1 files changed, 296 insertions, 0 deletions
diff --git a/git.go b/git.go
new file mode 100644
index 0000000..17eaa81
--- /dev/null
+++ b/git.go
@@ -0,0 +1,296 @@
+package main
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Git is driven by exec'ing it: cgit and git-http-backend already require the
+// binary, and a Go implementation could only disagree with it.
+
+// gitEnv neutralises the system and global configuration.
+func gitEnv(extra []string) []string {
+	env := append(os.Environ(), "GIT_CONFIG_NOSYSTEM=1", "GIT_CONFIG_GLOBAL=/dev/null")
+	return append(env, extra...)
+}
+
+// git returns stdout alone; stderr goes into the error, since stdout is
+// usually an object id.
+func git(dir string, env []string, args ...string) ([]byte, error) {
+	return gitIn(dir, nil, env, args...)
+}
+
+func gitIn(dir string, stdin []byte, env []string, args ...string) ([]byte, error) {
+	cmd := exec.Command("git", args...)
+	cmd.Dir = dir
+	cmd.Env = gitEnv(env)
+	if stdin != nil {
+		cmd.Stdin = bytes.NewReader(stdin)
+	}
+	var out, fail bytes.Buffer
+	cmd.Stdout, cmd.Stderr = &out, &fail
+	if err := cmd.Run(); err != nil {
+		// %w, so a caller can ask git's exit status.
+		if msg := bytes.TrimSpace(fail.Bytes()); len(msg) > 0 {
+			return out.Bytes(), fmt.Errorf("git %s: %w: %s", strings.Join(args, " "), err, msg)
+		}
+		return out.Bytes(), fmt.Errorf("git %s: %w", strings.Join(args, " "), err)
+	}
+	return out.Bytes(), nil
+}
+
+// gitInit uses an empty template, so no sample hooks or placeholder
+// description, and an explicit branch, so nothing is inherited from
+// init.defaultBranch.
+func gitInit(dir string, bare bool) error {
+	tmpl, err := os.MkdirTemp("", "git-forge-template")
+	if err != nil {
+		return err
+	}
+	defer os.RemoveAll(tmpl)
+
+	args := []string{"init", "--quiet", "--template=" + tmpl, "--initial-branch=master"}
+	if bare {
+		args = append(args, "--bare")
+	}
+	_, err = git("", nil, append(args, dir)...)
+	return err
+}
+
+// change is one path in a tree. Nil data removes it.
+type change struct {
+	path string
+	data []byte
+}
+
+// branch is the only branch git-forge ever writes.
+const branch = "refs/heads/master"
+
+// commitWith moves the branch with a compare and swap, retrying when it moved.
+// build is handed the commit the branch points at, "" when empty, on every
+// attempt: a sequential id must be read again after a lost race. It returns
+// the subject too.
+func commitWith(dir string, author, committer Author, build func(base string) ([]change, string, error)) error {
+	defer lockRepo(dir)()
+
+	var err error
+	for attempt := 0; attempt < 8; attempt++ {
+		if attempt > 0 {
+			// Git holds its ref lock for a moment.
+			time.Sleep(time.Duration(attempt) * 10 * time.Millisecond)
+		}
+		var base string
+		if base, err = resolveRef(dir, branch); err != nil {
+			return err
+		}
+		var changes []change
+		var subject string
+		if changes, subject, err = build(base); err != nil {
+			return err
+		}
+		var tree string
+		if tree, err = writeTree(dir, base, changes); err != nil {
+			return err
+		}
+		var commit string
+		if commit, err = commitTree(dir, tree, base, subject, author, committer); err != nil {
+			return err
+		}
+		// An empty old value means the branch must not exist yet.
+		if _, err = git(dir, nil, "update-ref", branch, commit, base); err == nil {
+			// Plumbing runs no maintenance, and nothing else collects these
+			// repositories.
+			gcAuto(dir)
+			return nil
+		}
+	}
+	return fmt.Errorf("%s: gave up after eight attempts: %v", dir, err)
+}
+
+// Git's own thresholds decide whether anything happens; gc.autoDetach keeps
+// the work off the request that triggered it.
+func gcAuto(dir string) {
+	env := []string{"GIT_CONFIG_COUNT=1", "GIT_CONFIG_KEY_0=gc.autoDetach", "GIT_CONFIG_VALUE_0=true"}
+	if _, err := git(dir, env, "gc", "--auto", "--quiet"); err != nil {
+		log.Printf("git: gc --auto in %s: %v", dir, err)
+	}
+}
+
+// lockRepo serialises this process's writers to one repository, so they do not
+// burn their retries on each other. Git's compare and swap is what makes them
+// correct.
+var writeLocks sync.Map // repository directory to *sync.Mutex
+
+func lockRepo(dir string) func() {
+	value, _ := writeLocks.LoadOrStore(dir, &sync.Mutex{})
+	mu := value.(*sync.Mutex)
+	mu.Lock()
+	return mu.Unlock
+}
+
+// resolveRef returns "" for a ref that is not there, and an error for a
+// repository that is not: rev-parse exits 1 for the first and 128 for the
+// second, and a missing directory must not read as an empty page.
