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>

1 files changed, 8 insertions, 8 deletions

diff --git a/lockfile.h b/lockfile.h
index c45d2db..6ed336e 100644
--- a/lockfile.h
+++ b/lockfile.h
@@ -111,7 +111,7 @@
 #include "tempfile.h"
 
 struct lock_file {
-	struct tempfile tempfile;
+	struct tempfile *tempfile;
 };
 
 /* String appended to a filename to derive the lockfile name: */
@@ -180,7 +180,7 @@ static inline int hold_lock_file_for_update(
  */
 static inline int is_lock_file_locked(struct lock_file *lk)
 {
-	return is_tempfile_active(&lk->tempfile);
+	return is_tempfile_active(lk->tempfile);
 }
 
 /*
@@ -208,7 +208,7 @@ extern NORETURN void unable_to_lock_die(const char *path, int err);
  */
 static inline FILE *fdopen_lock_file(struct lock_file *lk, const char *mode)
 {
-	return fdopen_tempfile(&lk->tempfile, mode);
+	return fdopen_tempfile(lk->tempfile, mode);
 }
 
 /*
@@ -217,17 +217,17 @@ static inline FILE *fdopen_lock_file(struct lock_file *lk, const char *mode)
  */
 static inline const char *get_lock_file_path(struct lock_file *lk)
 {
-	return get_tempfile_path(&lk->tempfile);
+	return get_tempfile_path(lk->tempfile);
 }
 
 static inline int get_lock_file_fd(struct lock_file *lk)
 {
-	return get_tempfile_fd(&lk->tempfile);
+	return get_tempfile_fd(lk->tempfile);
 }
 
 static inline FILE *get_lock_file_fp(struct lock_file *lk)
 {
-	return get_tempfile_fp(&lk->tempfile);
+	return get_tempfile_fp(lk->tempfile);
 }
 
 /*
@@ -246,7 +246,7 @@ extern char *get_locked_file_path(struct lock_file *lk);
  */
 static inline int close_lock_file_gently(struct lock_file *lk)
 {
-	return close_tempfile_gently(&lk->tempfile);
+	return close_tempfile_gently(lk->tempfile);
 }
 
 /*
@@ -270,7 +270,7 @@ static inline int close_lock_file_gently(struct lock_file *lk)
  */
 static inline int reopen_lock_file(struct lock_file *lk)
 {
-	return reopen_tempfile(&lk->tempfile);
+	return reopen_tempfile(lk->tempfile);
 }
 
 /*