win32: add a method to trigger the Crypto API to complete a certificate chain

I started a thread[1] on the mailing list awhile ago, but the short version is
that Windows doesn't ship with a full list of certificates[2].  Even if the
server sends the whole chain, if Windows doesn't have the appropriate
certificate pre-installed in its "Third-Party Root Certification Authorities"
store, connections mysteriously fail with:

  abort: error: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:661)

Windows expects the application to call the methods invoked here as part of the
certificate verification, triggering a call out to Windows update if necessary,
to complete the trust chain.  The python bug to add this support[3] hasn't had
any recent activity, and isn't targeting py27 anyway.

The only work around that I could find (besides figuring out the certificate and
walking through the import wizard) is to browse to the site in Internet
Explorer.  Opening the page with FireFox or Chrome didn't work.  That's a pretty
obscure way to fix a pretty obscure problem.  We go to great lengths to
demystify various SSL errors, but this case is clearly lacking.  Let's try to
make things easier to diagnose and fix.

When I had trouble figuring out how to get ctypes to work with all of the API
pointers, I found that there are other python projects[4] using this API to
achieve the same thing.

[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-April/096501.html
[2] https://support.microsoft.com/en-us/help/931125/how-to-get-a-root-certificate-update-for-windows
[3] https://bugs.python.org/issue20916
[4] https://github.com/nvaccess/nvda/blob/3b86bce2066b1934df14b96f2e83369900860ecf/source/updateCheck.py#L511

mercurial/win32.py

@@ -22,6 +22,7 b' from . import ('
 _kernel32 = ctypes.windll.kernel32
 _advapi32 = ctypes.windll.advapi32
 _user32 = ctypes.windll.user32
+_crypt32 = ctypes.windll.crypt32
 
 _BOOL = ctypes.c_long
 _WORD = ctypes.c_ushort
@@ -31,6 +32,7 b' from . import ('
 _LPCSTR = _LPSTR = ctypes.c_char_p
 _HANDLE = ctypes.c_void_p
 _HWND = _HANDLE
+_PCCERT_CONTEXT = ctypes.c_void_p
 
 _INVALID_HANDLE_VALUE = _HANDLE(-1).value
 
@@ -134,8 +136,74 b' class _CONSOLE_SCREEN_BUFFER_INFO(ctypes'
 _STD_OUTPUT_HANDLE = _DWORD(-11).value
 _STD_ERROR_HANDLE = _DWORD(-12).value
 
+# CERT_TRUST_STATUS dwErrorStatus
+CERT_TRUST_IS_PARTIAL_CHAIN = 0x10000
+
+# CertCreateCertificateContext encodings
+X509_ASN_ENCODING = 0x00000001
+PKCS_7_ASN_ENCODING = 0x00010000
+
+# These structs are only complete enough to achieve what we need.
+class CERT_CHAIN_CONTEXT(ctypes.Structure):
+    _fields_ = (
+        ("cbSize", _DWORD),
+
+        # CERT_TRUST_STATUS struct
+        ("dwErrorStatus", _DWORD),
+        ("dwInfoStatus", _DWORD),
+
+        ("cChain", _DWORD),
+        ("rgpChain", ctypes.c_void_p),
+        ("cLowerQualityChainContext", _DWORD),
+        ("rgpLowerQualityChainContext", ctypes.c_void_p),
+        ("fHasRevocationFreshnessTime", _BOOL),
+        ("dwRevocationFreshnessTime", _DWORD),
+    )
+
+class CERT_USAGE_MATCH(ctypes.Structure):
+    _fields_ = (
+        ("dwType", _DWORD),
+
+         # CERT_ENHKEY_USAGE struct
+        ("cUsageIdentifier", _DWORD),
+        ("rgpszUsageIdentifier", ctypes.c_void_p), # LPSTR *
+    )
+
+class CERT_CHAIN_PARA(ctypes.Structure):
+    _fields_ = (
+        ("cbSize", _DWORD),
+        ("RequestedUsage", CERT_USAGE_MATCH),
+        ("RequestedIssuancePolicy", CERT_USAGE_MATCH),
+        ("dwUrlRetrievalTimeout", _DWORD),
+        ("fCheckRevocationFreshnessTime", _BOOL),
+        ("dwRevocationFreshnessTime", _DWORD),
+        ("pftCacheResync", ctypes.c_void_p), # LPFILETIME
+        ("pStrongSignPara", ctypes.c_void_p), # PCCERT_STRONG_SIGN_PARA
+        ("dwStrongSignFlags", _DWORD),
+    )
+
 # types of parameters of C functions used (required by pypy)
 
