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git/tempfile.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

265 lines
9.0 KiB
C
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#ifndef TEMPFILE_H
#define TEMPFILE_H
#include "list.h"
/*
* Handle temporary files.
*
* The tempfile API allows temporary files to be created, deleted, and
* atomically renamed. Temporary files that are still active when the
* program ends are cleaned up automatically. Lockfiles (see
* "lockfile.h") are built on top of this API.
*
*
* Calling sequence
* ----------------
*
* The caller:
*
* * Attempts to create a temporary file by calling
* `create_tempfile()`. The resources used for the temporary file are
* managed by the tempfile API.
*
* * Writes new content to the file by either:
*
* * writing to the `tempfile->fd` file descriptor
*
* * calling `fdopen_tempfile()` to get a `FILE` pointer for the
* open file and writing to the file using stdio.
*
* Note that the file descriptor created by create_tempfile()
* is marked O_CLOEXEC, so the new contents must be written by
* the current process, not any spawned one.
*
* When finished writing, the caller can:
*
* * Close the file descriptor and remove the temporary file by
* calling `delete_tempfile()`.
*
* * Close the temporary file and rename it atomically to a specified
* filename by calling `rename_tempfile()`. This relinquishes
* control of the file.
*
* * Close the file descriptor without removing or renaming the
* temporary file by calling `close_tempfile_gently()`, and later call
* `delete_tempfile()` or `rename_tempfile()`.
*
* After the temporary file is renamed or deleted, the `tempfile`
* object is no longer valid and should not be reused.
*
* If the program exits before `rename_tempfile()` or
* `delete_tempfile()` is called, an `atexit(3)` handler will close
* and remove the temporary file.
*
* If you need to close the file descriptor yourself, do so by calling
* `close_tempfile_gently()`. You should never call `close(2)` or `fclose(3)`
* yourself, otherwise the `struct tempfile` structure would still
* think that the file descriptor needs to be closed, and a later
* cleanup would result in duplicate calls to `close(2)`. Worse yet,
* if you close and then later open another file descriptor for a
* completely different purpose, then the unrelated file descriptor
* might get closed.
*
*
* Error handling
* --------------
*
* `create_tempfile()` returns an allocated tempfile on success or NULL
* on failure. On errors, `errno` describes the reason for failure.
*
* `delete_tempfile()`, `rename_tempfile()`, and `close_tempfile_gently()`
* return 0 on success. On failure they set `errno` appropriately and return
* -1. `delete` and `rename` (but not `close`) do their best to delete the
* temporary file before returning.
*/
struct tempfile {
volatile struct volatile_list_head list;
volatile sig_atomic_t active;
volatile int fd;
FILE *volatile fp;
volatile pid_t owner;
struct strbuf filename;
};
/*
* Attempt to create a temporary file at the specified `path`. Return
* a tempfile (whose "fd" member can be used for writing to it), or
* NULL on error. It is an error if a file already exists at that path.
*/
extern struct tempfile *create_tempfile(const char *path);
/*
* Register an existing file as a tempfile, meaning that it will be
* deleted when the program exits. The tempfile is considered closed,
* but it can be worked with like any other closed tempfile (for
* example, it can be opened using reopen_tempfile()).
*/
extern struct tempfile *register_tempfile(const char *path);
/*
* mks_tempfile functions
*
* The following functions attempt to create and open temporary files
* with names derived automatically from a template, in the manner of
* mkstemps(), and arrange for them to be deleted if the program ends
* before they are deleted explicitly. There is a whole family of such
* functions, named according to the following pattern:
*
* x?mks_tempfile_t?s?m?()
*
* The optional letters have the following meanings:
*
* x - die if the temporary file cannot be created.
*
* t - create the temporary file under $TMPDIR (as opposed to
* relative to the current directory). When these variants are
* used, template should be the pattern for the filename alone,
* without a path.
*
* s - template includes a suffix that is suffixlen characters long.
*
* m - the temporary file should be created with the specified mode
* (otherwise, the mode is set to 0600).
*
* None of these functions modify template. If the caller wants to
* know the (absolute) path of the file that was created, it can be
* read from tempfile->filename.
*
* On success, the functions return a tempfile whose "fd" member is open
* for writing the temporary file. On errors, they return NULL and set
* errno appropriately (except for the "x" variants, which die() on
* errors).
