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git/lockfile.h
Jeff King 076aa2cbda tempfile: auto-allocate tempfiles on heap 
The previous commit taught the tempfile code to give up
ownership over tempfiles that have been renamed or deleted.
That makes it possible to use a stack variable like this:

  struct tempfile t;

  create_tempfile(&t, ...);
  ...
  if (!err)
          rename_tempfile(&t, ...);
  else
          delete_tempfile(&t);

But doing it this way has a high potential for creating
memory errors. The tempfile we pass to create_tempfile()
ends up on a global linked list, and it's not safe for it to
go out of scope until we've called one of those two
deactivation functions.

Imagine that we add an early return from the function that
forgets to call delete_tempfile(). With a static or heap
tempfile variable, the worst case is that the tempfile hangs
around until the program exits (and some functions like
setup_shallow_temporary rely on this intentionally, creating
a tempfile and then leaving it for later cleanup).

But with a stack variable as above, this is a serious memory
error: the variable goes out of scope and may be filled with
garbage by the time the tempfile code looks at it.  Let's
see if we can make it harder to get this wrong.

Since many callers need to allocate arbitrary numbers of
tempfiles, we can't rely on static storage as a general
solution. So we need to turn to the heap. We could just ask
all callers to pass us a heap variable, but that puts the
burden on them to call free() at the right time.

Instead, let's have the tempfile code handle the heap
allocation _and_ the deallocation (when the tempfile is
deactivated and removed from the list).

This changes the return value of all of the creation
functions. For the cleanup functions (delete and rename),
we'll add one extra bit of safety: instead of taking a
tempfile pointer, we'll take a pointer-to-pointer and set it
to NULL after freeing the object. This makes it safe to
double-call functions like delete_tempfile(), as the second
call treats the NULL input as a noop. Several callsites
follow this pattern.

The resulting patch does have a fair bit of noise, as each
caller needs to be converted to handle:

  1. Storing a pointer instead of the struct itself.

  2. Passing the pointer instead of taking the struct
     address.

  3. Handling a "struct tempfile *" return instead of a file
     descriptor.

We could play games to make this less noisy. For example, by
defining the tempfile like this:

  struct tempfile {
	struct heap_allocated_part_of_tempfile {
                int fd;
                ...etc
        } *actual_data;
  }

Callers would continue to have a "struct tempfile", and it
would be "active" only when the inner pointer was non-NULL.
But that just makes things more awkward in the long run.
There aren't that many callers, so we can simply bite
the bullet and adjust all of them. And the compiler makes it
easy for us to find them all.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2017-09-06 17:19:54 +09:00

308 lines
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C
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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_gently()`, 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_gently()`. 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 and return -1. The `commit` variants (but not `close`)
* do their best to delete the temporary file before returning.
*/
#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 silently returns -1 to the caller, or ...
*/
#define LOCK_DIE_ON_ERROR 1
/*
* ... this flag can be passed instead to return -1 and give the usual
* error message upon an error.
*/
#define LOCK_REPORT_ON_ERROR 4
/*
* 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);
}
/*
* Return a nonzero value iff `lk` is currently locked.
*/
static inline int is_lock_file_locked(struct lock_file *lk)
{
return is_tempfile_active(lk->tempfile);
}
/*
* 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_gently()` 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.
*/
static inline int close_lock_file_gently(struct lock_file *lk)
{
return close_tempfile_gently(lk->tempfile);
}
/*
* Re-open a lockfile that has been closed using `close_lock_file_gently()`
* 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_gently()` 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 */
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