Add structured logging and better error handling

internal/explain/explain.go

@@ -0,0 +1,88 @@
+package explain
+
+import (
+	"crypto/tls"
+	"crypto/x509"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"net"
+	"syscall"
+
+	"ddc/internal/guard"
+)
+
+var keptAsIs = []error{
+	guard.ErrBlockedAddress,
+	guard.ErrBlockedPort,
+	guard.ErrTooManyRedirects,
+	guard.ErrUnsupportedScheme,
+}
+
+func Error(err error) string {
+	if err == nil {
+		return ""
+	}
+	for _, known := range keptAsIs {
+		if errors.Is(err, known) {
+			return capitalize(known.Error())
+		}
+	}
+	if message := networkMessage(err); message != "" {
+		return message
+	}
+	if message := fileSystemMessage(err); message != "" {
+		return message
+	}
+	return capitalize(err.Error())
+}
+
+func networkMessage(err error) string {
+	var dnsError *net.DNSError
+	var certificateError *tls.CertificateVerificationError
+	var unknownAuthority x509.UnknownAuthorityError
+	var hostnameError x509.HostnameError
+	var timeoutError net.Error
+
+	switch {
+	case errors.As(err, &dnsError):
+		return fmt.Sprintf("Could not find the server %q. Check the link.", dnsError.Name)
+	case errors.As(err, &certificateError), errors.As(err, &unknownAuthority), errors.As(err, &hostnameError):
+		return "The server's security certificate is not valid."
+	case errors.Is(err, syscall.ECONNREFUSED):
+		return "The server refused the connection."
+	case errors.Is(err, syscall.ECONNRESET), errors.Is(err, io.ErrUnexpectedEOF):
+		return "The server closed the connection before the file was complete. Use Retry to continue."
+	case errors.Is(err, syscall.ENETUNREACH), errors.Is(err, syscall.EHOSTUNREACH):
+		return "The server could not be reached from this network."
+	case errors.As(err, &timeoutError) && timeoutError.Timeout():
+		return "The server took too long to respond."
+	default:
+		return ""
+	}
+}
+
+func fileSystemMessage(err error) string {
+	switch {
+	case errors.Is(err, syscall.ENOSPC):
+		return "The disk is full."
+	case errors.Is(err, fs.ErrPermission):
+		return "The server does not have permission to write there. Check the folder owner and permissions."
+	case errors.Is(err, syscall.EROFS):
+		return "The destination is on a read-only file system."
+	default:
+		return ""
+	}
+}
+
+func capitalize(message string) string {
+	if message == "" {
+		return message
+	}
+	first := message[0]
+	if first >= 'a' && first <= 'z' {
+		return string(first-'a'+'A') + message[1:]
+	}
+	return message
+}