+func resolveRef(dir, ref string) (string, error) {
+	out, err := git(dir, nil, "rev-parse", "--verify", "--quiet", ref)
+	if err == nil {
+		return strings.TrimSpace(string(out)), nil
+	}
+	var exit *exec.ExitError
+	if errors.As(err, &exit) && exit.ExitCode() == 1 {
+		return "", nil
+	}
+	return "", err
+}
+
+// writeTree uses a temporary index rather than mktree, because these paths
+// nest and update-index rewrites the intermediate trees. The index sits
+// outside the repository: nothing the forge writes goes inside one.
+func writeTree(dir, base string, changes []change) (string, error) {
+	// A directory, not a file: git refuses an index file that exists and is
+	// empty.
+	tmp, err := os.MkdirTemp("", "git-forge-index")
+	if err != nil {
+		return "", err
+	}
+	defer os.RemoveAll(tmp)
+	env := []string{"GIT_INDEX_FILE=" + filepath.Join(tmp, "index")}
+
+	if base != "" {
+		if _, err := git(dir, env, "read-tree", base); err != nil {
+			return "", err
+		}
+	}
+	for _, c := range changes {
+		if c.data == nil {
+			if _, err := git(dir, env, "update-index", "--force-remove", c.path); err != nil {
+				return "", err
+			}
+			continue
+		}
+		blob, err := gitIn(dir, c.data, nil, "hash-object", "-w", "--stdin")
+		if err != nil {
+			return "", err
+		}
+		id := strings.TrimSpace(string(blob))
+		if _, err := git(dir, env, "update-index", "--add", "--cacheinfo", "100644,"+id+","+c.path); err != nil {
+			return "", err
+		}
+	}
+	tree, err := git(dir, env, "write-tree")
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(tree)), nil
+}
+
+func commitTree(dir, tree, parent, subject string, author, committer Author) (string, error) {
+	args := []string{"commit-tree", tree, "-m", subject}
+	if parent != "" {
+		args = append(args, "-p", parent)
+	}
+	env := []string{
+		"GIT_AUTHOR_NAME=" + author.Name,
+		"GIT_AUTHOR_EMAIL=" + author.Email,
+		"GIT_COMMITTER_NAME=" + committer.Name,
+		"GIT_COMMITTER_EMAIL=" + committer.Email,
+	}
+	out, err := git(dir, env, args...)
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(out)), nil
+}
+
+// catFile reads one blob, named by its id or as commit:path.
+func catFile(dir, rev string) ([]byte, error) {
+	return git(dir, nil, "cat-file", "blob", rev)
+}
+
+func listTreeRecursive(dir, commit string) ([]string, error) {
+	return treePaths(git(dir, nil, "ls-tree", "-r", "--name-only", commit))
+}
+
+func listTree(dir, commit, prefix string) ([]string, error) {
+	return treePaths(git(dir, nil, "ls-tree", "--name-only", commit, prefix))
+}
+
+func treePaths(out []byte, err error) ([]string, error) {
+	if err != nil {
+		return nil, err
+	}
+	var paths []string
+	for _, line := range strings.Split(strings.TrimSpace(string(out)), "\n") {
+		if line != "" {
+			paths = append(paths, line)
+		}
+	}
+	return paths, nil
+}
+
+// catFiles reads many blobs in one exec, in the order asked; a missing path
+// yields nil.
+func catFiles(dir string, specs []string) ([][]byte, error) {
+	if len(specs) == 0 {
+		return nil, nil
+	}
+	out, err := gitIn(dir, []byte(strings.Join(specs, "\n")+"\n"), nil, "cat-file", "--batch")
+	if err != nil {
+		return nil, err
+	}
+
+	blobs := make([][]byte, 0, len(specs))
+	for rest := out; len(blobs) < len(specs); {
+		line, tail, ok := bytes.Cut(rest, []byte("\n"))
+		if !ok {
+			break
+		}
+		fields := bytes.Fields(line)
+		// "<oid> <type> <size>" for a hit, "<spec> missing" for a miss.
+		if len(fields) != 3 {
+			blobs = append(blobs, nil)
+			rest = tail
+			continue
+		}
+		size, err := strconv.Atoi(string(fields[2]))
+		if err != nil || size > len(tail) {
+			return nil, fmt.Errorf("cat-file --batch: unreadable record %q", line)
+		}
+		blobs = append(blobs, tail[:size])
+		rest = tail[size:]
+		rest = bytes.TrimPrefix(rest, []byte("\n"))
+	}
+	return blobs, nil
+}
+
+// grepTree is the whole search. A fixed string, not a regular expression: the
+// input is hostile.
+func grepTree(dir, commit, query, pathspec string) ([]string, error) {
+	out, err := git(dir, nil, "grep", "-i", "-l", "-F", "-e", query, commit, "--", pathspec)
+	if err != nil {
+		// Exit 1 is nothing matched; anything else must not read as an empty
+		// result.
+		var exit *exec.ExitError
+		if errors.As(err, &exit) && exit.ExitCode() == 1 {
+			return nil, nil
+		}
+		return nil, err
+	}
+	var paths []string
+	for _, line := range strings.Split(strings.TrimSpace(string(out)), "\n") {
+		// Lines are "<commit>:<path>".
+		if _, path, ok := strings.Cut(line, ":"); ok {
+			paths = append(paths, path)
+		}
+	}
+	return paths, nil
+}