*/
/* See "mks_tempfile functions" above. */
extern struct tempfile *mks_tempfile_sm(const char *template,
int suffixlen, int mode);
/* See "mks_tempfile functions" above. */
static inline struct tempfile *mks_tempfile_s(const char *template,
int suffixlen)
{
return mks_tempfile_sm(template, suffixlen, 0600);
}
/* See "mks_tempfile functions" above. */
static inline struct tempfile *mks_tempfile_m(const char *template, int mode)
{
return mks_tempfile_sm(template, 0, mode);
}
/* See "mks_tempfile functions" above. */
static inline struct tempfile *mks_tempfile(const char *template)
{
return mks_tempfile_sm(template, 0, 0600);
}
/* See "mks_tempfile functions" above. */
extern struct tempfile *mks_tempfile_tsm(const char *template,
int suffixlen, int mode);
/* See "mks_tempfile functions" above. */
static inline struct tempfile *mks_tempfile_ts(const char *template,
int suffixlen)
{
return mks_tempfile_tsm(template, suffixlen, 0600);
}
/* See "mks_tempfile functions" above. */
static inline struct tempfile *mks_tempfile_tm(const char *template, int mode)
{
return mks_tempfile_tsm(template, 0, mode);
}
/* See "mks_tempfile functions" above. */
static inline struct tempfile *mks_tempfile_t(const char *template)
{
return mks_tempfile_tsm(template, 0, 0600);
}
/* See "mks_tempfile functions" above. */
extern struct tempfile *xmks_tempfile_m(const char *template, int mode);
/* See "mks_tempfile functions" above. */
static inline struct tempfile *xmks_tempfile(const char *template)
{
return xmks_tempfile_m(template, 0600);
}
/*
* Associate a stdio stream with the temporary file (which must still
* be open). Return `NULL` (*without* deleting the file) on error. The
* stream is closed automatically when `close_tempfile_gently()` is called or
* when the file is deleted or renamed.
*/
extern FILE *fdopen_tempfile(struct tempfile *tempfile, const char *mode);
static inline int is_tempfile_active(struct tempfile *tempfile)
{
return tempfile && tempfile->active;
}
/*
* 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.
*/
extern const char *get_tempfile_path(struct tempfile *tempfile);
extern int get_tempfile_fd(struct tempfile *tempfile);
extern FILE *get_tempfile_fp(struct tempfile *tempfile);
/*
* If the temporary file is still open, close it (and the file pointer
* too, if it has been opened using `fdopen_tempfile()`) without
* deleting the file. Return 0 upon success. On failure to `close(2)`,
* return a negative value. Usually `delete_tempfile()` or `rename_tempfile()`
* should eventually be called regardless of whether `close_tempfile_gently()`
* succeeds.
*/
extern int close_tempfile_gently(struct tempfile *tempfile);
/*
* Re-open a temporary file that has been closed using
* `close_tempfile_gently()` but not yet deleted or renamed. This can be used
* to implement a sequence of operations like the following:
*
* * Create temporary file.
*
* * Write new contents to file, then `close_tempfile_gently()` to cause the
* contents to be written to disk.
*
* * Pass the name of the temporary file to another program to allow
* it (and nobody else) to inspect or even modify the file's
* contents.
*
* * `reopen_tempfile()` to reopen the temporary file. Make further
* updates to the contents.
*
* * `rename_tempfile()` to move the file to its permanent location.
*/
extern int reopen_tempfile(struct tempfile *tempfile);
/*
* Close the file descriptor and/or file pointer and remove the
* temporary file associated with `tempfile`. It is a NOOP to call
* `delete_tempfile()` for a `tempfile` object that has already been
* deleted or renamed.
*/
extern void delete_tempfile(struct tempfile **tempfile_p);
/*
* Close the file descriptor and/or file pointer if they are still
* open, and atomically rename the temporary file to `path`. `path`
* must be on the same filesystem as the lock file. Return 0 on
* success. On failure, delete the temporary 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 `rename_tempfile()` for a
* `tempfile` object that is not currently active.
*/
extern int rename_tempfile(struct tempfile **tempfile_p, const char *path);
#endif /* TEMPFILE_H */
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