httpclient: a refused server certificate says which check refused it

src/clients/ioxide.httpclient/Tls/OpenSsl.cs

@@ -170,4 +170,10 @@ public static string LastError()
         }
         return message;
     }
+
+    [LibraryImport(Crypto)] private static partial nint X509_verify_cert_error_string(long n);
+
+    /// <summary>What an X509 verify result means: "certificate has expired", "hostname mismatch", ...</summary>
+    public static string VerifyResultText(long result)
+        => Marshal.PtrToStringUTF8(X509_verify_cert_error_string(result)) ?? $"X509 verify result {result}";
 }

src/clients/ioxide.httpclient/Tls/TlsClientStream.cs

@@ -148,15 +148,20 @@ private async ValueTask RunHandshakeAsync(TlsClientOptions options, string serve
         {
             int ret = OpenSsl.Connect(_ssl, out int err);
 
+            // Read before the alert's flush, during which other connections' OpenSSL calls clear this thread's queue.
+            string? failure = ret != 1 && err != OpenSsl.SSL_ERROR_WANT_READ && err != OpenSsl.SSL_ERROR_WANT_WRITE
+                ? HandshakeFailure(options)
+                : null;
+
             await FlushCiphertextAsync();
 
             if (ret == 1)
             {
                 break;
             }
-            if (err != OpenSsl.SSL_ERROR_WANT_READ && err != OpenSsl.SSL_ERROR_WANT_WRITE)
+            if (failure is not null)
             {
-                throw new IOException($"TLS handshake to '{serverName}' failed: {OpenSsl.LastError()}");
+                throw new IOException($"TLS handshake to '{serverName}' failed: {failure}");
             }
 
             // The deadline is checked between flights rather than raced against the recv: a ring
@@ -192,6 +197,16 @@ private async ValueTask RunHandshakeAsync(TlsClientOptions options, string serve
         NegotiatedAlpn = ReadNegotiatedAlpn(_ssl);
     }
 
+    // The queue says the handshake failed; a refused certificate's verify result says which check refused it.
+    private string HandshakeFailure(TlsClientOptions options)
+    {
+        string error = OpenSsl.LastError();
+        long verify = OpenSsl.SSL_get_verify_result(_ssl);
+        return options.VerifyCertificate && verify != OpenSsl.X509_V_OK
+            ? $"{error}: {OpenSsl.VerifyResultText(verify)}"
+            : error;
+    }
+
     /// <summary>
     /// Encrypt and send. Returns the number of PLAINTEXT bytes accepted, matching
     /// <see cref="RingSocket.SendAsync"/>, so a caller looping until everything is sent works

tests/Ioxide.Tests.Http/TlsClientVerificationTests.cs

@@ -213,12 +213,12 @@ private static void RegisterChain(Runner runner)
                 $"should say the connection closed, got: {body}");
         });
 
-        runner.Pending("tls client: a verification failure says which check failed", () =>
+        runner.Test("tls client: a verification failure says which check failed", () =>
         {
             // Expired, self-signed and unknown-issuer are three different X509 verify codes (10, 18
             // and 20), and a caller acts on them differently: one is a clock or a renewal, one is
             // an origin misconfigured, one is trust configured wrongly on OUR side. All three
-            // arrive as the same sentence.
+            // used to arrive as the same sentence.
             using Authority trusted = Authority.Mint();
             using Authority stranger = Authority.Mint();
 
@@ -240,9 +240,7 @@ private static void RegisterChain(Runner runner)
             string[] reasons = [Reason(expired), Reason(selfSigned), Reason(unknownIssuer)];
             Assert.True(reasons.Distinct().Count() == 3,
                 "three different verification failures reported the same thing: " + reasons[0]);
-        }, "the X509 reason is thrown away: SSL_get_verify_result is consulted only AFTER a handshake "
-           + "that succeeded, where it can only be X509_V_OK, and never on the failure path where it "
-           + "holds the code");
+        });
     }
 
     // --- driving the client ----------------------------------------------------------------------