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diff --git a/lockfile.h b/lockfile.h
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+#ifndef LOCKFILE_H
+#define LOCKFILE_H
+
+/*
+ * File write-locks as used by Git.
+ *
+ * The lockfile API serves two purposes:
+ *
+ * * Mutual exclusion and atomic file updates. When we want to change
+ *   a file, we create a lockfile `<filename>.lock`, write the new
+ *   file contents into it, and then rename the lockfile to its final
+ *   destination `<filename>`. We create the `<filename>.lock` file
+ *   with `O_CREAT|O_EXCL` so that we can notice and fail if somebody
+ *   else has already locked the file, then atomically rename the
+ *   lockfile to its final destination to commit the changes and
+ *   unlock the file.
+ *
+ * * Automatic cruft removal. If the program exits after we lock a
+ *   file but before the changes have been committed, we want to make
+ *   sure that we remove the lockfile. This is done by remembering the
+ *   lockfiles we have created in a linked list and setting up an
+ *   `atexit(3)` handler and a signal handler that clean up the
+ *   lockfiles. This mechanism ensures that outstanding lockfiles are
+ *   cleaned up if the program exits (including when `die()` is
+ *   called) or if the program is terminated by a signal.
+ *
+ * Please note that lockfiles only block other writers. Readers do not
+ * block, but they are guaranteed to see either the old contents of
+ * the file or the new contents of the file (assuming that the
+ * filesystem implements `rename(2)` atomically).
+ *
+ * Most of the heavy lifting is done by the tempfile module (see
+ * "tempfile.h").
+ *
+ * Calling sequence
+ * ----------------
+ *
+ * The caller:
+ *
+ * * Allocates a `struct lock_file` either as a static variable or on
+ *   the heap, initialized to zeros. Once you use the structure to
+ *   call the `hold_lock_file_for_*()` family of functions, it belongs
+ *   to the lockfile subsystem and its storage must remain valid
+ *   throughout the life of the program (i.e. you cannot use an
+ *   on-stack variable to hold this structure).
+ *
+ * * Attempts to create a lockfile by calling `hold_lock_file_for_update()`.
+ *
+ * * Writes new content for the destination file by either:
+ *
+ *   * writing to the file descriptor returned by the
+ *     `hold_lock_file_for_*()` functions (also available via
+ *     `lock->fd`).
+ *
+ *   * calling `fdopen_lock_file()` to get a `FILE` pointer for the
+ *     open file and writing to the file using stdio.
+ *
+ *   Note that the file descriptor returned by hold_lock_file_for_update()
+ *   is marked O_CLOEXEC, so the new contents must be written by the
+ *   current process, not a spawned one.
+ *
+ * When finished writing, the caller can:
+ *
+ * * Close the file descriptor and rename the lockfile to its final
+ *   destination by calling `commit_lock_file()` or
+ *   `commit_lock_file_to()`.
+ *
+ * * Close the file descriptor and remove the lockfile by calling
+ *   `rollback_lock_file()`.
+ *
+ * * Close the file descriptor without removing or renaming the
+ *   lockfile by calling `close_lock_file()`, and later call
+ *   `commit_lock_file()`, `commit_lock_file_to()`,
+ *   `rollback_lock_file()`, or `reopen_lock_file()`.
+ *
+ * Even after the lockfile is committed or rolled back, the
+ * `lock_file` object must not be freed or altered by the caller.
+ * However, it may be reused; just pass it to another call of
+ * `hold_lock_file_for_update()`.
+ *
+ * If the program exits before `commit_lock_file()`,
+ * `commit_lock_file_to()`, or `rollback_lock_file()` is called, the
+ * tempfile module will close and remove the lockfile, thereby rolling
+ * back any uncommitted changes.
+ *
+ * If you need to close the file descriptor you obtained from a
+ * `hold_lock_file_for_*()` function yourself, do so by calling
+ * `close_lock_file()`. See "tempfile.h" for more information.
+ *
+ *
+ * Under the covers, a lockfile is just a tempfile with a few helper
+ * functions. In particular, the state diagram and the cleanup
+ * machinery are all implemented in the tempfile module.
+ *
+ *
+ * Error handling
+ * --------------
+ *
+ * The `hold_lock_file_for_*()` functions return a file descriptor on
+ * success or -1 on failure (unless `LOCK_DIE_ON_ERROR` is used; see
+ * "flags" below). On errors, `errno` describes the reason for
+ * failure. Errors can be reported by passing `errno` to
+ * `unable_to_lock_message()` or `unable_to_lock_die()`.
+ *
+ * Similarly, `commit_lock_file`, `commit_lock_file_to`, and
+ * `close_lock_file` return 0 on success. On failure they set `errno`
+ * appropriately, do their best to roll back the lockfile, and return
+ * -1.
+ */
+
+#include "tempfile.h"
+
+struct lock_file {
+	struct tempfile tempfile;
+};
+
+/* String appended to a filename to derive the lockfile name: */
+#define LOCK_SUFFIX ".lock"
+#define LOCK_SUFFIX_LEN 5
+
+
+/*
+ * Flags
+ * -----
+ *
+ * The following flags can be passed to `hold_lock_file_for_update()`.
+ */
+
+/*
+ * If a lock is already taken for the file, `die()` with an error
+ * message. If this flag is not specified, trying to lock a file that
+ * is already locked returns -1 to the caller.
+ */
+#define LOCK_DIE_ON_ERROR 1
+
+/*
+ * Usually symbolic links in the destination path are resolved. This
+ * means that (1) the lockfile is created by adding ".lock" to the
+ * resolved path, and (2) upon commit, the resolved path is
+ * overwritten. However, if `LOCK_NO_DEREF` is set, then the lockfile
+ * is created by adding ".lock" to the path argument itself. This
+ * option is used, for example, when detaching a symbolic reference,
+ * which for backwards-compatibility reasons, can be a symbolic link
+ * containing the name of the referred-to-reference.
