 SECURITY: This should always use "siteConfig.RedactedContents" and never
+		// "siteConfig.Contents" to generate the diff because we do not want to leak secrets in the
+		// diff.
+		prevRedactedContents = r.previousSiteConfig.RedactedContents
+	}
+
+	prettyID := func(id int32) string { return fmt.Sprintf("ID: %d", id) }
+
+	// We're not diffing a file, so set an empty string for the URI argument.
+	edits := myers.ComputeEdits("", prevRedactedContents, r.siteConfig.RedactedContents)
+	diff := fmt.Sprint(gotextdiff.ToUnified(prettyID(prevID), prettyID(r.siteConfig.ID), prevRedactedContents, edits))
 
-	// return cmp.Diff(r.siteConfig.Contents, r.previousSiteConfig.Contents)
-	return ""
+	return &diff
 }
 
 func (r SiteConfigurationChangeResolver) CreatedAt() gqlutil.DateTime {
@@ -59,3 +89,8 @@ func (r SiteConfigurationChangeResolver) CreatedAt() gqlutil.DateTime {
 func (r SiteConfigurationChangeResolver) UpdatedAt() gqlutil.DateTime {
 	return gqlutil.DateTime{Time: r.siteConfig.UpdatedAt}
 }
+
+// One line wrapper to be able to use in tests as well.
+func marshalSiteConfigurationChangeID(id int32) graphql.ID {
+	return relay.MarshalID(siteConfigurationChangeKind, &id)
+}