[PATCH 09/20] tempfile: handle NULL tempfile pointers gracefully

[Jeff King]

The tempfile functions all take pointers to tempfile
objects, but do not check whether the argument is NULL.
This isn't a big deal in practice, since the lifetime of any
tempfile object is defined to last for the whole program. So
even if we try to call delete_tempfile() on an
already-deleted tempfile, our "active" check will tell us
that it's a noop.

In preparation for transitioning to a new system that
loosens the "tempfile objects can never be freed" rule,
let's tighten up our active checks:

  1. A NULL pointer is now defined as "inactive" (so it will
     BUG for most functions, but works as a silent noop for
     things like delete_tempfile).

  2. Functions should always do the "active" check before
     looking at any of the struct fields.

Signed-off-by: Jeff King <p...@peff.net>
---
 tempfile.c | 12 +++++++-----
 tempfile.h |  2 +-
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/tempfile.c b/tempfile.c
index 964c66d504..861f817133 100644
--- a/tempfile.c
+++ b/tempfile.c
@@ -236,13 +236,15 @@ FILE *get_tempfile_fp(struct tempfile *tempfile)
 
 int close_tempfile_gently(struct tempfile *tempfile)
 {
-       int fd = tempfile->fd;
-       FILE *fp = tempfile->fp;
+       int fd;
+       FILE *fp;
        int err;
 
-       if (fd < 0)
+       if (!is_tempfile_active(tempfile) || tempfile->fd < 0)
                return 0;
 
+       fd = tempfile->fd;
+       fp = tempfile->fp;
        tempfile->fd = -1;
        if (fp) {
                tempfile->fp = NULL;
@@ -262,10 +264,10 @@ int close_tempfile_gently(struct tempfile *tempfile)
 
 int reopen_tempfile(struct tempfile *tempfile)
 {
-       if (0 <= tempfile->fd)
-               die("BUG: reopen_tempfile called for an open object");
        if (!is_tempfile_active(tempfile))
                die("BUG: reopen_tempfile called for an inactive object");
+       if (0 <= tempfile->fd)
+               die("BUG: reopen_tempfile called for an open object");
        tempfile->fd = open(tempfile->filename.buf, O_WRONLY);
        return tempfile->fd;
 }
diff --git a/tempfile.h b/tempfile.h
index d854dcdd3e..d30663182d 100644
--- a/tempfile.h
+++ b/tempfile.h
@@ -211,7 +211,7 @@ extern FILE *fdopen_tempfile(struct tempfile *tempfile, 
const char *mode);
 
 static inline int is_tempfile_active(struct tempfile *tempfile)
 {
-       return tempfile->active;
+       return tempfile && tempfile->active;
 }
 
 /*
-- 
2.14.1.721.gc5bc1565f1