Add `NewCredentialSSHKeyFromSigner` (#706)

This change adds `NewCredentialSSHKeyFromSigner`, which allows idiomatic
use of SSH keys from Go. This also lets us spin off an SSH server in the
tests.

1 files changed, 62 insertions, 0 deletions

diff --git a/credentials.go b/credentials.go
index b1051b9..deb399a 100644
--- a/credentials.go
+++ b/credentials.go
@@ -3,15 +3,20 @@ package git
 /*
 #include <git2.h>
 #include <git2/credential.h>
+#include <git2/sys/credential.h>
 
 git_credential_t _go_git_credential_credtype(git_credential *cred);
+void _go_git_populate_credential_ssh_custom(git_credential_ssh_custom *cred);
 */
 import "C"
 import (
+	"crypto/rand"
 	"fmt"
 	"runtime"
 	"strings"
 	"unsafe"
+
+	"golang.org/x/crypto/ssh"
 )
 
 // CredentialType is a bitmask of supported credential types.
@@ -192,6 +197,63 @@ func NewCredentialSSHKeyFromAgent(username string) (*Credential, error) {
 	return cred, nil
 }
 
+type credentialSSHCustomData struct {
+	signer ssh.Signer
+}
+
+//export credentialSSHCustomFree
+func credentialSSHCustomFree(cred *C.git_credential_ssh_custom) {
+	if cred == nil {
+		return
+	}
+
+	C.free(unsafe.Pointer(cred.username))
+	C.free(unsafe.Pointer(cred.publickey))
+	pointerHandles.Untrack(cred.payload)
+	C.free(unsafe.Pointer(cred))
+}
+
+//export credentialSSHSignCallback
+func credentialSSHSignCallback(
+	errorMessage **C.char,
+	sig **C.uchar,
+	sig_len *C.size_t,
+	data *C.uchar,
+	data_len C.size_t,
+	handle unsafe.Pointer,
+) C.int {
+	signer := pointerHandles.Get(handle).(*credentialSSHCustomData).signer
+	signature, err := signer.Sign(rand.Reader, C.GoBytes(unsafe.Pointer(data), C.int(data_len)))
+	if err != nil {
+		return setCallbackError(errorMessage, err)
+	}
+	*sig = (*C.uchar)(C.CBytes(signature.Blob))
+	*sig_len = C.size_t(len(signature.Blob))
+	return C.int(ErrorCodeOK)
+}
+
+// NewCredentialSSHKeyFromSigner creates new SSH credentials using the provided signer.
+func NewCredentialSSHKeyFromSigner(username string, signer ssh.Signer) (*Credential, error) {
+	publicKey := signer.PublicKey().Marshal()
+
+	ccred := (*C.git_credential_ssh_custom)(C.calloc(1, C.size_t(unsafe.Sizeof(C.git_credential_ssh_custom{}))))
+	ccred.parent.credtype = C.GIT_CREDENTIAL_SSH_CUSTOM
+	ccred.username = C.CString(username)
+	ccred.publickey = (*C.char)(C.CBytes(publicKey))
+	ccred.publickey_len = C.size_t(len(publicKey))
+	C._go_git_populate_credential_ssh_custom(ccred)
+
+	data := credentialSSHCustomData{
+		signer: signer,
+	}
+	ccred.payload = pointerHandles.Track(&data)
+
+	cred := newCredential()
+	cred.ptr = &ccred.parent
+
+	return cred, nil
+}
+
 func NewCredentialDefault() (*Credential, error) {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()