diff options
Diffstat (limited to 'src/common/pg_lzcompress.c')
| -rw-r--r-- | src/common/pg_lzcompress.c | 52 |
1 files changed, 41 insertions, 11 deletions
diff --git a/src/common/pg_lzcompress.c b/src/common/pg_lzcompress.c index 3adad62d9a4..78f105447ce 100644 --- a/src/common/pg_lzcompress.c +++ b/src/common/pg_lzcompress.c @@ -714,11 +714,13 @@ pglz_decompress(const char *source, int32 slen, char *dest, if (ctrl & 1) { /* - * Otherwise it contains the match length minus 3 and the - * upper 4 bits of the offset. The next following byte - * contains the lower 8 bits of the offset. If the length is - * coded as 18, another extension tag byte tells how much - * longer the match really was (0-255). + * Set control bit means we must read a match tag. The match + * is coded with two bytes. First byte uses lower nibble to + * code length - 3. Higher nibble contains upper 4 bits of the + * offset. The next following byte contains the lower 8 bits + * of the offset. If the length is coded as 18, another + * extension tag byte tells how much longer the match really + * was (0-255). */ int32 len; int32 off; @@ -731,16 +733,44 @@ pglz_decompress(const char *source, int32 slen, char *dest, /* * Now we copy the bytes specified by the tag from OUTPUT to - * OUTPUT. It is dangerous and platform dependent to use - * memcpy() here, because the copied areas could overlap - * extremely! + * OUTPUT (copy len bytes from dp - off to dp). The copied + * areas could overlap, to preven possible uncertainty, we + * copy only non-overlapping regions. */ len = Min(len, destend - dp); - while (len--) + while (off < len) { - *dp = dp[-off]; - dp++; + /*--------- + * When offset is smaller than length - source and + * destination regions overlap. memmove() is resolving + * this overlap in an incompatible way with pglz. Thus we + * resort to memcpy()-ing non-overlapping regions. + * + * Consider input: 112341234123412341234 + * At byte 5 here ^ we have match with length 16 and + * offset 4. 11234M(len=16, off=4) + * We are decoding first period of match and rewrite match + * 112341234M(len=12, off=8) + * + * The same match is now at position 9, it points to the + * same start byte of output, but from another position: + * the offset is doubled. + * + * We iterate through this offset growth until we can + * proceed to usual memcpy(). If we would try to decode + * the match at byte 5 (len=16, off=4) by memmove() we + * would issue memmove(5, 1, 16) which would produce + * 112341234XXXXXXXXXXXX, where series of X is 12 + * undefined bytes, that were at bytes [5:17]. + *--------- + */ + memcpy(dp, dp - off, off); + len -= off; + dp += off; + off += off; } + memcpy(dp, dp - off, len); + dp += len; } else { |