+ */
+#define LOCK_NO_DEREF 2
+
+/*
+ * Attempt to create a lockfile for the file at `path` and return a
+ * file descriptor for writing to it, or -1 on error. If the file is
+ * currently locked, retry with quadratic backoff for at least
+ * timeout_ms milliseconds. If timeout_ms is 0, try exactly once; if
+ * timeout_ms is -1, retry indefinitely. The flags argument and error
+ * handling are described above.
+ */
+extern int hold_lock_file_for_update_timeout(
+		struct lock_file *lk, const char *path,
+		int flags, long timeout_ms);
+
+/*
+ * Attempt to create a lockfile for the file at `path` and return a
+ * file descriptor for writing to it, or -1 on error. The flags
+ * argument and error handling are described above.
+ */
+static inline int hold_lock_file_for_update(
+		struct lock_file *lk, const char *path,
+		int flags)
+{
+	return hold_lock_file_for_update_timeout(lk, path, flags, 0);
+}
+
+/*
+ * Append an appropriate error message to `buf` following the failure
+ * of `hold_lock_file_for_update()` to lock `path`. `err` should be the
+ * `errno` set by the failing call.
+ */
+extern void unable_to_lock_message(const char *path, int err,
+				   struct strbuf *buf);
+
+/*
+ * Emit an appropriate error message and `die()` following the failure
+ * of `hold_lock_file_for_update()` to lock `path`. `err` should be the
+ * `errno` set by the failing
+ * call.
+ */
+extern NORETURN void unable_to_lock_die(const char *path, int err);
+
+/*
+ * Associate a stdio stream with the lockfile (which must still be
+ * open). Return `NULL` (*without* rolling back the lockfile) on
+ * error. The stream is closed automatically when `close_lock_file()`
+ * is called or when the file is committed or rolled back.
+ */
+static inline FILE *fdopen_lock_file(struct lock_file *lk, const char *mode)
+{
+	return fdopen_tempfile(&lk->tempfile, mode);
+}
+
+/*
+ * Return the path of the lockfile. The return value is a pointer to a
+ * field within the lock_file object and should not be freed.
+ */
+static inline const char *get_lock_file_path(struct lock_file *lk)
+{
+	return get_tempfile_path(&lk->tempfile);
+}
+
+static inline int get_lock_file_fd(struct lock_file *lk)
+{
+	return get_tempfile_fd(&lk->tempfile);
+}
+
+static inline FILE *get_lock_file_fp(struct lock_file *lk)
+{
+	return get_tempfile_fp(&lk->tempfile);
+}
+
+/*
+ * Return the path of the file that is locked by the specified
+ * lock_file object. The caller must free the memory.
+ */
+extern char *get_locked_file_path(struct lock_file *lk);
+
+/*
+ * If the lockfile is still open, close it (and the file pointer if it
+ * has been opened using `fdopen_lock_file()`) without renaming the
+ * lockfile over the file being locked. Return 0 upon success. On
+ * failure to `close(2)`, return a negative value and roll back the
+ * lock file. Usually `commit_lock_file()`, `commit_lock_file_to()`,
+ * or `rollback_lock_file()` should eventually be called if
+ * `close_lock_file()` succeeds.
+ */
+static inline int close_lock_file(struct lock_file *lk)
+{
+	return close_tempfile(&lk->tempfile);
+}
+
+/*
+ * Re-open a lockfile that has been closed using `close_lock_file()`
+ * but not yet committed or rolled back. This can be used to implement
+ * a sequence of operations like the following:
+ *
+ * * Lock file.
+ *
+ * * Write new contents to lockfile, then `close_lock_file()` to
+ *   cause the contents to be written to disk.
+ *
+ * * Pass the name of the lockfile to another program to allow it (and
+ *   nobody else) to inspect the contents you wrote, while still
+ *   holding the lock yourself.
+ *
+ * * `reopen_lock_file()` to reopen the lockfile. Make further updates
+ *   to the contents.
+ *
+ * * `commit_lock_file()` to make the final version permanent.
+ */
+static inline int reopen_lock_file(struct lock_file *lk)
+{
+	return reopen_tempfile(&lk->tempfile);
+}
+
+/*
+ * Commit the change represented by `lk`: close the file descriptor
+ * and/or file pointer if they are still open and rename the lockfile
+ * to its final destination. Return 0 upon success. On failure, roll
+ * back the lock file and return -1, with `errno` set to the value
+ * from the failing call to `close(2)` or `rename(2)`. It is a bug to
+ * call `commit_lock_file()` for a `lock_file` object that is not
+ * currently locked.
+ */
+extern int commit_lock_file(struct lock_file *lk);
+
+/*
+ * Like `commit_lock_file()`, but rename the lockfile to the provided
+ * `path`. `path` must be on the same filesystem as the lock file.
+ */
+static inline int commit_lock_file_to(struct lock_file *lk, const char *path)
+{
+	return rename_tempfile(&lk->tempfile, path);
+}
+
+/*
+ * Roll back `lk`: close the file descriptor and/or file pointer and
+ * remove the lockfile. It is a NOOP to call `rollback_lock_file()`
+ * for a `lock_file` object that has already been committed or rolled
+ * back.
+ */
+static inline void rollback_lock_file(struct lock_file *lk)
+{
+	delete_tempfile(&lk->tempfile);
+}
+
+#endif /* LOCKFILE_H */