[cloud] Improve cloud shell error messages (#548)

## Summary

This shows better error messages when cloud shell fails. A possibly
common case is when a nix package doesn't exist or doesn't build
correctly. Ideally we don't end the shell and let the user fix it, but
in the meantime this shows a more relevant error message.

To implement this efficiently and extensibly, this uses an io.Writer to
collect pre-defined ssh session errors without explicitly storing the
output.

## How was it tested?

* Added a bad package to my devbox.json and did `devbox cloud shell`
* To test generic failures did `devbox cloud shell`, turned off my wifi
and waited for a timeout

<img width="1200" alt="image"
src="https://user-images.githubusercontent.com/544948/215221477-63380ba9-8fd1-440b-9056-32e585d2131d.png">

Author: mikeland73

devbox.json

@@ -17,4 +17,4 @@
   "nixpkgs": {
     "commit": "3954218cf613eba8e0dcefa9abe337d26bc48fd0"
   }
-}
+}

internal/boxcli/usererr/usererr.go

@@ -59,6 +59,17 @@ func WithUserMessage(source error, msg string, args ...any) error {
 	}
 }
 
+func WithLoggedUserMessage(source error, msg string, args ...any) error {
+	if source == nil || HasUserMessage(source) {
+		return source
+	}
+	return &combined{
+		logged:      true,
+		source:      source,
+		userMessage: fmt.Sprintf(msg, args...),
+	}
+}
+
 func HasUserMessage(err error) bool {
 	c := &combined{}
 	return errors.As(err, &c) // note double pointer

internal/cloud/cloud.go

@@ -360,7 +360,8 @@ func shell(username, hostname, projectDir string, shellStartTime time.Time) erro
 		ShellStartTime: telemetry.UnixTimestampFromTime(shellStartTime),
 		Username:       username,
 	}
-	return client.Shell()
+	sessionErrors := newSSHSessionErrors()
+	return cloudShellErrorHandler(client.Shell(sessionErrors), sessionErrors)
 }
 
 // relativeProjectPathInVM refers to the project path relative to the user's

internal/cloud/errors.go

@@ -0,0 +1,64 @@
+package cloud
+
+import (
+	"io"
+	"strings"
+
+	"go.jetpack.io/devbox/internal/boxcli/usererr"
+)
+
+const errApplyNixDerivationString = "Error: apply Nix derivation:"
+
+var sshSessionErrorStrings = []string{
+	errApplyNixDerivationString,
+}
+
+// sshSessionErrors is a helper struct to collect errors from ssh sessions.
+// For performance and privacy it doesn't actually keep any content from the
+// sessions, but instead just keeps track of which errors were encountered.
+type sshSessionErrors struct {
+	errors map[string]bool
+}
+
+var _ io.Writer = (*sshSessionErrors)(nil)
+
+func newSSHSessionErrors() *sshSessionErrors {
+	return &sshSessionErrors{
+		errors: make(map[string]bool),
+	}
+}
+
+func (s *sshSessionErrors) Write(p []byte) (n int, err error) {
+	for _, errorString := range sshSessionErrorStrings {
+		if strings.Contains(string(p), errorString) {
+			s.errors[errorString] = true
+		}
+	}
+	return len(p), nil
+}
+
+// cloudShellErrorHandler is a helper function to handle ssh errors that
+// may contain nix errors in them. For now being cautious and logging them
+// to Sentry even though they may be due to user action.
+func cloudShellErrorHandler(err error, sessionErrors *sshSessionErrors) error {
+	if err == nil {
+		return nil
+	}
+
+	// This usually on initial setup when running start_devbox_shell.sh
+	if found := sessionErrors.errors[errApplyNixDerivationString]; found {
+		return usererr.WithLoggedUserMessage(
+			err,
+			"Failed to apply Nix derivation. This can happen if your devbox (nix) "+
+				"packages don't exist or failed to build. Please check your "+
+				"devbox.json and try again",
+		)
+	}
+
+	// This can happen due to connection issues or any other unforeseen errors
+	return usererr.WithLoggedUserMessage(
+		err,
+		"Your cloud shell terminated unexpectedly. Please check your connection "+
+			"and devbox.json and try again",
+	)
+}

internal/cloud/openssh/client.go

@@ -6,6 +6,7 @@ package openssh
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"io/fs"
 	"net"
 	"os"
@@ -23,7 +24,7 @@ type Client struct {
 	Username       string
 }
 
-func (c *Client) Shell() error {
+func (c *Client) Shell(w io.Writer) error {
 
 	cmd := c.cmd("-t")
 	remoteCmd := fmt.Sprintf(
@@ -33,8 +34,8 @@ func (c *Client) Shell() error {
 	)
 	cmd.Args = append(cmd.Args, remoteCmd)
 	cmd.Stdin = os.Stdin
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
+	cmd.Stdout = io.MultiWriter(os.Stdout, w)
+	cmd.Stderr = io.MultiWriter(os.Stderr, w)
 	return logCmdRun(cmd)
 }