feat(librarian): add ability to create and fetch a gist

internal/github/github.go

@@ -336,3 +336,51 @@ func (c *Client) ClosePullRequest(ctx context.Context, number int) error {
 	})
 	return err
 }
+
+// Gist represents a created GitHub gist.
+type Gist struct {
+	// ID is the guid of the gist.
+	ID string
+	// Owner is the GitHub user name that owns the gist.
+	Owner string
+	// Url is the HTML Url of the created gist.
+	Url string
+}
+
+// CreateGist creates a new gist with one or more files.
+func (c *Client) CreateGist(ctx context.Context, contents map[string]string, isPublic bool) (*Gist, error) {
+	var files = make(map[github.GistFilename]github.GistFile, len(contents))
+	for filename, content := range contents {
+		files[github.GistFilename(filename)] = github.GistFile{
+			Content: github.Ptr(content),
+		}
+	}
+	gist, _, err := c.Gists.Create(ctx, &github.Gist{
+		Files:  files,
+		Public: github.Ptr(isPublic),
+	})
+	if err != nil {
+		return nil, err
+	}
+	return &Gist{
+		ID:    gist.GetID(),
+		Owner: gist.GetOwner().GetName(),
+		Url:   gist.GetHTMLURL(),
+	}, nil
+}
+
+// GetGistContent fetches all the file contents for a specific gist.
+func (c *Client) GetGistContent(ctx context.Context, gistID string) (map[string]string, error) {
+	gist, _, err := c.Gists.Get(ctx, gistID)
+	if err != nil {
+		return nil, err
+	}
+
+	files := gist.GetFiles()
+	var contents = make(map[string]string, len(files))
+	for _, file := range files {
+		contents[file.GetFilename()] = file.GetContent()
+	}
+
+	return contents, nil
+}

internal/librarian/command.go

@@ -88,6 +88,8 @@ type GitHubClient interface {
 	CreateRelease(ctx context.Context, tagName, name, body, commitish string) (*github.RepositoryRelease, error)
 	CreateIssueComment(ctx context.Context, number int, comment string) error
 	CreateTag(ctx context.Context, tag, commitish string) error
+	CreateGist(ctx context.Context, contents map[string]string, isPublic bool) (*github.Gist, error)
+	GetGistContent(ctx context.Context, gistID string) (map[string]string, error)
 }
 
 // ContainerClient is an abstraction over the Docker client.

internal/librarian/mocks_test.go

@@ -42,6 +42,8 @@ type mockGitHubClient struct {
 	createReleaseCalls      int
 	createIssueCalls        int
 	createTagCalls          int
+	createGistCalls         int
+	getGistContentCalls     int
 	createPullRequestErr    error
 	addLabelsToIssuesErr    error
 	getLabelsErr            error
@@ -51,7 +53,11 @@ type mockGitHubClient struct {
 	createReleaseErr        error
 	createIssueErr          error
 	createTagErr            error
+	createGistError         error
+	getGistError            error
 	createdPR               *github.PullRequestMetadata
+	createdGist             *github.Gist
+	getGistContent          map[string]string
 	labels                  []string
 	pullRequests            []*github.PullRequest
 	pullRequest             *github.PullRequest
@@ -115,6 +121,16 @@ func (m *mockGitHubClient) CreateTag(ctx context.Context, tagName, commitish str
 	return m.createTagErr
 }
 
+func (m *mockGitHubClient) CreateGist(ctx context.Context, contents map[string]string, isPublic bool) (*github.Gist, error) {
+	m.createGistCalls++
+	return m.createdGist, m.createGistError
+}
+
+func (m *mockGitHubClient) GetGistContent(ctx context.Context, gistID string) (map[string]string, error) {
+	m.getGistContentCalls++
+	return m.getGistContent, m.getGistError
+}
+
 // mockContainerClient is a mock implementation of the ContainerClient interface for testing.
 type mockContainerClient struct {
 	ContainerClient

internal/librarian/pull_request_serializer.go

@@ -0,0 +1,77 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//	https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package librarian
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"regexp"
+)
+
+// GistOverflowHandler handles storing overflow content in a gist.
+type GistOverflowHandler struct {
+	github         GitHubClient
+	maxContentSize int
+}
+
+const maxPullRequestBodySize = 65536
+
+var overflowPullRequestRegex = regexp.MustCompile(`See full release notes at: https://gist.github.com/[^\/]+/([0-9a-f]+)`)
+
+// NewGistOverflowHandler returns a handler for storing overflow content in a gist.
+func NewGistOverflowHandler(github GitHubClient, maxContentSize int) (*GistOverflowHandler, error) {
+	if maxContentSize == 0 {
+		maxContentSize = maxPullRequestBodySize
+	}
+	return &GistOverflowHandler{
+		github:         github,
+		maxContentSize: maxContentSize,
+	}, nil
+}
+
+// SavePullRequestBody stores content in a gist if it's too big and returns a minimized version.
+func (g *GistOverflowHandler) SavePullRequestBody(ctx context.Context, body string) (string, error) {
+	if len(body) > g.maxContentSize {
+		slog.Info("content is too big, saving to gist", slog.Int("len", len(body)))
+		contents := map[string]string{
+			"release-notes.md": body,
+		}
+		gist, err := g.github.CreateGist(ctx, contents, true)
+		if err != nil {
+			return "", err
+		}
+		return fmt.Sprintf("See full release notes at: %s", gist.Url), nil
+	}
+	return body, nil
+}
+
+// FetchPullRequestBody restores content from a stroed gist.
+func (g *GistOverflowHandler) FetchPullRequestBody(ctx context.Context, body string) (string, error) {
+	matches := overflowPullRequestRegex.FindStringSubmatch(body)
+	if len(matches) == 2 {
+		slog.Info("found gist in pull request body", "gistID", matches[1])
+		contents, err := g.github.GetGistContent(ctx, matches[1])
+		if err != nil {
+			return "", err
+		}
+		content, found := contents["release-notes.md"]
+		if found {
+			return content, nil
+		}
+		return "", fmt.Errorf("unable to find release-notes.md from gist %s", matches[1])
+	}
+	return body, nil
+}