From John Stebbins original post (with pathc, Thanks!):

Author: erik Date: Fri Oct 7 19:06:24 2011 New Revision: 1239 Log: Always attempt to move forward on a seek libdvdnav has a problem in that it is difficult to gracefully recover from a read error and continue on to subsequent blocks on the disc. An application would like to seek forward past the current block after getting a read error in order to attempt to get past the bad block(s). But dvdnav_sector_search() does not guarantee that a requested forward seek will actually move the current position forward. It truncates down to the nearest VOBU which will almost always cause a forward seek request by a single block (or a small number of blocks) to move the current position backward. This behaviour puts the application into a loop of: read failure, attempted forward seek (which results in backward seek), read failure ... Modified: trunk/libdvdnav/src/searching.c Modified: trunk/libdvdnav/src/searching.c ============================================================================== --- trunk/libdvdnav/src/searching.c Fri Oct 7 18:56:02 2011 (r1238) +++ trunk/libdvdnav/src/searching.c Fri Oct 7 19:06:24 2011 (r1239) @@ -47,7 +47,7 @@ /* Return placed in vobu. */ /* Returns error status */ /* FIXME: Maybe need to handle seeking outside current cell. */ -static dvdnav_status_t dvdnav_scan_admap(dvdnav_t *this, int32_t domain, uint32_t seekto_block, uint32_t *vobu) { +static dvdnav_status_t dvdnav_scan_admap(dvdnav_t *this, int32_t domain, uint32_t seekto_block, int next, uint32_t *vobu) { vobu_admap_t *admap = NULL; #ifdef LOG_DEBUG @@ -89,7 +89,7 @@ static dvdnav_status_t dvdnav_scan_admap vobu_start = next_vobu; address++; } - *vobu = vobu_start; + *vobu = next ? next_vobu : vobu_start; return DVDNAV_STATUS_OK; } fprintf(MSG_OUT, "libdvdnav: admap not located\n"); @@ -160,7 +160,7 @@ dvdnav_status_t dvdnav_time_search(dvdna fprintf(MSG_OUT, "libdvdnav: Seeking to cell %i from choice of %i to %i\n", cell_nr, first_cell_nr, last_cell_nr); #endif - if (dvdnav_scan_admap(this, state->domain, target, &vobu) == DVDNAV_STATUS_OK) { + if (dvdnav_scan_admap(this, state->domain, target, 0, &vobu) == DVDNAV_STATUS_OK) { uint32_t start = state->pgc->cell_playback[cell_nr-1].first_sector; if (vm_jump_cell_block(this->vm, cell_nr, vobu - start)) { @@ -184,9 +184,13 @@ dvdnav_status_t dvdnav_time_search(dvdna dvdnav_status_t dvdnav_sector_search(dvdnav_t *this, uint64_t offset, int32_t origin) { uint32_t target = 0; + uint32_t current_pos; + uint32_t cur_sector; + uint32_t cur_cell_nr; uint32_t length = 0; uint32_t first_cell_nr, last_cell_nr, cell_nr; int32_t found; + int forward = 0; cell_playback_t *cell; dvd_state_t *state; dvdnav_status_t result; @@ -213,6 +217,10 @@ dvdnav_status_t dvdnav_sector_search(dvd fprintf(MSG_OUT, "libdvdnav: Before cellN=%u blockN=%u\n", state->cellN, state->blockN); #endif + current_pos = target; + cur_sector = this->vobu.vobu_start + this->vobu.blockN; + cur_cell_nr = state->cellN; + switch(origin) { case SEEK_SET: if(offset >= length) { @@ -244,6 +252,7 @@ dvdnav_status_t dvdnav_sector_search(dvd pthread_mutex_unlock(&this->vm_lock); return DVDNAV_STATUS_ERR; } + forward = target > current_pos; this->cur_cell_time = 0; if (this->pgc_based) { @@ -270,6 +279,27 @@ dvdnav_status_t dvdnav_sector_search(dvd } else { /* convert the target sector from Cell-relative to absolute physical sector */ target += cell->first_sector; + if (forward && (cell_nr == cur_cell_nr)) { + uint32_t vobu; + /* if we are seeking forward from the current position, make sure + * we move to a new position that is after our current position. + * simply truncating to the vobu will go backwards */ + if (dvdnav_scan_admap(this, state->domain, target, 0, &vobu) != DVDNAV_STATUS_OK) + break; + if (vobu <= cur_sector) { + if (dvdnav_scan_admap(this, state->domain, target, 1, &vobu) != DVDNAV_STATUS_OK) + break; + if (vobu > cell->last_sector) { + if (cell_nr == last_cell_nr) + break; + cell_nr++; + cell = &(state->pgc->cell_playback[cell_nr-1]); + target = cell->first_sector; + } else { + target = vobu; + } + } + } found = 1; break; } @@ -281,7 +311,7 @@ dvdnav_status_t dvdnav_sector_search(dvd fprintf(MSG_OUT, "libdvdnav: Seeking to cell %i from choice of %i to %i\n", cell_nr, first_cell_nr, last_cell_nr); #endif - if (dvdnav_scan_admap(this, state->domain, target, &vobu) == DVDNAV_STATUS_OK) { + if (dvdnav_scan_admap(this, state->domain, target, 0, &vobu) == DVDNAV_STATUS_OK) { int32_t start = state->pgc->cell_playback[cell_nr-1].first_sector; if (vm_jump_cell_block(this->vm, cell_nr, vobu - start)) {