r.to.vect fails on a properly thinned tif via r.thin/r.null

[hbgdal]

Hi, This happens only on a small number of tif files. Most of the tif files are processed successfully. Commands to reproduce problem with 7.2.0 are pasted below. I would appreciate any workaround/patch(for 7.2.0 or any other version) for this fix.

Problem: For some tifs, the r.to.vect fails complaining that the image needs to be thinned, and then hangs with a message about busy SQLite DB.

Expected behavior: Either r.to.vect should extract lines from thinned image, or r.thin should thin the image further.

Env: Ubuntu 14.04. 7.2.0 compiled

Commands to run:

#This is "binary" image with two colors black and white
r.in.gdal input=/tmp/grass_problem.tif output=grass_problem location=grass_problem

#Replace "white" color with null for r.thin.
r.null --verbose map=grass_problem@PERMANENT setnull=255 

#r.thin succeeds in less than 10 iterations.(See output below)
r.thin --verbose input=grass_problem@PERMANENT output=grass_thin  

#r.to.vect complains.              
r.to.vect --verbose input=grass_thin@PERMANENT output=vec_lines type=line 


===== r.thin output showing success ====
Raster map <grass_problem@PERMANENT> - 4028 rows X 3288 columns
Bounding box:  l = 3, r = 3284, t = 7, b = 4028
Pass number 1
Deleted 240415 pixels
Pass number 2
Deleted 5154 pixels
Pass number 3
Deleted 115 pixels
Pass number 4
Deleted 5 pixels
Pass number 5
Deleted 0 pixels
Thinning completed successfully.
Output map 4028 rows X 3288 columns
Window 4028 rows X 3288 columns
(Thu Aug  3 11:17:10 2017) Command finished (2 sec)      

=== r.to.vect output showing failure ===

r.to.vect --verbose input=grass_thin@PERMANENT output=vec_lines type=line       
Using native format
Extracting lines...
Crowded cell at (362.125000, 12883.540000): row 424, col 1721, count 7
ERROR: Raster map is not thinned properly.
Please run r.thin.
WARNING: Busy SQLITE db, already waiting for 10 seconds.
....

[hbgdal]

part 00 of gzipped tif file

[hbgdal]

part 01 of gzipped tif file