+_crypt32.CertCreateCertificateContext.argtypes = [_DWORD, # cert encoding
+                                                  ctypes.c_char_p, # cert
+                                                  _DWORD] # cert size
+_crypt32.CertCreateCertificateContext.restype = _PCCERT_CONTEXT
+
+_crypt32.CertGetCertificateChain.argtypes = [
+        ctypes.c_void_p, # HCERTCHAINENGINE
+        _PCCERT_CONTEXT,
+        ctypes.c_void_p, # LPFILETIME
+        ctypes.c_void_p, # HCERTSTORE
+        ctypes.c_void_p, # PCERT_CHAIN_PARA
+        _DWORD,
+        ctypes.c_void_p, # LPVOID
+        ctypes.c_void_p  # PCCERT_CHAIN_CONTEXT *
+    ]
+_crypt32.CertGetCertificateChain.restype = _BOOL
+
+_crypt32.CertFreeCertificateContext.argtypes = [_PCCERT_CONTEXT]
+_crypt32.CertFreeCertificateContext.restype = _BOOL
+
 _kernel32.CreateFileA.argtypes = [_LPCSTR, _DWORD, _DWORD, ctypes.c_void_p,
     _DWORD, _DWORD, _HANDLE]
 _kernel32.CreateFileA.restype = _HANDLE
@@ -234,6 +302,51 b' def _getfileinfo(name):'
     finally:
         _kernel32.CloseHandle(fh)
 
+def checkcertificatechain(cert, build=True):
+    '''Tests the given certificate to see if there is a complete chain to a
+       trusted root certificate.  As a side effect, missing certificates are
+       downloaded and installed unless ``build=False``.  True is returned if a
+       chain to a trusted root exists (even if built on the fly), otherwise
+       False.  NB: A chain to a trusted root does NOT imply that the certificate
+       is valid.
+    '''
+
+    chainctxptr = ctypes.POINTER(CERT_CHAIN_CONTEXT)
+
+    pchainctx = chainctxptr()
+    chainpara = CERT_CHAIN_PARA(cbSize=ctypes.sizeof(CERT_CHAIN_PARA),
+                                RequestedUsage=CERT_USAGE_MATCH())
+
+    certctx = _crypt32.CertCreateCertificateContext(X509_ASN_ENCODING, cert,
+                                                    len(cert))
+    if certctx is None:
+        _raiseoserror('CertCreateCertificateContext')
+
+    flags = 0
+
+    if not build:
+        flags |= 0x100  # CERT_CHAIN_DISABLE_AUTH_ROOT_AUTO_UPDATE
+
+    try:
+        # Building the certificate chain will update root certs as necessary.
+        if not _crypt32.CertGetCertificateChain(None,      # hChainEngine
+                                                certctx,   # pCertContext
+                                                None,      # pTime
+                                                None,      # hAdditionalStore
+                                                ctypes.byref(chainpara),
+                                                flags,
+                                                None,      # pvReserved
+                                                ctypes.byref(pchainctx)):
+            _raiseoserror('CertGetCertificateChain')
+
+        chainctx = pchainctx.contents
+
+        return chainctx.dwErrorStatus & CERT_TRUST_IS_PARTIAL_CHAIN == 0
+    finally:
+        if pchainctx:
+            _crypt32.CertFreeCertificateChain(pchainctx)
+        _crypt32.CertFreeCertificateContext(certctx)
+
 def oslink(src, dst):
     try:
         if not _kernel32.CreateHardLinkA(dst, src, None):