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, 38 insertions, 47 deletions

diff --git a/tempfile.h b/tempfile.h
index e32b4df..b8f4b5e 100644
--- a/tempfile.h
+++ b/tempfile.h
@@ -17,22 +17,18 @@
  *
  * The caller:
  *
- * * Allocates a `struct tempfile`. Once the structure is passed to
- *   `create_tempfile()`, its storage must remain valid until
- *   `delete_tempfile()` or `rename_tempfile()` is called on it.
- *
  * * Attempts to create a temporary file by calling
- *   `create_tempfile()`.
+ *   `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 file descriptor returned by `create_tempfile()`
- *     (also available via `tempfile->fd`).
+ *   * 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 returned by create_tempfile()
+ *   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.
  *
@@ -50,7 +46,7 @@
  *   `delete_tempfile()` or `rename_tempfile()`.
  *
  * After the temporary file is renamed or deleted, the `tempfile`
- * object may be reused or freed.
+ * 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
@@ -69,8 +65,8 @@
  * Error handling
  * --------------
  *
- * `create_tempfile()` returns a file descriptor on success or -1 on
- * failure. On errors, `errno` describes the reason for failure.
+ * `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
@@ -89,10 +85,10 @@ struct tempfile {
 
 /*
  * Attempt to create a temporary file at the specified `path`. Return
- * a file descriptor for writing to it, or -1 on error. It is an error
- * if a file already exists at that path.
+ * 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 int create_tempfile(struct tempfile *tempfile, const char *path);
+extern struct tempfile *create_tempfile(const char *path);
 
 /*
  * Register an existing file as a tempfile, meaning that it will be
@@ -100,7 +96,7 @@ extern int create_tempfile(struct tempfile *tempfile, const char *path);
  * but it can be worked with like any other closed tempfile (for
  * example, it can be opened using reopen_tempfile()).
  */
-extern void register_tempfile(struct tempfile *tempfile, const char *path);
+extern struct tempfile *register_tempfile(const char *path);
 
 
 /*
@@ -132,70 +128,65 @@ extern void register_tempfile(struct tempfile *tempfile, const char *path);
  * know the (absolute) path of the file that was created, it can be
  * read from tempfile->filename.
  *
- * On success, the functions return a file descriptor that is open for
- * writing the temporary file. On errors, they return -1 and set errno
- * appropriately (except for the "x" variants, which die() on errors).
+ * 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 int mks_tempfile_sm(struct tempfile *tempfile,
-			   const char *template, int suffixlen, int mode);
+extern struct tempfile *mks_tempfile_sm(const char *template,
+					int suffixlen, int mode);
 
 /* See "mks_tempfile functions" above. */
-static inline int mks_tempfile_s(struct tempfile *tempfile,
-				 const char *template, int suffixlen)
+static inline struct tempfile *mks_tempfile_s(const char *template,
+					      int suffixlen)
 {
-	return mks_tempfile_sm(tempfile, template, suffixlen, 0600);
+	return mks_tempfile_sm(template, suffixlen, 0600);
 }
 
 /* See "mks_tempfile functions" above. */
-static inline int mks_tempfile_m(struct tempfile *tempfile,
-				 const char *template, int mode)
+static inline struct tempfile *mks_tempfile_m(const char *template, int mode)
 {
-	return mks_tempfile_sm(tempfile, template, 0, mode);
+	return mks_tempfile_sm(template, 0, mode);
 }
 
 /* See "mks_tempfile functions" above. */
-static inline int mks_tempfile(struct tempfile *tempfile,
-			       const char *template)
+static inline struct tempfile *mks_tempfile(const char *template)
 {
-	return mks_tempfile_sm(tempfile, template, 0, 0600);
+	return mks_tempfile_sm(template, 0, 0600);
 }
 
 /* See "mks_tempfile functions" above. */
-extern int mks_tempfile_tsm(struct tempfile *tempfile,
-			    const char *template, int suffixlen, int mode);
+extern struct tempfile *mks_tempfile_tsm(const char *template,
+					 int suffixlen, int mode);
 
 /* See "mks_tempfile functions" above. */
-static inline int mks_tempfile_ts(struct tempfile *tempfile,
-				  const char *template, int suffixlen)
+static inline struct tempfile *mks_tempfile_ts(const char *template,
+					       int suffixlen)
 {
-	return mks_tempfile_tsm(tempfile, template, suffixlen, 0600);
+	return mks_tempfile_tsm(template, suffixlen, 0600);
 }
 
 /* See "mks_tempfile functions" above. */
-static inline int mks_tempfile_tm(struct tempfile *tempfile,
-				  const char *template, int mode)
+static inline struct tempfile *mks_tempfile_tm(const char *template, int mode)
 {
-	return mks_tempfile_tsm(tempfile, template, 0, mode);
+	return mks_tempfile_tsm(template, 0, mode);
 }
 
 /* See "mks_tempfile functions" above. */
-static inline int mks_tempfile_t(struct tempfile *tempfile,
-				 const char *template)
+static inline struct tempfile *mks_tempfile_t(const char *template)
 {
-	return mks_tempfile_tsm(tempfile, template, 0, 0600);
+	return mks_tempfile_tsm(template, 0, 0600);
 }
 
 /* See "mks_tempfile functions" above. */
-extern int xmks_tempfile_m(struct tempfile *tempfile,
-			   const char *template, int mode);
+extern struct tempfile *xmks_tempfile_m(const char *template, int mode);
 
 /* See "mks_tempfile functions" above. */
-static inline int xmks_tempfile(struct tempfile *tempfile,
-				const char *template)
+static inline struct tempfile *xmks_tempfile(const char *template)
 {
-	return xmks_tempfile_m(tempfile, template, 0600);
+	return xmks_tempfile_m(template, 0600);
 }
 
 /*
@@ -257,7 +248,7 @@ extern int reopen_tempfile(struct tempfile *tempfile);
  * `delete_tempfile()` for a `tempfile` object that has already been
  * deleted or renamed.
  */
-extern void delete_tempfile(struct tempfile *tempfile);
+extern void delete_tempfile(struct tempfile **tempfile_p);
 
 /*
  * Close the file descriptor and/or file pointer if they are still
@@ -268,6 +259,6 @@ extern void delete_tempfile(struct tempfile *tempfile);
  * `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, const char *path);
+extern int rename_tempfile(struct tempfile **tempfile_p, const char *path);
 
 #endif /* TEMPFILE_H */