[PATCH 06/13] dir.c: always copy input to add_pattern()

The add_pattern() function has a subtle and undocumented gotcha: the pattern string you pass in must remain valid as long as the pattern_list is in use (and nor do we take ownership of it). This is easy to get wrong, causing either subtle bugs (because you free or reuse the string buffer) or leaks (because you copy the string, but don't track ownership separately).

All of this "pattern" code was originally the "exclude" mechanism. So this _usually_ works OK because you add entries in one of two ways:

  1. From the command-line (e.g., "--exclude"), in which case we're
     pointing to an argv entry which remains valid for the lifetime of
     the program.

  2. From a file (e.g., ".gitignore"), in which case we read the whole
     file into a buffer, attach it to the pattern_list's "filebuf"
     entry, then parse the buffer in-place (adding NULs). The strings
     point into the filebuf, which is cleaned up when the whole
     pattern_list goes away.

But other code, like sparse-checkout, reads individual lines from stdin and passes them one by one to add_pattern(), leaking each. We could fix this by refactoring it to take in the whole buffer at once, like (2) above, and stuff it in "filebuf". But given how subtle the interface is, let's just fix it to always copy the string.

That seems at first like we'd be wasting extra memory, but we can mitigate that:

  a. The path_pattern struct already uses a FLEXPTR, since we sometimes
     make a copy (when we see "foo/", we strip off the trailing slash,
     requiring a modifiable copy of the string).

     Since we'll now always embed the string inside the struct, we can
     switch to the regular FLEX_ARRAY pattern, saving us 8 bytes of
     pointer. So patterns with a trailing slash and ones under 8 bytes
     actually get smaller.

  b. Now that we don't need the original string to hang around, we can
     get rid of the "filebuf" mechanism entirely, and just free the file
     contents after parsing. Since files are the sources we'd expect to
     have the largest pattern sets, we should mostly break even on
     stuffing the same data into the individual structs.

This patch just adjusts the add_pattern() interface; it doesn't fix any leaky callers yet.

Signed-off-by: Jeff King <peff@peff.net>
---
 dir.c | 15 +++++----------
 dir.h |  3 ++-
 2 files changed, 7 insertions(+), 11 deletions(-)