do not check odb_mkstemp return value for errors

The odb_mkstemp function does not return an error; it dies
on failure instead. But many of its callers compare the
resulting descriptor against -1 and die themselves.

Mostly this is just pointless, but it does raise a question
when looking at the callers: if they show the results of the
"template" buffer after a failure, what's in it? The answer
is: it doesn't matter, because it cannot happen.

So let's make that clear by removing the bogus error checks.
In bitmap_writer_finish(), we can drop the error-handling
code entirely. In the other two cases, it's shared with the
open() in another code path; we can just move the
error-check next to that open() call.

And while we're at it, let's flesh out the function's
docstring a bit to make the error behavior clear.
Signed-off-by: NJeff King <peff@peff.net>

builtin/index-pack.c

@@ -311,10 +311,11 @@ static const char *open_pack_file(const char *pack_name)
 			output_fd = odb_mkstemp(tmp_file, sizeof(tmp_file),
 						"pack/tmp_pack_XXXXXX");
 			pack_name = xstrdup(tmp_file);
-		} else
+		} else {
 			output_fd = open(pack_name, O_CREAT|O_EXCL|O_RDWR, 0600);
-		if (output_fd < 0)
-			die_errno(_("unable to create '%s'"), pack_name);
+			if (output_fd < 0)
+				die_errno(_("unable to create '%s'"), pack_name);
+		}
 		nothread_data.pack_fd = output_fd;
 	} else {
 		input_fd = open(pack_name, O_RDONLY);

cache.h

@@ -1674,7 +1674,10 @@ extern struct packed_git *find_sha1_pack(const unsigned char *sha1,
 extern void pack_report(void);
 
 /*
- * Create a temporary file rooted in the object database directory.
+ * Create a temporary file rooted in the object database directory, or
+ * die on failure. The filename is taken from "pattern", which should have the
+ * usual "XXXXXX" trailer, and the resulting filename is written into the
+ * "template" buffer. Returns the open descriptor.
  */
 extern int odb_mkstemp(char *template, size_t limit, const char *pattern);
 

pack-bitmap-write.c

@@ -517,8 +517,6 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 
 	int fd = odb_mkstemp(tmp_file, sizeof(tmp_file), "pack/tmp_bitmap_XXXXXX");
 
-	if (fd < 0)
-		die_errno("unable to create '%s'", tmp_file);
 	f = sha1fd(fd, tmp_file);
 
 	memcpy(header.magic, BITMAP_IDX_SIGNATURE, sizeof(BITMAP_IDX_SIGNATURE));

pack-write.c

@@ -77,9 +77,9 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
 		} else {
 			unlink(index_name);
 			fd = open(index_name, O_CREAT|O_EXCL|O_WRONLY, 0600);
+			if (fd < 0)
+				die_errno("unable to create '%s'", index_name);
 		}
-		if (fd < 0)
-			die_errno("unable to create '%s'", index_name);
 		f = sha1fd(fd, index_name);
 	}