AIS: Add better handling of non byte aligned data blobs

I ran into a problem with the type 8 unstructured data. When the number
of bits available is not dividable by 8, the last byte may contain
random bits. I would expect the bits beyond the valid range to be 0.

An example of this can be found in the following message:
!AIVDM,3,1,4,A,85PH6TAKfDOkp95`nCRt5w<:qFUiaihFhBc7s4AHGsQ,0*40
!AIVDM,3,2,4,A,DcMJM18k6<=m7rwVm3b5RRWEskwJWej8uP<0:W5K6PA,0*61
!AIVDM,3,3,4,A,gPs<dwulp,4*14

The current version decodes this to:

{"class":"AIS","device":"stdin","type":8,"repeat":0,"mmsi":369493649,"scaled":true,"dac":366,"fid":57,"data":"510:47f3e09168d938bc17f30ae56971a71c16c12ac7ec44585fb854add69d048cc630dd47ebf9b50ea1628a757bcff6a7b7223d80c00a9c56c6811be0eccb3ff74e"}

The 510 bits of data are 63 full bytes and then 6 remaining bits. The
last byte has value: 4e, which is 0100 1110. I would expect 4c, 0100 1100.

To fix this, the following patch could be applied. Please note that the
problem might also be present in other message types, but I have not
encountered any problems in other message types in the 1 million
messages I have in my test dataset. Message types that could be impacted
are: 6, 17, 25 and 26, as they all use the same memcpy to get
unstructured data.

Identified by: Stefan Roels <sroels-gpsd-dev@42solutions.nl>

Added new sample entry to sample.aivdm that exercises this case
Had to regenerate large ais test data log

1 files changed, 10 insertions, 4 deletions

diff --git a/driver_ais.c b/driver_ais.c
index 535a420f..a5c26c0a 100644
--- a/driver_ais.c
+++ b/driver_ais.c
@@ -736,10 +736,16 @@ bool ais_binary_decode(const struct gpsd_errout_t *errout,
 	    }
 	}
 	/* land here if we failed to match a known DAC/FID */
-	if (!ais->type8.structured)
-	    (void)memcpy(ais->type8.bitdata,
-			 (char *)bits + (56 / CHAR_BIT),
-			 BITS_TO_BYTES(ais->type8.bitcount));
+	if (!ais->type8.structured) {
+		size_t number_of_bytes = BITS_TO_BYTES(ais->type8.bitcount);
+		(void)memcpy(ais->type8.bitdata,
+				(char *)bits + (56 / CHAR_BIT),
+				number_of_bytes);
+		size_t valid_bits_in_last_byte = ais->type8.bitcount % CHAR_BIT;
+		if(valid_bits_in_last_byte>0)
+			ais->type8.bitdata[number_of_bytes-1] &= (0xFF
+					<< (8-valid_bits_in_last_byte));
+	}
 	break;
     case 9: /* Standard SAR Aircraft Position Report */
 	PERMISSIVE_LENGTH_CHECK(168);