fix(ssh): wrap agent signers to continue to next identity on signing failure

When an SSH agent signer fails to sign (e.g. user cancels a FIDO2/security-key
user-presence prompt), golang.org/x/crypto/ssh treats the error as fatal and
aborts the entire publickey authentication, preventing remaining identities
from being tried. This deviates from OpenSSH behavior where a declined
signature simply moves on to the next key.

This change wraps agent-backed signers in a failoverSigner that returns a
deliberately invalid signature on signing failure instead of propagating the
error. The server rejects the invalid signature as a normal auth failure,
allowing RetryableAuthMethod to continue with the next identity.

Also adds conndebug logging for:
- Agent socket dial failures
- Agent key listing errors
- Number of identities provided by the agent
- Each agent identity being attempted
- Per-key signing failures

Fixes #3365

Author: Jason-Shen2

pkg/remote/sshclient.go

@@ -310,6 +310,8 @@ func createPublicKeyCallback(connCtx context.Context, sshKeywords *wconfig.ConnK
 		if len(*authSockSignersPtr) != 0 {
 			authSockSigner := (*authSockSignersPtr)[0]
 			*authSockSignersPtr = (*authSockSignersPtr)[1:]
+			blocklogger.Infof(connCtx, "[conndebug] trying agent identity %s %s...\n",
+				authSockSigner.PublicKey().Type(), ssh.FingerprintSHA256(authSockSigner.PublicKey()))
 			return []ssh.Signer{authSockSigner}, nil
 		}
 
@@ -772,10 +774,19 @@ func createClientConfig(connCtx context.Context, sshKeywords *wconfig.ConnKeywor
 	if !utilfn.SafeDeref(sshKeywords.SshIdentitiesOnly) && agentPath != "" {
 		conn, err := dialIdentityAgent(agentPath)
 		if err != nil {
+			blocklogger.Infof(connCtx, "[conndebug] failed to open identity agent socket %q: %v\n", agentPath, err)
 			log.Printf("Failed to open Identity Agent Socket %q: %v", agentPath, err)
 		} else {
 			agentClient = agent.NewClient(conn)
-			authSockSigners, _ = agentClient.Signers()
+			agentSigners, err := agentClient.Signers()
+			if err != nil {
+				blocklogger.Infof(connCtx, "[conndebug] failed to list identity agent keys: %v\n", err)
+				log.Printf("Failed to list identity agent keys: %v", err)
+			}
+			blocklogger.Infof(connCtx, "[conndebug] identity agent provided %d identities\n", len(agentSigners))
+			for _, agentSigner := range agentSigners {
+				authSockSigners = append(authSockSigners, failoverSigner{signer: agentSigner, connCtx: connCtx})
+			}
 		}
 	}
 

pkg/remote/sshsigners.go

@@ -0,0 +1,51 @@
+// Copyright 2025, Command Line Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+package remote
+
+import (
+	"context"
+	"io"
+
+	"github.com/wavetermdev/waveterm/pkg/blocklogger"
+	"golang.org/x/crypto/ssh"
+)
+
+type failoverSigner struct {
+	signer  ssh.Signer
+	connCtx context.Context
+}
+
+func (f failoverSigner) PublicKey() ssh.PublicKey {
+	return f.signer.PublicKey()
+}
+
+func (f failoverSigner) Sign(rand io.Reader, data []byte) (*ssh.Signature, error) {
+	sig, err := f.signer.Sign(rand, data)
+	if err == nil {
+		return sig, nil
+	}
+	blocklogger.Infof(f.connCtx, "[conndebug] agent signing failed for key %s %s (%v); continuing with next identity\n",
+		f.signer.PublicKey().Type(), ssh.FingerprintSHA256(f.signer.PublicKey()), err)
+	return f.invalidSignature(), nil
+}
+
+func (f failoverSigner) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*ssh.Signature, error) {
+	if as, ok := f.signer.(ssh.AlgorithmSigner); ok {
+		sig, err := as.SignWithAlgorithm(rand, data, algorithm)
+		if err == nil {
+			return sig, nil
+		}
+		blocklogger.Infof(f.connCtx, "[conndebug] agent signing failed for key %s %s (%v); continuing with next identity\n",
+			f.signer.PublicKey().Type(), ssh.FingerprintSHA256(f.signer.PublicKey()), err)
+		return f.invalidSignature(), nil
+	}
+	return f.Sign(rand, data)
+}
+
+func (f failoverSigner) invalidSignature() *ssh.Signature {
+	return &ssh.Signature{
+		Format: f.signer.PublicKey().Type(),
+		Blob:   []byte("invalid-signature-identity-skipped"),
+	}
+}