Merge branch 'mh/lockfile'

The lockfile API and its users have been cleaned up.

* mh/lockfile: (38 commits)
  lockfile.h: extract new header file for the functions in lockfile.c
  hold_locked_index(): move from lockfile.c to read-cache.c
  hold_lock_file_for_append(): restore errno before returning
  get_locked_file_path(): new function
  lockfile.c: rename static functions
  lockfile: rename LOCK_NODEREF to LOCK_NO_DEREF
  commit_lock_file_to(): refactor a helper out of commit_lock_file()
  trim_last_path_component(): replace last_path_elm()
  resolve_symlink(): take a strbuf parameter
  resolve_symlink(): use a strbuf for internal scratch space
  lockfile: change lock_file::filename into a strbuf
  commit_lock_file(): use a strbuf to manage temporary space
  try_merge_strategy(): use a statically-allocated lock_file object
  try_merge_strategy(): remove redundant lock_file allocation
  struct lock_file: declare some fields volatile
  lockfile: avoid transitory invalid states
  git_config_set_multivar_in_file(): avoid call to rollback_lock_file()
  dump_marks(): remove a redundant call to rollback_lock_file()
  api-lockfile: document edge cases
  commit_lock_file(): rollback lock file on failure to rename
  ...

1 files changed, 84 insertions, 0 deletions

diff --git a/lockfile.h b/lockfile.h
new file mode 100644
index 0000000000..9059e8958f
--- /dev/null
+++ b/lockfile.h
@@ -0,0 +1,84 @@
+#ifndef LOCKFILE_H
+#define LOCKFILE_H
+
+/*
+ * File write-locks as used by Git.
+ *
+ * For an overview of how to use the lockfile API, please see
+ *
+ *     Documentation/technical/api-lockfile.txt
+ *
+ * This module keeps track of all locked files in lock_file_list for
+ * use at cleanup. This list and the lock_file objects that comprise
+ * it must be kept in self-consistent states at all time, because the
+ * program can be interrupted any time by a signal, in which case the
+ * signal handler will walk through the list attempting to clean up
+ * any open lock files.
+ *
+ * A lockfile is owned by the process that created it. The lock_file
+ * object has an "owner" field that records its owner. This field is
+ * used to prevent a forked process from closing a lockfile created by
+ * its parent.
+ *
+ * The possible states of a lock_file object are as follows:
+ *
+ * - Uninitialized.  In this state the object's on_list field must be
+ *   zero but the rest of its contents need not be initialized.  As
+ *   soon as the object is used in any way, it is irrevocably
+ *   registered in the lock_file_list, and on_list is set.
+ *
+ * - Locked, lockfile open (after hold_lock_file_for_update(),
+ *   hold_lock_file_for_append(), or reopen_lock_file()). In this
+ *   state:
+ *   - the lockfile exists
+ *   - active is set
+ *   - filename holds the filename of the lockfile
+ *   - fd holds a file descriptor open for writing to the lockfile
+ *   - owner holds the PID of the process that locked the file
+ *
+ * - Locked, lockfile closed (after successful close_lock_file()).
+ *   Same as the previous state, except that the lockfile is closed
+ *   and fd is -1.
+ *
+ * - Unlocked (after commit_lock_file(), commit_lock_file_to(),
+ *   rollback_lock_file(), a failed attempt to lock, or a failed
+ *   close_lock_file()).  In this state:
+ *   - active is unset
+ *   - filename is empty (usually, though there are transitory
+ *     states in which this condition doesn't hold). Client code should
+ *     *not* rely on the filename being empty in this state.
+ *   - fd is -1
+ *   - the object is left registered in the lock_file_list, and
+ *     on_list is set.
+ */
+
+struct lock_file {
+	struct lock_file *volatile next;
+	volatile sig_atomic_t active;
+	volatile int fd;
+	volatile pid_t owner;
+	char on_list;
+	struct strbuf filename;
+};
+
+/* String appended to a filename to derive the lockfile name: */
+#define LOCK_SUFFIX ".lock"
+#define LOCK_SUFFIX_LEN 5
+
+#define LOCK_DIE_ON_ERROR 1
+#define LOCK_NO_DEREF 2
+
+extern int unable_to_lock_error(const char *path, int err);
+extern void unable_to_lock_message(const char *path, int err,
+				   struct strbuf *buf);
+extern NORETURN void unable_to_lock_die(const char *path, int err);
+extern int hold_lock_file_for_update(struct lock_file *, const char *path, int);
+extern int hold_lock_file_for_append(struct lock_file *, const char *path, int);
+extern char *get_locked_file_path(struct lock_file *);
+extern int commit_lock_file_to(struct lock_file *, const char *path);
+extern int commit_lock_file(struct lock_file *);
+extern int reopen_lock_file(struct lock_file *);
+extern int close_lock_file(struct lock_file *);
+extern void rollback_lock_file(struct lock_file *);
+
+#endif /* LOCKFILE_H */