Fix managed http/https transports

For http transport, perviously, an initial request without any
credentials was being sent and upon unauthorized response,
credentials were fetched and the request was retried with the
obtained credentials. This appeared to have resulted in the remote
server closing the connection, resulting in clone failure error:

```
unable to clone: Post "http://test-user:***@127.0.0.1:40463/bar/test-reponame/git-upload-pack": io: read/write on closed pipe
```

Querying the credentials at the very beginning and not failing fixes the
closed pipe issue.

For https transport, since the go transport doesn't have access to the
certificate, it results in the following failure:

```
unable to clone: Get "https://127.0.0.1:44185/bar/test-reponame/info/refs?service=git-upload-pack": x509: certificate signed by unknown authority
```

Since the go smart transport is private, there seems to be no way to
pass the certificates to it. Unlike the credentials, there's no method
to fetch the certificate from libgit2.
Some past discussions in libgit2 talks about keeping the data outside of
libgit2 when using an external transport.
With the current structure of the code, it's hard to pass the
certificate to the go transport.
This change introduces a global CA certs pool that can be populated by
the users of git2go and the smart transport can lookup for the presence
of any certificate in the global pool before making any https requests.
This solves the cloning issue due to cert signing.

http.go

@@ -1,6 +1,8 @@
 package git
 
 import (
+	"crypto/tls"
+	"crypto/x509"
 	"errors"
 	"fmt"
 	"io"
@@ -190,8 +192,19 @@ func (self *httpSmartSubtransportStream) sendRequest() error {
 
 	var resp *http.Response
 	var err error
-	var userName string
-	var password string
+
+	// Obtain the credentials and use them.
+	cred, err := self.owner.transport.SmartCredentials("", CredentialTypeUserpassPlaintext)
+	if err != nil {
+		return err
+	}
+	defer cred.Free()
+
+	userName, password, err := cred.GetUserpassPlaintext()
+	if err != nil {
+		return err
+	}
+
 	for {
 		req := &http.Request{
 			Method: self.req.Method,
@@ -204,30 +217,38 @@ func (self *httpSmartSubtransportStream) sendRequest() error {
 		}
 
 		req.SetBasicAuth(userName, password)
-		resp, err = http.DefaultClient.Do(req)
-		if err != nil {
-			return err
-		}
 
-		if resp.StatusCode == http.StatusOK {
-			break
-		}
+		c := http.Client{}
 
-		if resp.StatusCode == http.StatusUnauthorized {
-			resp.Body.Close()
+		cap := x509.NewCertPool()
 
-			cred, err := self.owner.transport.SmartCredentials("", CredentialTypeUserpassPlaintext)
-			if err != nil {
-				return err
-			}
-			defer cred.Free()
+		// NOTE: self.req.URL.Host returns only host without port. To be
+		// able to fetch the correct certs from the global certs, parse again
+		// and get host+port with url.Host.
+		u, err := url.Parse(self.req.URL.String())
+		if err != nil {
+			return fmt.Errorf("failed to parse URL: %w", err)
+		}
 
-			userName, password, err = cred.GetUserpassPlaintext()
-			if err != nil {
-				return err
+		// Use CA cert if found.
+		if cert, found := globalCACertPool.certPool[u.Host]; found {
+			if ok := cap.AppendCertsFromPEM(cert); !ok {
+				return fmt.Errorf("failed to parse CA cert")
 			}
+			c.Transport = &http.Transport{
+				TLSClientConfig: &tls.Config{
+					RootCAs: cap,
+				},
+			}
+		}
 
-			continue
+		resp, err = c.Do(req)
+		if err != nil {
+			return err
+		}
+
+		if resp.StatusCode == http.StatusOK {
+			break
 		}
 
 		// Any other error we treat as a hard error and punt back to the caller

transport.go

@@ -24,6 +24,7 @@ import "C"
 import (
 	"fmt"
 	"io"
+	"net/url"
 	"reflect"
 	"runtime"
 	"sync"
@@ -39,8 +40,38 @@ var (
 	}{
 		transports: make(map[string]*RegisteredSmartTransport),
 	}
+	// globalCACertPool is a mapping of global CA certs used by git2go-managed
+	// transports. The map's key is hostname+port and the value is a
+	// corresponding CA cert.
+	// Since the git2go-managed transports aren't public, this can be used to
+	// provide certs to the subtransports that can be looked up for a given
+	// host.
+	globalCACertPool = struct {
+		sync.Mutex
+		certPool map[string][]byte
+	}{
+		certPool: make(map[string][]byte),
+	}
 )
 
+// RegisterCACerts registers CA cert associated with an address in the
+// globalCACertPool.
+func RegisterCACerts(address string, caBundle []byte) error {
+	globalCACertPool.Lock()
+	defer globalCACertPool.Unlock()
+	// Ignore empty CA bundles.
+	if len(caBundle) == 0 {
+		return nil
+	}
+	u, err := url.Parse(address)
+	if err != nil {
+		return err
+	}
+	// Store the certificate based on host+port, e.g.: 127.0.0.1:42107.
+	globalCACertPool.certPool[u.Host] = caBundle
+	return nil
+}
+
 // unregisterManagedTransports unregisters all git2go-managed transports.
 func unregisterManagedTransports() error {
 	globalRegisteredSmartTransports.Lock()