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libctf: fix CTF dict compression

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Commit 483546ce4f ("libctf: make ctf_serialize() actually serialize")
accidentally broke dict compression.  There were two bugs:

 - ctf_arc_write_one_ctf was still making its own decision about
   whether to compress the dict via direct ctf_size comparison, which is
   unfortunate because now that it no longer calls ctf_serialize itself,
   ctf_size is always zero when it does this: it should let the writing
   functions decide on the threshold, which they contain code to do which is
   simply not used for lack of one trivial wrapper to write to an fd and
   also provide a compression threshold

 - ctf_write_mem, the function underlying all writing as of the commit
   above, was calling zlib's compressBound and avoiding compression if this
   returned a value larger than the input.  Unfortunately compressBound does
   not do a trial compression and determine whether the result is
   compressible: it just adds zlib header sizes to the value passed in, so
   our test would *always* have concluded that the value was incompressible!
   Avoid by simply always compressing if the raw size is larger than the
   threshold: zlib is quite clever enough to avoid actually compressing
   if the data is incompressible.

Add a testcase for this.

libctf/
	* ctf-impl.h (ctf_write_thresholded): New...
	* ctf-serialize.c (ctf_write_thresholded): ... defined here,
        a wrapper around...
        (ctf_write_mem): ... this.  Don't check compressibility.
	(ctf_compress_write): Reimplement as a ctf_write_thresholded
        wrapper.
	(ctf_write): Likewise.
	* ctf-archive.c (arc_write_one_ctf): Just call
        ctf_write_thresholded rather than trying to work out whether
        to compress.
	* testsuite/libctf-writable/ctf-compressed.*: New test.
This commit is contained in:
Nick Alcock
2024-07-15 19:55:40 +01:00
parent 87b2f67310
commit 36c771b179
5 changed files with 179 additions and 42 deletions
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2
libctf/ctf-impl.h
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@@ -762,6 +762,8 @@ extern int ctf_flip (ctf_dict_t *, ctf_header_t *, unsigned char *, int);
extern int ctf_import_unref (ctf_dict_t *fp, ctf_dict_t *pfp);
extern int ctf_write_thresholded (ctf_dict_t *fp, int fd, size_t threshold);
_libctf_malloc_
extern void *ctf_mmap (size_t length, size_t offset, int fd);
extern void ctf_munmap (void *, size_t);