On my macOS 10.12 machine, test_extract_subject fails consistently. It should parse the test certificate and extract its subject, producing this string:

"C=US,ST=California,L=Palo Alto,O=MongoDB,OU=Drivers,CN=server"

Instead, it produces:

"C=(null),ST=(null),L=(null) Alto,O=(null),OU=(null),CN=(null)"

The failure occurs when the the driver tries to get a C string pointer from the Core Foundation string reference:

CFStringGetCStringPtr (str, CFStringGetFastestEncoding (str));

For some reason that returns NULL on my machine. I've found a similar unresolved bug report on StackOverflow.

Making an ASCII-decoded copy of the string is slower and more complicated but works reliably, I think the performance hit is insignificant:

CFStringEncoding encoding = kCFStringEncodingASCII;
CFIndex maxSize = CFStringGetMaximumSizeForEncoding (length, encoding) + 1;
char *cs = bson_malloc ((size_t) maxSize);
CFStringGetCString (str, cs, maxSize, encoding);